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ArticleApril 24, 2020

Aerosol Filtration Efficiency of Common Fabrics Used in Respiratory Cloth Masks
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Abhiteja Konda
Abhiteja Konda
Center for Nanoscale Materials, Argonne National Laboratory, Lemont, Illinois 60439, United States
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http://orcid.org/0000-0003-3362-0583
Abhinav Prakash
Abhinav Prakash
Center for Nanoscale Materials, Argonne National Laboratory, Lemont, Illinois 60439, United States
Pritzker School of Molecular Engineering, University of Chicago, Chicago, Illinois 60637, United States
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http://orcid.org/0000-0002-8899-0568
Gregory A. Moss
Gregory A. Moss
Worker Safety & Health Division, Argonne National Laboratory, Lemont, Illinois 60439, United States
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Michael Schmoldt
Michael Schmoldt
Center for Nanoscale Materials, Argonne National Laboratory, Lemont, Illinois 60439, United States
Worker Safety & Health Division, Argonne National Laboratory, Lemont, Illinois 60439, United States
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Gregory D. Grant
Gregory D. Grant
Pritzker School of Molecular Engineering, University of Chicago, Chicago, Illinois 60637, United States
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Supratik Guha*
Supratik Guha
Pritzker School of Molecular Engineering, University of Chicago, Chicago, Illinois 60637, United States
Argonne National Laboratory, Lemont, Illinois 60439, United States
*Telephone: (914)-325-5147. Email: [email protected]
More by Supratik Guha
http://orcid.org/0000-0001-5071-8318

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ACS Nano

Cite this: ACS Nano 2020, 14, 5, 6339–6347

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https://doi.org/10.1021/acsnano.0c03252

Published April 24, 2020

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Abstract

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The emergence of a pandemic affecting the respiratory system can result in a significant demand for face masks. This includes the use of cloth masks by large sections of the public, as can be seen during the current global spread of COVID-19. However, there is limited knowledge available on the performance of various commonly available fabrics used in cloth masks. Importantly, there is a need to evaluate filtration efficiencies as a function of aerosol particulate sizes in the 10 nm to 10 μm range, which is particularly relevant for respiratory virus transmission. We have carried out these studies for several common fabrics including cotton, silk, chiffon, flannel, various synthetics, and their combinations. Although the filtration efficiencies for various fabrics when a single layer was used ranged from 5 to 80% and 5 to 95% for particle sizes of <300 nm and >300 nm, respectively, the efficiencies improved when multiple layers were used and when using a specific combination of different fabrics. Filtration efficiencies of the hybrids (such as cotton–silk, cotton–chiffon, cotton–flannel) was >80% (for particles <300 nm) and >90% (for particles >300 nm). We speculate that the enhanced performance of the hybrids is likely due to the combined effect of mechanical and electrostatic-based filtration. Cotton, the most widely used material for cloth masks performs better at higher weave densities (i.e., thread count) and can make a significant difference in filtration efficiencies. Our studies also imply that gaps (as caused by an improper fit of the mask) can result in over a 60% decrease in the filtration efficiency, implying the need for future cloth mask design studies to take into account issues of “fit” and leakage, while allowing the exhaled air to vent efficiently. Overall, we find that combinations of various commonly available fabrics used in cloth masks can potentially provide significant protection against the transmission of aerosol particles.

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Note

The units in Figure 2 were corrected April 27, 2020.

Note

This article is made available via the ACS COVID-19 subset for unrestricted RESEARCH re-use and analyses in any form or by any means with acknowledgement of the original source. These permissions are granted for the duration of the World Health Organization (WHO) declaration of COVID-19 as a global pandemic.

The use of cloth masks, many of them homemade, (1,2) has become widely prevalent in response to the 2019–2020 SARS-CoV-2 outbreak, where the virus can be transmitted via respiratory droplets. (3−6) The use of such masks is also an anticipated response of the public in the face of future pandemics related to the respiratory tract. However, there is limited data available today on the performance of common cloth materials used in such cloth masks, (7−12) particularly their filtration efficiencies as a function of different aerosol sizes ranging from ∼10 nm to ∼10 μm scale sizes. This is also of current significance as the relative effectiveness of different droplet sizes in transmitting the SARS-CoV-2 virus is not clear, and understanding the filtration response across a large bracketed size distribution is therefore important. (13−16) In this paper, we report the results of experiments where we measure the filtration efficiencies of a number of common fabrics, as well as selective combinations for use as hybrid cloth masks, as a function of aerosol sizes ranging from ∼10 nm to 6 μm. These include cotton, the most widely used fabric in cloth masks, as well as fabric fibers that can be electrostatically charged, such as natural silk.

Respiratory droplets can be of various sizes (17,18) and are commonly classified as aerosols (made of droplets that are <5 μm) and droplets that are greater than 5 μm. (3) Although the fate of these droplets largely depends on environmental factors such as humidity, temperature, etc., in general, the larger droplets settle due to gravity and do not travel distances more than 1–2 m. (19) However, aerosols remain suspended in the air for longer durations due to their small size and play a key role in spreading infection. (14−16) The use of physical barriers such as respiratory masks can be highly effective in mitigating this spread via respiratory droplets. (20−22) Filtration of aerosols follows five basic mechanisms: gravity sedimentation, inertial impaction, interception, diffusion, and electrostatic attraction. (23,24) For aerosols larger than ∼1 μm to 10 μm, the first two mechanisms play a role, where ballistic energy or gravity forces are the primary influence on the large exhaled droplets. As the aerosol size decreases, diffusion by Brownian motion and mechanical interception of particles by the filter fibers is a predominant mechanism in the 100 nm to 1 μm range. For nanometer-sized particles, which can easily slip between the openings in the network of filter fibers, electrostatic attraction predominates the removal of low mass particles which are attracted to and bind to the fibers. Electrostatic filters are generally most efficient at low velocities such as the velocity encountered by breathing through a face mask. (25)

There have been a few studies reported on the use of cloth face masks mainly during or after the Influenza Pandemic in 2009; (8−12,26) However, there is still a lack of information that includes (i) the performance of various fabrics as a function of particle size from the nanoscale to the micron sized (particularly important because this covers the ∼10 nm to ∼5 μm size scale for aerosols) and (ii) the effect of hybrid multilayer approaches for masks that can combine the benefits of different filtering mechanisms across different aerosol size ranges. (9,26) These have been the objectives of the experimental work described in this paper. In addition, we also point out the importance of fit (that leads to gaps) while using the face mask. (27,28)

The experimental apparatus (see Figure 1) consists of an aerosol generation and mixing chamber and a downstream collection chamber. The air flows from the generation chamber to the collection chamber through the cloth sample that is mounted on a tube connecting the two chambers. The aerosol particles are generated using a commercial sodium chloride (NaCl) aerosol generator (TSI Particle Generator, model #8026), producing particles in the range of a few tens of nanometers to approximately 10 μm. The NaCl aerosol based testing is widely used for testing face respirators in compliance with the NIOSH 42 CFR Part 84 test protocol. (29,30) Two different particle analyzers are used to determine particle size dimensions and concentrations: a TSI Nanoscan SMPS nanoparticle sizer (Nanoscan, model #3910) and a TSI optical particle sizer (OPS, model #3330) for measurements in the range of 10 to 300 nm and 300 nm to 6 μm, respectively.

Figure 1. Schematic of the experimental setup. A polydisperse NaCl aerosol is introduced into the mixing chamber, where it is mixed and passed through the material being tested (“test specimen”). The test specimen is held in place using a clamp for a better seal. The aerosol is sampled before (upstream, C_u) and after (downstream, C_d) it passes through the specimen. The pressure difference is measured using a manometer, and the aerosol flow velocity is measured using a velocity meter. We use two circular holes with a diameter of 0.635 cm to simulate the effect of gaps on the filtration efficiency. The sampled aerosols are analyzed using particle analyzers (OPS and Nanoscan), and the resultant particle concentrations are used to determine filter efficiencies.

Particles are generated upstream of the cloth sample, whose filtration properties are to be tested, and the air is drawn through the cloth using a blower fan which can be controlled in order to vary the airflow rate. Effective area of the cloth sample during the tests was ∼59 cm². Measurements of particle size and distribution were made by sampling air at a distance of 7.5 cm upstream and 15 cm downstream of the cloth sample. The differential pressures and air velocities were measured using a TSI digital manometer (model #AXD620) and a TSI Hot Wire anemometer (model #AVM410). The differential pressure (ΔP) across the sample material is an indicator of the comfort and breathability of the material when used as a face mask. (31) Tests were carried out at two different airflows: 1.2 and 3.2 CFM, representative of respiration rates at rest (∼35 L/min) and during moderate exertion (∼90 L/min), respectively. (32)

The effect of gaps between the contour of the face and the mask as caused by an improper fit will affect the efficiency of any face mask. (21,27,28,33) This is of particular relevance to cloth and surgical masks that are used by the public and which are generally not “fitted”, unlike N95 masks or elastomeric respirators. A preliminary study of this effect was explored by drilling holes (symmetrically) in the connecting tube onto which the fabric (or a N95 or surgical mask) is mounted. The holes, in proximity to the sample (Figure 1), resulted in openings of area ∼0.5−2% of the active sample area. This, therefore, represented “leakage” of the air around the mask.

Although the detailed transmission specifics of SARS-CoV-2 virus are not well understood yet, droplets that are below 5 μm are considered the primary source of transmission in a respiratory infection, (13,15,34) and droplets that are smaller than 1 μm tend to stay in the environment as aerosols for longer durations of up to 8 h. (19) Aerosol droplets containing the SARS-CoV-2 virus have been shown to remain suspended in air for ∼3 h. (13,35) We have therefore targeted our experimental measurements in the important particle size range between ∼10 nm and 6 μm.

We tested the performance of over 15 natural and synthetic fabrics that included materials such as cotton with different thread counts, silk, flannel, and chiffon. The complete list is provided in the Materials and Methods section. For comparison, we also tested a N95 respirator and surgical masks. Additionally, as appropriate, we tested the efficiency of multiple layers of a single fabric or a combination of multiple fabrics for hybrid cloth masks in order to explore combinations of physical filtering as well as electrostatic filtering.

Results and Discussion

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We determine the filtration efficiency of a particular cloth as a function of particle size (Figure 2) by measuring the concentration of the particles upstream, C_u (Figure 2a,b) and the concentration of the particle downstream, C_d (Figure 2c,d). Concentrations were measured in the size ranges of 10–178 nm (using the nanoscan tool) and 300 nm to 6 μm (using the optical particle sizer tool). The representative example in Figure 2 shows the case for a single layer of silk fabric, where the measurements of C_u and C_d were carried out at a flow rate of 1.2 CFM. Following the procedure detailed in the Materials and Methods section, we then estimated the filtration efficiency of a cloth from C_u and C_d as a function of aerosol particle size.

Figure 2. Particle concentration as a function of particle size at a flow rate of 1.2 CFM. Plots showing the particle concentration (in arbitrary units) upstream and downstream through a single layer of natural silk for particle sizes <300 nm (a,c) and between 300 nm and 6 μm (b,d). Each bin shows the particle concentration for at least six trials. The particle concentrations in panels (b) and (d) are given in log scale for better representation of the data. The y-axis scales are the same for panels "a" and "c"; and for panels "b" and "d".

The results plotted in Figure 3a are the filtration efficiencies for cotton (the most common material used in cloth masks) with different thread counts (rated in threads per inch—TPI—and representative of the coarseness or fineness of the fabric). We compare a moderate (80 TPI) thread count quilter’s cotton (often used in do-it-yourself masks) with a high (600 TPI) cotton fabric sample. Additionally, we also measured the transmission through a traditional cotton quilt where two 120 TPI quilter’s cotton sheets sandwich a ∼0.5 cm batting (90% cotton–5% polyester–5% other fibers). Comparing the two cotton sheets with different thread counts, the 600 TPI cotton is clearly superior with >65% efficiency at <300 nm and >90% efficiency at >300 nm, which implies a tighter woven cotton fabric may be preferable. In comparison, the single-layer 80 TPI cotton does not perform as well, with efficiencies varying from ∼5 to ∼55% depending on the particle size across the entire range. The quilt, a commonly available household material, with a fibrous cotton batting also provided excellent filtration across the range of particle sizes (>80% for <300 nm and >90% for >300 nm).

Figure 3. Filtration efficiency of individual fabrics at a flow rate of 1.2 CFM (without gap). (a) Plot showing the filtration efficiencies of a cotton quilt consisting of two 120 threads per inch (TPI) cotton sheets enclosing a ∼0.5 cm thick cotton batting, 80 TPI quilters cotton (Q Cotton 80 TPI), and a 600 TPI cotton (cotton 600 TPI). (b) Plot showing the filtration efficiencies of one layer of natural silk (Silk-1L), four layers of natural silk (Silk-4L), one layer of flannel, and one layer of chiffon. The error bars on the <300 nm measurements are higher, particularly for samples with high filtration efficiencies because of the small number of particles generated in this size range, the relatively poorer counting efficiency of the detector at <300 nm particle size, and the very small counts downstream of the sample. The sizes of the error bars for some of the data points (>300 nm) are smaller than the symbol size and hence not clearly visible.

Electrostatic interactions are commonly observed in various natural and synthetic fabrics. (36,37) For instance, polyester woven fabrics can retain more static charge compared to natural fibers or cotton due to their lower water adsorption properties. (36) The electrostatic filtering of aerosols have been well studied. (38) As a result, we investigated three fabrics expected to possess moderate electrostatic discharge value: natural silk, chiffon (polyester–Spandex), and flannel (cotton–polyester). (36) The results for these are shown in Figure 3b. In the case of silk, we made measurements through one, two, and four layers of the fabric as silk scarves are often wrapped in multiple layers around the face (the results for two layers of silk are presented in Figure S1 (Supporting Information) and omitted from this figure). In all of these cases, the performance in filtering nanosized particles <300 nm is superior to performance in the 300 nm to 6 μm range and particularly effective below ∼30 nm, consistent with the expectations from the electrostatic effects of these materials. Increasing the number of layers (as shown for silk in Figure 3b), as expected, improves the performance. We performed additional experiments to validate this using the 600 TPI cotton and chiffon (Figure S1). We note that the performance of a four-layer silk composite offers >80% filtration efficiency across the entire range, from 10 nm to 6 μm.

In Figure 4a, we combine the nanometer-sized aerosol effectiveness (for silk, chiffon, and flannel) and wearability (of silk and chiffon because of their sheer nature) with the overall high performance of the 600 TPI cotton to examine the filtration performance of hybrid approaches. We made measurements for three variations: combining one layer 600 TPI cotton with two layers of silk, two layers of chiffon, and one layer of flannel. The results are also compared with the performance of a standard N95 mask. All three hybrid combinations performed well, exceeding 80% efficiency in the <300 nm range, and >90% in the >300 nm range. These cloth hybrids are slightly inferior to the N95 mask above 300 nm, but superior for particles smaller than 300 nm. The N95 respirators are designed and engineered to capture more than 95% of the particles that are above 300 nm, (39,40) and therefore, their underperformance in filtering particles below 300 nm is not surprising.

Figure 4. Filtration efficiency of hybrid fabrics at a flow rate of 1.2 CFM. (a) Plot showing the filtration efficiencies without gap for an N95 respirator and a combination of different fabrics: 1 layer of 600 threads per inch (TPI) cotton and 2 layers of silk (cotton/silk), 1 layer of 600 TPI cotton and 2 layers of chiffon (cotton/chiffon), and 1 layer of 600 TPI cotton and 1 layer of flannel (cotton/flannel). (b) Plot showing the filtration efficiencies of a surgical mask and cotton/silk with (dashed) and without a gap (solid). The gap used is ∼1% of the active mask surface area. The error bars on the <300 nm measurements are higher, particularly for samples with high filtration efficiencies because of the small number of particles generated in this size range, the relatively poorer counting efficiency of the detector at <300 nm particle size, and the very small counts downstream of the sample. The sizes of the error bars for some of the data points (>300 nm) are smaller than the symbol size and hence not clearly visible.

It is important to note that in the realistic situation of masks worn on the face without elastomeric gasket fittings (such as the commonly available cloth and surgical masks), the presence of gaps between the mask and the facial contours will result in “leakage” reducing the effectiveness of the masks. It is well recognized that the “fit” is a critical aspect of a high-performance mask. (27,28,33,41) Earlier researchers have attempted to examine this qualitatively in cloth and other masks through feedback on “fit” from human trials. (11,12) In our case, we have made a preliminary examination of this effect via the use of cross-drilled holes on the tube holding the mask material (see Figure 1) that represents leakage of air. For example, in Figure 4b, we compare the performance of the surgical mask and the cotton/silk hybrid sample with and without a hole that represents about ∼1% of the mask area. Whereas the surgical mask provides moderate (>60%) and excellent (close to 100%) particle exclusion below and above 300 nm, respectively, the tests carried out with the 1% opening surprisingly resulted in significant drops in the mask efficiencies across the entire size range (60% drop in the >300 nm range). In this case, the two holes were ∼0.635 cm in diameter and the mask area was ∼59 cm². Similar trends in efficiency drops are seen in the cotton/silk hybrid sample, as well. Hole size also had an influence on the filtration efficiency. In the case of an N95 mask, increasing hole size from 0.5 to 2% of the cloth sample area reduced the weighted average filtration efficiency from ∼60 to 50% for a particle of size <300 nm. It is unclear at this point whether specific aerodynamic effects exacerbate the “leakage” effects when simulated by holes. Its determination is outside the scope of this paper. However, our measurements at both the high flow (3.2 CFM) and low flow (1.2 CFM) rates show substantial drop in effectiveness when holes are present. The results in Figures 2–4 highlight materials with good performance. Several fabrics were tested that did not provide strong filtration protection (<30%), and examples include satin and synthetic silk (Table S1). The filtration efficiencies of all of the samples that we measured at both 1.2 CFM and 3.2 CFM are detailed in the Supporting Information (Figures S2–S4).

In Table 1, we summarize the key findings from the various fabrics and approaches that we find promising. Average filtration efficiencies (see Materials and Methods section for further detail) in the 10–178 nm and 300 nm to 6 μm range are presented along with the differential pressures measured across the cloths, which represents the breathability and degree of comfort of the masks. The average differential pressure across all of the fabrics at a flow rate of 1.2 CFM was found to be 2.5 ± 0.4 Pa, indicating a low resistance and represent conditions for good breathability (Table 1). (31) As expected, we observed an increase in the average differential pressures for the higher flow rate (3.2 CFM) case (Table S1).

Table 1. Filtration Efficiencies of Various Test Specimens at a Flow Rate of 1.2 CFM and the Corresponding Differential Pressure (ΔP) across the Specimen^a

	flow rate: 1.2 CFM
	filter efficiency (%)		pressure differential
sample/fabric	<300 nm average ± error	>300 nm average ± error	ΔP (Pa)
N95 (no gap)	85 ± 15	99.9 ± 0.1	2.2
N95 (with gap)	34 ± 15	12 ± 3	2.2
surgical mask (no gap)	76 ± 22	99.6 ± 0.1	2.5
surgical mask (with gap)	50 ± 7	44 ± 3	2.5
cotton quilt	96 ± 2	96.1 ± 0.3	2.7
quilter’s cotton (80 TPI), 1 layer	9 ± 13	14 ± 1	2.2
quilter’s cotton (80 TPI), 2 layers	38 ± 11	49 ± 3	2.5
flannel	57 ± 8	44 ± 2	2.2
cotton (600 TPI), 1 layer	79 ± 23	98.4 ± 0.2	2.5
cotton (600 TPI), 2 layers	82 ± 19	99.5 ± 0.1	2.5
chiffon, 1 layer	67 ± 16	73 ± 2	2.7
chiffon, 2 layers	83 ± 9	90 ± 1	3.0
natural silk, 1 layer	54 ± 8	56 ± 2	2.5
natural silk, 2 layers	65 ± 10	65 ± 2	2.7
natural silk, 4 layers	86 ± 5	88 ± 1	2.7
hybrid 1: cotton/chiffon	97 ± 2	99.2 ± 0.2	3.0
hybrid 2: cotton/silk (no gap)	94 ± 2	98.5 ± 0.2	3.0
hybrid 2: cotton/silk (gap)	37 ± 7	32 ± 3	3.0
hybrid 3: cotton/flannel	95 ± 2	96 ± 1	3.0

^{^a}

The filtration efficiencies are the weighted averages for each size range—less than 300 nm and more than 300 nm.

Guidance

We highlight a few observations from our studies for cloth mask design:

Fabric with tight weaves and low porosity, such as those found in cotton sheets with high thread count, are preferable. For instance, a 600 TPI cotton performed better than an 80 TPI cotton. Fabrics that are porous should be avoided.

Materials such as natural silk, a chiffon weave (we tested a 90% polyester–10% Spandex fabric), and flannel (we tested a 65% cotton–35% polyester blend) can likely provide good electrostatic filtering of particles. We found that four layers of silk (as maybe the case for a wrapped scarf) provided good protection across the 10 nm to 6 μm range of particulates.

Combining layers to form hybrid masks, leveraging mechanical and electrostatic filtering may be an effective approach. This could include high thread count cotton combined with two layers of natural silk or chiffon, for instance. A quilt consisting of two layers of cotton sandwiching a cotton−polyester batting also worked well. In all of these cases, the filtration efficiency was >80% for <300 nm and >90% for >300 nm sized particles.

The filtration properties noted in (i) through (iii) pertain to the intrinsic properties of the mask material and do not take into account the effect of air leaks that arise due to improper “fit” of a mask on the user’s face. It is critically important that cloth mask designs also take into account the quality of this “fit” to minimize leakage of air between the mask and the contours of the face, while still allowing the exhaled air to be vented effectively. Such leakage can significantly reduce mask effectiveness and are a reason why properly worn N95 masks and masks with elastomeric fittings work so well.

Conclusions

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In conclusion, we have measured the filtration efficiencies of various commonly available fabrics for use as cloth masks in filtering particles in the significant (for aerosol-based virus transmission) size range of ∼10 nm to ∼6 μm and have presented filtration efficiency data as a function of aerosol particle size. We find that cotton, natural silk, and chiffon can provide good protection, typically above 50% in the entire 10 nm to 6.0 μm range, provided they have a tight weave. Higher threads per inch cotton with tighter weaves resulted in better filtration efficiencies. For instance, a 600 TPI cotton sheet can provide average filtration efficiencies of 79 ± 23% (in the 10 nm to 300 nm range) and 98.4 ± 0.2% (in the 300 nm to 6 μm range). A cotton quilt with batting provides 96 ± 2% (10 nm to 300 nm) and 96.1 ± 0.3% (300 nm to 6 μm). Likely the highly tangled fibrous nature of the batting aids in the superior performance at small particle sizes. Materials such as silk and chiffon are particularly effective (considering their sheerness) at excluding particles in the nanoscale regime (<∼100 nm), likely due to electrostatic effects that result in charge transfer with nanoscale aerosol particles. A four-layer silk (used, for instance, as a scarf) was surprisingly effective with an average efficiency of >85% across the 10 nm −6 μm particle size range. As a result, we found that hybrid combinations of cloths such as high threads-per-inch cotton along with silk, chiffon, or flannel can provide broad filtration coverage across both the nanoscale (<300 nm) and micron scale (300 nm to 6 μm) range, likely due to the combined effects of electrostatic and physical filtering. Finally, it is important to note that openings and gaps (such as those between the mask edge and the facial contours) can degrade the performance. Our findings indicate that leakages around the mask area can degrade efficiencies by ∼50% or more, pointing out the importance of “fit”. Opportunities for future studies include cloth mask design for better “fit” and the role of factors such as humidity (arising from exhalation) and the role of repeated use and washing of cloth masks. In summary, we find that the use of cloth masks can potentially provide significant protection against the transmission of particles in the aerosol size range.

Materials and Methods

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Materials

All of the fabrics used as well as the surgical masks and N95 respirators tested are commercially available. We used 15 different types of fabrics. This included different types of cotton (80 and 600 threads per inch), cotton quilt, flannel (65% cotton and 35% polyester), synthetic silk (100% polyester), natural silk, Spandex (52% nylon, 39% polyester, and 9% Spandex), satin (97% polyester and 3% Spandex), chiffon (90% polyester and 10% Spandex), and different polyester and polyester–cotton blends. Specific information on the composition, microstructure, and other parameters can be found in the Supporting Information (Table S2).

Polydisperse Aerosol Generation

A polydisperse, nontoxic NaCl aerosol was generated using a particle generator and introduced into the mixing chamber along with an inlet for air. The aerosol is then mixed in the mixing chamber with the help of a portable fan. The particle generator produces particles sizes in the ranges of 10 nm to 10 μm.

Detection of Aerosol Particles

The particles were sampled both upstream (C_u, before the aerosol passes through the test specimen) and downstream (C_d, after the aerosol passes through the test specimen) for 1 min. The samples collected from the upstream and downstream are separately sent to the two particle sizers to determine a particle concentration (pt/cc). Each sample is tested seven times following the minimum sample size recommended by the American Industrial Hygiene Association exposure assessment sampling guidelines. (42) We observed a significantly lower particle count in the upper size distribution for both of the data sets, that is, for particles greater than 178 nm for the data from the TSI Nanoscan analyzer and greater than 6 μm for the data from TSI OPS analyzer. We exclude the data above these thresholds for all of the studies reported due to the extremely low counts. We categorize our data based on these two particle analyzers—individually the two plots (Figure 2a,b) show two size distributions—particles smaller than 300 nm and particles larger than 300 nm. Two different flow rates of 1.2 CFM (a face velocity of 0.1 m/s) and 3.2 CFM (a face velocity of 0.26 m/s) were used that corresponded to rates observed at rest to moderate activity, respectively. The velocity of the aerosol stream was measured at ∼5 cm behind where the test specimen would be mounted using a velocity meter.

Differential Pressure

The differential pressure (ΔP) across the test specimen was measured ∼7.5 cm away on either side of the material being tested using a micromanometer. The ΔP value is an estimate of the breathability of the fabric.

Data Analysis

The particle concentrations from seven consecutive measurements were recorded and divided into multiple bins—10 for nanoparticle sizer (dimensions in nm: 10–13, 13–18, 18–24, 24–32, 32–42, 42–56, 56–75, 75–100, 100–133, 133–178) and 6 for optical particle sizer (dimensions in μm: 0.3–0.6, 0.6–1.0, 1.0–2.0, 2.0–3.0, 3.0–4.0, 4.0–6.0). The seven measurements for each bin were subjected to one iteration of the Grubbs’ test with a 95% confidence interval to remove at most one outlier per bin. This improves the statistical viability of the data. Following Grubbs’ test, average concentrations were used to calculate the filtration efficiencies as described below.

Filtration Efficiency

The filtration efficiency (FE) of different masks was calculated using the following formula:

where C_u and C_d are the mean particle concentrations per bin upstream and downstream, respectively. To account for any possible drifts in the aerosol generation, we measured upstream concentrations before and after the downstream measurement and used the average of these two upstream values to calculate C_u (for runs that did not include a gap). We do not measure upstream concentration twice when the run included a gap. The error in FE was calculated using the quadrature rule of error propagation. Due to noise in the measurements, some FE values were below 0, which is unrealistic. As such, negative FE values were removed from consideration in figures and further calculations. In addition to the FE curves, we computed an aggregate filter efficiency for each test specimen. To do this, we took a weighted average of FE values weighted by the bin width for the two particle size ranges (<300 nm and >300 nm). These values are reported in Table 1 and Table S1.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsnano.0c03252.

Filtration efficiencies for various fabrics tested at two different flow rates and the effect of layering on the filtration efficiencies of chiffon, silk, and 600 TPI cotton; detailed information on various fabrics used (PDF)

nn0c03252_si_001.pdf (866.43 kb)

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Author Information

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Corresponding Author
- Supratik Guha - Pritzker School of Molecular Engineering, University of Chicago, Chicago, Illinois 60637, United States; Argonne National Laboratory, Lemont, Illinois 60439, United States; http://orcid.org/0000-0001-5071-8318; Email: [email protected]
Authors
- Abhiteja Konda - Center for Nanoscale Materials, Argonne National Laboratory, Lemont, Illinois 60439, United States; http://orcid.org/0000-0003-3362-0583
- Abhinav Prakash - Center for Nanoscale Materials, Argonne National Laboratory, Lemont, Illinois 60439, United States; Pritzker School of Molecular Engineering, University of Chicago, Chicago, Illinois 60637, United States; http://orcid.org/0000-0002-8899-0568
- Gregory A. Moss - Worker Safety & Health Division, Argonne National Laboratory, Lemont, Illinois 60439, United States
- Michael Schmoldt - Center for Nanoscale Materials, Argonne National Laboratory, Lemont, Illinois 60439, United States; Worker Safety & Health Division, Argonne National Laboratory, Lemont, Illinois 60439, United States
- Gregory D. Grant - Pritzker School of Molecular Engineering, University of Chicago, Chicago, Illinois 60637, United States
Author Contributions
A.K. and A.P. contributed equally.
Funding
Department of Defense Vannevar Bush Fellowship Grant No. N00014-18-1-2869.
Notes
The authors declare no competing financial interest.

Acknowledgments

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Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. S.G. acknowledges the Vannevar Bush Fellowship under the program sponsored by the Office of the Undersecretary of Defense for Research and Engineering [OUSD (R&E)] and the Office of Naval Research as the executive manager for the grant. A.K. acknowledges and thanks Prof. Anindita Basu for helpful discussion and support.

References

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This article references 42 other publications.

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Ma, N.; Jeffrey, S. S. How to Sew a Fabric Face Mask. Science 2020, 367, 1424, DOI: 10.1126/science.abb0736
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Deciphering cancer clues from blood
Ma, Ning; Jeffrey, Stefanie S.
Science (Washington, DC, United States) (2020), 367 (6485), 1424-1425CODEN: SCIEAS; ISSN:1095-9203. (American Association for the Advancement of Science)
Cancer is assocd. with considerable morbidity and mortality, and despite therapeutic advances, it still represents the second leading cause of death worldwide (). As cancers grow, evolve, and spread, they shed circulating tumor cells (CTCs), as well as other tumor-assocd. cells and products, into the bloodstream. Capturing and analyzing CTCs or other tumor-assocd. cells and products from a patient's blood sample can provide insight into a particular cancer's biol., response to treatment, and/ or potential therapeutic targets (). CTCs are heterogeneous; a pressing question concerns which CTCs represent those directly involved in metastasis, the major cause of cancer-related death. On page 1468 of this issue, Ebright et al. () identify genes in patient-derived CTCs encoding ribosomal proteins (RPs) that were assocd. with metastatic progression in mouse models, poor outcome in patients, and alterations in global translation. These findings could point to potential biomarkers or targets for future metastatic cancer therapies.
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Centers for Disease Control and Prevention. Coronavirus Disease 2019 (COVID-19). Use of Cloth Face Coverings to Help Slow the Spread of COVID-19; CS316353B, available at https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/diy-cloth-face-coverings.html, 2020; pp 1– 3.
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Kutter, J. S.; Spronken, M. I.; Fraaij, P. L.; Fouchier, R. A.; Herfst, S. Transmission Routes of Respiratory Viruses Among Humans. Curr. Opin. Virol. 2018, 28, 142– 151, DOI: 10.1016/j.coviro.2018.01.001
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Transmission routes of respiratory viruses among humans
Kutter, Jasmin S.; Spronken, Monique I.; Fraaij, Pieter L.; Fouchier, Ron AM; Herfst, Sander
Current Opinion in Virology (2018), 28 (), 142-151CODEN: COVUAF; ISSN:1879-6257. (Elsevier B.V.)
A review. Respiratory tract infections can be caused by a wide variety of viruses. Airborne transmission via droplets and aerosols enables some of these viruses to spread efficiently among humans, causing outbreaks that are difficult to control. Many outbreaks have been investigated retrospectively to study the possible routes of inter-human virus transmission. The results of these studies are often inconclusive and at the same time data from controlled expts. is sparse. Therefore, fundamental knowledge on transmission routes that could be used to improve intervention strategies is still missing. We present an overview of the available data from exptl. and observational studies on the transmission routes of respiratory viruses between humans, identify knowledge gaps, and discuss how the available knowledge is currently implemented in isolation guidelines in health care settings.
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Stelzer-Braid, S.; Oliver, B. G.; Blazey, A. J.; Argent, E.; Newsome, T. P.; Rawlinson, W. D.; Tovey, E. R. Exhalation of Respiratory Viruses by Breathing, Coughing, and Talking. J. Med. Virol. 2009, 81, 1674– 1679, DOI: 10.1002/jmv.21556
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Exhalation of respiratory viruses by breathing, coughing, and talking
Stelzer-Braid Sacha; Oliver Brian G; Blazey Angus J; Argent Elizabeth; Newsome Timothy P; Rawlinson William D; Tovey Euan R
Journal of medical virology (2009), 81 (9), 1674-9 ISSN:.
There is a lack of quantitative information about the generation of virus aerosols by infected subjects. The exhaled aerosols generated by coughing, talking, and breathing were sampled in 50 subjects using a novel mask, and analyzed using PCR for nine respiratory viruses. The exhaled samples from a subset of 10 subjects who were PCR positive for rhinovirus were also examined by cell culture for this virus. Of the 50 subjects, among the 33 with symptoms of upper respiratory tract infections, 21 had at least one virus detected by PCR, while amongst the 17 asymptomatic subjects, 4 had a virus detected by PCR. Overall, rhinovirus was detected in 19 subjects, influenza in 4 subjects, parainfluenza in 2 subjects, and human metapneumovirus in 1 subject. Two subjects were co-infected. Of the 25 subjects who had virus-positive nasal mucus, the same virus type was detected in 12 breathing samples, 8 talking samples, and in 2 coughing samples. In the subset of exhaled samples from 10 subjects examined by culture, infective rhinovirus was detected in 2. These data provide further evidence that breathing may be a source of respirable particles carrying infectious virus.
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Milton, D. K.; Fabian, M. P.; Cowling, B. J.; Grantham, M. L.; McDevitt, J. J. Influenza Virus Aerosols in Human Exhaled Breath: Particle Size, Culturability, and Effect of Surgical Masks. PLoS Pathog. 2013, 9, e1003205 DOI: 10.1371/journal.ppat.1003205
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Influenza virus aerosols in human exhaled breath: particle size, culturability, and effect of surgical masks
Milton, Donald K.; Patricia Fabian, M.; Cowling, Benjamin J.; Grantham, Michael L.; McDevitt, James J.
PLoS Pathogens (2013), 9 (3), e1003205CODEN: PPLACN; ISSN:1553-7374. (Public Library of Science)
The CDC recommends that healthcare settings provide influenza patients with facemasks as a means of reducing transmission to staff and other patients, and a recent report suggested that surgical masks can capture influenza virus in large droplet spray. However, there is minimal data on influenza virus aerosol shedding, the infectiousness of exhaled aerosols, and none on the impact of facemasks on viral aerosol shedding from patients with seasonal influenza. We collected samples of exhaled particles (one with and one without a facemask) in two size fractions ("coarse">5 μm, "fine" ≤5 μm) from 37 volunteers within 5 days of seasonal influenza onset, measured viral copy no. using quant. RT-PCR, and tested the fine-particle fraction for culturable virus. Fine particles contained 8.8 (95% CI 4.1 to 19) fold more viral copies than did coarse particles. Surgical masks reduced viral copy nos. in the fine fraction by 2.8 fold (95% CI 1.5 to 5.2) and in the coarse fraction by 25 fold (95% CI 3.5 to 180). Overall, masks produced a 3.4 fold (95% CI 1.8 to 6.3) redn. in viral aerosol shedding. Correlations between nasopharyngeal swab and the aerosol fraction copy nos. were weak (r = 0.17, coarse; r = 0.29, fine fraction). Copy nos. in exhaled breath declined rapidly with day after onset of illness. Two subjects with the highest copy nos. gave culture pos. fine particle samples. Surgical masks worn by patients reduce aerosols shedding of virus. The abundance of viral copies in fine particle aerosols and evidence for their infectiousness suggests an important role in seasonal influenza transmission. Monitoring exhaled virus aerosols will be important for validation of exptl. transmission studies in humans.
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National Academies of Sciences. Medicine. Rapid Expert Consultation on the Possibility of Bioaerosol Spread of SARS-CoV-2 for the COVID-19 Pandemic; The National Academies Press: Washington, DC, 2020; p 3.
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National Academies of Sciences. Medicine. Rapid Expert Consultation on the Effectiveness of Fabric Masks for the COVID-19 Pandemic; The National Academies Press: Washington, DC, 2020; p 8.
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MacIntyre, C. R.; Seale, H.; Dung, T. C.; Hien, N. T.; Nga, P. T.; Chughtai, A. A.; Rahman, B.; Dwyer, D. E.; Wang, Q. A Cluster Randomised Trial of Cloth Masks Compared With Medical Masks in Healthcare Workers. BMJ. Open 2015, 5, e006577 DOI: 10.1136/bmjopen-2014-006577
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Shakya, K. M.; Noyes, A.; Kallin, R.; Peltier, R. E. Evaluating the Efficacy of Cloth Facemasks in Reducing Particulate Matter Exposure. J. Exposure Sci. Environ. Epidemiol. 2017, 27, 352– 357, DOI: 10.1038/jes.2016.42
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Evaluating the efficacy of cloth facemasks in reducing particulate matter exposure
Shakya, Kabindra M.; Noyes, Alyssa; Kallin, Randa; Peltier, Richard E.
Journal of Exposure Science & Environmental Epidemiology (2017), 27 (3), 352-357CODEN: JESEBS; ISSN:1559-0631. (Nature Publishing Group)
Inexpensive cloth masks are widely used in developing countries to protect from particulate pollution albeit limited data on their efficacy exists. This study examd. the efficiency of four types of masks (three types of cloth masks and one type of surgical mask) commonly worn in the developing world. Five monodispersed aerosol sphere size (30, 100, and 500 nm, and 1 and 2.5 μm) and dild. whole diesel exhaust was used to assess facemask performance. Among the three cloth mask types, a cloth mask with an exhaust valve performed best with filtration efficiency of 80-90% for the measured polystyrene latex (PSL) particle sizes. Two styles of com. available fabric masks were the least effective with a filtration efficiency of 39-65% for PSL particles, and they performed better as the particle size increased. When the cloth masks were tested against lab-generated whole diesel particles, the filtration efficiency for three particle sizes (30, 100, and 500 nm) ranged from 15% to 57%. Std. N95 mask performance was used as a control to compare the results with cloth masks, and our results suggest that cloth masks are only marginally beneficial in protecting individuals from particles <2.5 μm. Compared with cloth masks, disposable surgical masks are more effective in reducing particulate exposure.
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Rengasamy, S.; Eimer, B.; Shaffer, R. E. Simple Respiratory Protection--Evaluation of the Filtration Performance of Cloth Masks and Common Fabric Materials Against 20–1000 nm Size Particles. Ann. Occup. Hyg. 2010, 54, 789– 798, DOI: 10.1093/annhyg/meq044
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Simple Respiratory Protection-Evaluation of the Filtration Performance of Cloth Masks and Common Fabric Materials Against 20-1000 nm Size Particles
Rengasamy, Samy; Eimer, Benjamin; Shaffer, Ronald E.
Annals of Occupational Hygiene (2010), 54 (7), 789-798CODEN: AOHYA3; ISSN:0003-4878. (Oxford University Press)
A shortage of disposable filtering facepiece respirators can be expected during a pandemic respiratory infection such as influenza A. Some individuals may want to use common fabric materials for respiratory protection because of shortage or affordability reasons. To address the filtration performance of common fabric materials against nano-size particles including viruses, five major categories of fabric materials including sweatshirts, T-shirts, towels, scarves, and cloth masks were tested for polydisperse and monodisperse aerosols (20-1000 nm) at two different face velocities (5.5 and 16.5 cm s-1) and compared with the penetration levels for N95 respirator filter media. The results showed that cloth masks and other fabric materials tested in the study had 40-90% instantaneous penetration levels against polydisperse NaCl aerosols employed in the National Institute for Occupational Safety and Health particulate respirator test protocol at 5.5 cm s-1. Similarly, varying levels of penetrations (9-98%) were obtained for different size monodisperse NaCl aerosol particles in the 20-1000 nm range. The penetration levels of these fabric materials against both polydisperse and monodisperse aerosols were much higher than the penetrations for the control N95 respirator filter media. At 16.5 cm s-1 face velocity, monodisperse aerosol penetrations slightly increased, while polydisperse aerosol penetrations showed no significant effect except one fabric mask with an increase. Results obtained in the study show that common fabric materials may provide marginal protection against nanoparticles including those in the size ranges of virus-contg. particles in exhaled breath.
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Davies, A.; Thompson, K. A.; Giri, K.; Kafatos, G.; Walker, J.; Bennett, A. Testing the Efficacy of Homemade Masks: Would They Protect in an Influenza Pandemic?. Disaster Med. Public Health Prep. 2013, 7, 413– 418, DOI: 10.1017/dmp.2013.43
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Testing the efficacy of homemade masks: would they protect in an influenza pandemic?
Davies Anna; Thompson Katy-Anne; Giri Karthika; Kafatos George; Walker Jimmy; Bennett Allan
Disaster medicine and public health preparedness (2013), 7 (4), 413-8 ISSN:.
OBJECTIVE: This study examined homemade masks as an alternative to commercial face masks. METHODS: Several household materials were evaluated for the capacity to block bacterial and viral aerosols. Twenty-one healthy volunteers made their own face masks from cotton t-shirts; the masks were then tested for fit. The number of microorganisms isolated from coughs of healthy volunteers wearing their homemade mask, a surgical mask, or no mask was compared using several air-sampling techniques. RESULTS: The median-fit factor of the homemade masks was one-half that of the surgical masks. Both masks significantly reduced the number of microorganisms expelled by volunteers, although the surgical mask was 3 times more effective in blocking transmission than the homemade mask. CONCLUSION: Our findings suggest that a homemade mask should only be considered as a last resort to prevent droplet transmission from infected individuals, but it would be better than no protection.
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van der Sande, M.; Teunis, P.; Sabel, R. Professional and Home-Made Face Masks Reduce Exposure to Respiratory Infections Among the General Population. PLoS One 2008, 3, e2618 DOI: 10.1371/journal.pone.0002618
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van Doremalen, N.; Bushmaker, T.; Morris, D. H.; Holbrook, M. G.; Gamble, A.; Williamson, B. N.; Tamin, A.; Harcourt, J. L.; Thornburg, N. J.; Gerber, S. I.; Lloyd-Smith, J. O.; de Wit, E.; Munster, V. J. Aerosol and Surface Stability of SARS-CoV-2 as Compared With SARS-CoV-1. N. Engl. J. Med. 2020, 382, 1564, DOI: 10.1056/NEJMc2004973
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Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1
van Doremalen Neeltje; Bushmaker Trenton; Holbrook Myndi G; Williamson Brandi N; de Wit Emmie; Munster Vincent J; Morris Dylan H; Gamble Amandine; Tamin Azaibi; Harcourt Jennifer L; Thornburg Natalie J; Gerber Susan I; Lloyd-Smith James O
The New England journal of medicine (2020), 382 (16), 1564-1567 ISSN:.
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Morawska, L.; Cao, J. Airborne Transmission of SARS-Cov-2: The World Should Face the Reality. Environ. Int. 2020, 139, 105730, DOI: 10.1016/j.envint.2020.105730
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Airborne transmission of SARS-CoV-2: The world should face the reality
Morawska, Lidia; Cao, Junji
Environment International (2020), 139 (), 105730CODEN: ENVIDV; ISSN:0160-4120. (Elsevier Ltd.)
Hand washing and maintaining social distance are the main measures recommended by the World Health Organization (WHO) to avoid contracting COVID-19. Unfortunately, these measured do not prevent infection by inhalation of small droplets exhaled by an infected person that can travel distance of meters or tens of meters in the air and carry their viral content. Science explains the mechanisms of such transport and there is evidence that this is a significant route of infection in indoor environments. Despite this, no countries or authorities consider airborne spread of COVID-19 in their regulations to prevent infections transmission indoors. It is therefore extremely important, that the national authorities acknowledge the reality that the virus spreads through air, and recommend that adequate control measures be implemented to prevent further spread of the SARS-CoV-2 virus, in particularly removal of the virus-laden droplets from indoor air by ventilation.
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Wang, J.; Du, G. COVID-19 May Transmit Through Aerosol. Ir. J. Med. Sci. 2020, 1– 2, DOI: 10.1007/s11845-020-02218-2
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Documentary Research of Human Respiratory Droplet Characteristics
Zhang Hualing; Li Dan; Xie Ling; Xiao Yimin; Zhang Hualing; Xiao Yimin; Li Dan
Procedia engineering (2015), 121 (), 1365-1374 ISSN:1877-7058.
Respiratory droplet characteristics are key to determine the droplet-borne pathogen transmission, which provide scientific basis for formulating the disease prevention from droplet transmission and control measures. Through studying the data information from existing documents, this paper gives the respiratory droplet characteristics, like size, concentration, velocity, etc. Meanwhile, droplet evaporation, droplet-borne pathogen activity and their transmission are discussed. The droplet size is no significant difference with human health level, gender and age. The size of droplets produced by health people is between 0.1 and 10 μm, it produced by patients is between 0.05 and 10 μm, and the patients' droplet concentration is higher. The coughed droplet concentrations change with the size into a peak rule. The velocity of the cough droplets is the biggest, the range of 10 to 25m/s, the transmission distance is more than 2m.
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World Health Organization. Annex C - Respiratory droplets. In Natural Ventilation for Infection Control in Health-Care Settings; Atkinson, J., Chartier, Y., Pessoa-Silva, C. L., Jensen, P., Li, Y., Seto, W. H., Eds.; World Health Organization: Geneva, 2009; pp 77– 82.
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Morawska, L. Droplet Fate in Indoor Environments, or Can We Prevent the Spread of Infection?. Indoor Air 2006, 16, 335– 347, DOI: 10.1111/j.1600-0668.2006.00432.x
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Droplet fate in indoor environments, or can we prevent the spread of infection?
Morawska L
Indoor air (2006), 16 (5), 335-47 ISSN:0905-6947.
UNLABELLED: When considering how people are infected and what can be done to prevent the infections, answers from many disciplines are sought: microbiology, epidemiology, medicine, engineering, and physics. There are many pathways to infection spread, and among the most significant from the epidemiological point of view is airborne transport. Microorganisms can become airborne when droplets are generated during speech, coughing, sneezing, vomiting, or atomization of feces during sewage removal. The fate of the droplets is governed by the physical principles of transport, with droplet size being the most important factor affecting their dispersion, deposition on surfaces and determining the survival of microorganisms within the droplets. In addition, physical characteristics of the indoor environment as well as the design and operation of building ventilation systems are of critical importance. Do we understand the mechanisms of infection spread and can we quantify the droplet dynamics under various indoor conditions? Unfortunately no, as this aspect of infection spread has attracted surprisingly little scientific interest. However, investigations of numerous cases in which a large number of people were infected show how critical the physics of microorganism spread can be. This paper reviews the state of knowledge regarding mechanisms of droplet spread and solutions available to minimize the spread and prevent infections. PRACTICAL IMPLICATIONS: Every day tens of millions of people worldwide suffer from viral infections of different severity at immense economic cost. There is, however, only minimal understanding of the dynamics of virus-laden aerosols, and so the ability to control and prevent virus spread is severely reduced, as was clearly demonstrated during the recent severe acute respiratory syndrome epidemic. This paper proposes the direction to significantly advance fundamental and applied knowledge of the pathways of viral infection spread in indoor atmospheric systems, through a comprehensive multidisciplinary approach and application of state-of-the-art scientific methods. Knowledge gained will have the potential to bring unprecedented economical gains worldwide by minimizing/reducing the spread of disease.
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Ching, W.-H.; Leung, M. K. H.; Leung, D. Y. C.; Li, Y.; Yuen, P. L. Reducing Risk of Airborne Transmitted Infection in Hospitals by Use of Hospital Curtains. Indoor Built Environ. 2008, 17, 252– 259, DOI: 10.1177/1420326X08091957
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Lai, A. C. K.; Poon, C. K. M.; Cheung, A. C. T. Effectiveness of Facemasks to Reduce Exposure Hazards for Airborne Infections Among General Populations. J. R. Soc., Interface 2012, 9, 938– 948, DOI: 10.1098/rsif.2011.0537
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Effectiveness of facemasks to reduce exposure hazards for airborne infections among general populations
Lai A C K; Poon C K M; Cheung A C T
Journal of the Royal Society, Interface (2012), 9 (70), 938-48 ISSN:.
Facemasks are widely used as a protective measure by general public to prevent inhalation of airborne pathogens including seasonal, swine and other forms of influenza and severe acute respiratory syndrome (SARS), etc. However, scientific data on effectiveness of facemasks in reducing infections in the community are extremely limited and even inconsistent. In this work, two manikins labelled as 'source' and 'susceptible' were used to measure the protection provided by facemasks under various emission scenarios. The source was modified to generate polydisperse ultrafine particles, whereas the susceptible was modified to mimic a realistic breathing pattern. The facemask was challenged by both pseudo-steady and highly transient emissions generated by an expiratory process where parameters, such as separation distance between manikins, emission velocity and expiratory duration, were controlled and measured systematically. Performances of four different types of facemask fits, varying from ideal to normal wearing practice, were also investigated. Under the pseudo-steady concentration environment, facemask protection was found to be 45 per cent, while under expiratory emissions, protection varied from 33 to 100 per cent. It was also observed that the separation between the source and the manikin was the most influential parameter affecting facemask protection.
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Leung, N. H. L.; Chu, D. K. W.; Shiu, E. Y. C.; Chan, K.-H.; McDevitt, J. J.; Hau, B. J. P.; Yen, H.-L.; Li, Y.; Ip, D. K. M.; Peiris, J. S. M.; Seto, W.-H.; Leung, G. M.; Milton, D. K.; Cowling, B. J. Respiratory Virus Shedding in Exhaled Breath and Efficacy of Face Masks. Nat. Med. 2020, DOI: 10.1038/s41591-020-0843-2
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Hinds, W. C. 9 - Filtration. In Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles, 2nd ed.; John Wiley & Sons: New York, 1999; pp 182– 205.
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Vincent, J. H. 21 - Aerosol Sample Applications and Field Studies. In Aerosol Sampling. Science, Standards, Instrumentation and Applications; Vincent, J. H., Ed.; John Wiley & Sons: New York, 2007; pp 528– 529.
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Jung, H.; Kim, J.; Lee, S.; Lee, J.; Kim, J.; Tsai, P.; Yoon, C. Comparison of Filtration Efficiency and Pressure Drop in Anti-Yellow Sand Masks, Quarantine Masks, Medical Masks, General Masks, and Handkerchiefs. Aerosol Air Qual. Res. 2014, 14, 991– 1002, DOI: 10.4209/aaqr.2013.06.0201
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Holton, P. M.; Tackett, D. L.; Willeke, K. Particle Size-Dependent Leakage and Losses of Aerosols in Respirators. Am. Ind. Hyg. Assoc. J. 1987, 48, 848– 854, DOI: 10.1080/15298668791385697
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Particle size-dependent leakage and losses of aerosols in respirators
Holton, Patricia M.; Tackett, Denise Lynne; Willeke, Klaus
American Industrial Hygiene Association Journal (1958-1999) (1987), 48 (10), 848-54CODEN: AIHAAP; ISSN:0002-8894.
Measuring particle size-dependent leakage into and losses inside a respirator shows the deposition mechanisms occurring at the leak site and the flow dynamics inside the respirator. This study investigated particle size-dependent leakage and deposition within the mask by examg. the leakage into the mask for different hole locations, probe locations, hole shapes, hole lengths, and hole sizes. The shape of the leak has an effect on particle size-dependent leakage. Probe and leak location tests indicated that not only does the total measured leakage change but that the size-dependence of the leakage also changes, depending on the leak and probe locations. When the leak site is in the chin area, the clean air entering through the filters at the chin helps to carry the inward leakage into the breathing zone. Particle size-dependent leakage does occur and is due to both inertial entry losses at the leak site and within the mask, and diffusional losses within the mask and leak site. Particle size-dependent curves change shape as the hole size changes, with relatively more larger particles entering through the small hole size.
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Rengasamy, S.; Eimer, B. C. Nanoparticle Penetration Through Filter Media and Leakage Through Face Seal Interface of N95 Filtering Facepiece Respirators. Ann. Occup. Hyg. 2012, 56, 568– 580, DOI: 10.1093/annhyg/mer122
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Nanoparticle penetration through filter media and leakage through face seal interface of N95 filtering facepiece respirators
Rengasamy Samy; Eimer Benjamin C
The Annals of occupational hygiene (2012), 56 (5), 568-80 ISSN:.
National Institute for Occupational Safety and Health recommends the use of particulate respirators for protection against nanoparticles (<100 nm size). Protection afforded by a filtering facepiece particulate respirator is a function of the filter efficiency and the leakage through the face-to-facepiece seal. The combination of particle penetration through filter media and particle leakage through face seal and any component interfaces is considered as total inward leakage (TIL). Although the mechanisms and extent of nanoparticle penetration through filter media have been well documented, information concerning nanoparticle leakage through face seal is lacking. A previous study in our laboratory measured filter penetration and TIL for specific size particles. The results showed higher filter penetration and TIL for 50 nm size particles, i.e. the most penetrating particle size (MPPS) than for 8 and 400 nm size particles. To better understand the significance of particle penetration through filter media and through face seal leakage, this study was expanded to measure filter penetration at sealed condition and TIL with artificially introduced leaks for 20-800 nm particles at 8-40 l minute volumes for four N95 models of filtering facepiece respirators (FFRs) using a breathing manikin. Results showed that the MPPS was ~45 nm for all four respirator models. Filter penetration for 45 nm size particles was significantly (P < 0.05) higher than the values for 400 nm size particles. A consistent increase in filter penetrations for 45 and 400 nm size particles was obtained with increasing breathing minute volumes. Artificial leakage of test aerosols (mode size ~75 nm) through increasing size holes near the sealing area of FFRs showed higher TIL values for 45 nm size particles at different minute volumes, indicating that the induced leakage allows the test aerosols, regardless of particle size, inside the FFR, while filter penetration determines the TIL for different size particles. TIL values obtained for 45 nm size particles were significantly (P < 0.05) higher than the values obtained for 400 nm size particles for all four models. Models with relatively small filter penetration values showed lower TIL values than the models with higher filter penetrations at smaller leak sizes indicating the dependence of TIL values on filter penetration. When the electrostatic charge was removed, the FFRs showed a shift in the MPPS to ~150 nm with the same test aerosols (mode size ~75 nm) at different hole sizes and breathing minute volumes, confirming the interaction between filter penetration and face seal leakage processes. The shift in the MPPS from 45 to 150 nm for the charge removed filters indicates that mechanical filters may perform better against nanoparticles than electrostatic filters rated for the same filter efficiency. The results suggest that among the different size particles that enter inside the N95 respirators, relatively high concentration of the MPPS particles in the breathing zone of respirators can be expected in workplaces with high concentration of nanoparticles. Overall, the data obtained in the study suggest that good fitting respirators with lower filter penetration values would provide better protection against nanoparticles.
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Rengasamy, S.; Zhuang, Z.; Niezgoda, G.; Walbert, G.; Lawrence, R.; Boutin, B.; Hudnall, J.; Monaghan, W. P.; Bergman, M.; Miller, C.; Harris, J.; Coffey, C. A Comparison of Total Inward Leakage Measured Using Sodium Chloride (NaCl) and Corn Oil Aerosol Methods for Air-Purifying Respirators. J. Occup. Environ. Hyg. 2018, 15, 616– 627, DOI: 10.1080/15459624.2018.1479064
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A comparison of total inward leakage measured using sodium chloride (NaCl) and corn oil aerosol methods for air-purifying respirators
Rengasamy, Samy; Zhuang, Ziqing; Niezgoda, George; Walbert, Gary; Lawrence, Robert; Boutin, Brenda; Hudnall, Judith; Monaghan, William P.; Bergman, Michael; Miller, Colleen; Harris, James; Coffey, Christopher
Journal of Occupational and Environmental Hygiene (2018), 15 (8), 616-627CODEN: JOEHA2; ISSN:1545-9624. (Taylor & Francis, Inc.)
The International Organization for Standardization (ISO) std. 16900-1:2014 specifies the use of sodium chloride (NaCl) and corn oil aerosols and sulfur hexafluoride gas for measuring total inward leakage (TIL). However, a comparison of TIL between different agents is lacking. The objective of this study was to measure and compare TIL for respirators using corn oil and NaCl aerosols. TIL was measured with 10 subjects donning two models of filtering facepiece respirators (FFRs) including FFP1, N95, P100, and elastomeric half-mask respirators (ERs) in NaCl and corn oil aerosol test chambers, using continuous sampling methods. After fit testing with a PortaCount (TSI, Inc., St. Paul, MN) using the Occupational Safety and Health Administration (OSHA) protocol, five subjects were tested in the NaCl chamber first and then in the corn oil chamber, while other subjects tested in the reverse order. TIL was measured as a ratio of mass-based aerosol concns. in-mask to the test chamber, while the subjects performed ISO 16900-1-defined exercises. The concn. of NaCl aerosol was measured using two flame photometers, and corn oil aerosol was measured with one light scattering photometer. The same instruments were used to measure filter penetration in both chambers using a Plexiglas setup. The size distribution of aerosols was detd. using a scanning mobility particle sizer and charge was measured with an electrometer. Filter efficiency was measured using an 8130 Automated Filter Tester (TSI). Results showed the geometric mean TIL for corn oil aerosol for one model each of all respirator categories, except P100, were significantly (p < 0.05) greater than for NaCl aerosol. Filter penetration in the two test chambers showed a trend similar to TIL. The count median diam. was ∼82 nm for NaCl and ∼200 nm for corn oil aerosols. The net pos. charge for NaCl aerosol was relatively larger. Both fit factor and filter efficiency influence TIL measurement. Overall, TIL detn. with aerosols of different size distributions and charges using different methodologies may produce dissimilar results.
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Electronic Code of Federal Regulations (eCFR), Title 42: Public Health, Part 84—Approval of Respiratory Protective Devices. Code of Federal Regulations , April 2020.
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Grinshpun, S. A.; Haruta, H.; Eninger, R. M.; Reponen, T.; McKay, R. T.; Lee, S.-A. Performance of an N95 Filtering Facepiece Particulate Respirator and a Surgical Mask During Human Breathing: Two Pathways for Particle Penetration. J. Occup. Environ. Hyg. 2009, 6, 593– 603, DOI: 10.1080/15459620903120086
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Performance of an N95 filtering facepiece particulate respirator and a surgical mask during human breathing: two pathways for particle penetration
Grinshpun Sergey A; Haruta Hiroki; Eninger Robert M; Reponen Tiina; McKay Roy T; Lee Shu-An
Journal of occupational and environmental hygiene (2009), 6 (10), 593-603 ISSN:.
The protection level offered by filtering facepiece particulate respirators and face masks is defined by the percentage of ambient particles penetrating inside the protection device. There are two penetration pathways: (1) through the faceseal leakage, and the (2) filter medium. This study aimed at differentiating the contributions of these two pathways for particles in the size range of 0.03-1 microm under actual breathing conditions. One N95 filtering facepiece respirator and one surgical mask commonly used in health care environments were tested on 25 subjects (matching the latest National Institute for Occupational Safety and Health fit testing panel) as the subjects performed conventional fit test exercises. The respirator and the mask were also tested with breathing manikins that precisely mimicked the prerecorded breathing patterns of the tested subjects. The penetration data obtained in the human subject- and manikin-based tests were compared for different particle sizes and breathing patterns. Overall, 5250 particle size- and exercise-specific penetration values were determined. For each value, the faceseal leakage-to-filter ratio was calculated to quantify the relative contributions of the two penetration pathways. The number of particles penetrating through the faceseal leakage of the tested respirator/mask far exceeded the number of those penetrating through the filter medium. For the N95 respirator, the excess was (on average) by an order of magnitude and significantly increased with an increase in particle size (p < 0.001): approximately 7-fold greater for 0.04 microm, approximately 10-fold for 0.1 microm, and approximately 20-fold for 1 microm. For the surgical mask, the faceseal leakage-to-filter ratio ranged from 4.8 to 5.8 and was not significantly affected by the particle size for the tested submicrometer fraction. Facial/body movement had a pronounced effect on the relative contribution of the two penetration pathways. Breathing intensity and facial dimensions showed some (although limited) influence. Because most of the penetrated particles entered through the faceseal, the priority in respirator/mask development should be shifted from improving the efficiency of the filter medium to establishing a better fit that would eliminate or minimize faceseal leakage.
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Sanchez, Andres L.; Hubbard, Joshua A.; Dellinger, Jennifer G.; Servantes, Brandon L.
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Aerosol collection efficiency was studied for electrostatically charged fibrous filters (3M Filtrete, BMF-20F). In this study, collection efficiencies at moderate filter face velocities (0.5-2.5 m/s) representative of some high vol. sampling applications was characterized. Exptl. data and anal. theories of filter performance are less common in this flow regime since the viscous flow field assumption may not be representative of actual flow through the filter mat. Addnl., electrostatic fiber charge d. is difficult to quantify, and measurements of aerosol collection efficiency are often used to calc. this fundamental parameter. The purpose of this study was to assess the relative influence of diffusion, inertial impaction, interception, and electrostatic filtration on overall filter performance. The effects of fiber charge d. were quantified by comparing efficiency data for charged and uncharged filter media, where an isopropanol bath was used to eliminate electrostatic charge. The effects of particle charge were also quantified by test aerosols brought into the equil. Boltzmann charge distribution, and then using an electrostatic precipitator to sep. out only those test particles with a charge of zero. Electrostatically charged filter media had collection efficiencies as high as 70-85% at 30 nm. Filter performance was reduced significantly (40-50% collection efficiency) when the electrostatic filtration component was eliminated. Expts. performed with zero charged NaCl particles showed that a significant increase in filter performance is attributable to an induction effect, where electrostatic fiber charge polarizes aerosol particles without charge. As filter face velocity increased the electrostatic filtration efficiency decreased since aerosol particles had less time to drift toward electrostatically charged fibers. Finally, exptl. data at 0.5 m/s were compared to theor. predictions and good agreement was found for both electrostatic and nonelectrostatic effects.
39
Bałazy, A.; Toivola, M.; Adhikari, A.; Sivasubramani, S. K.; Reponen, T.; Grinshpun, S. A. Do N95 Respirators Provide 95% Protection Level Against Airborne Viruses, and How Adequate are Surgical Masks?. Am. J. Infect. Control 2006, 34, 51– 57, DOI: 10.1016/j.ajic.2005.08.018
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Do N95 respirators provide 95% protection level against airborne viruses, and how adequate are surgical masks?
Balazy Anna; Toivola Mika; Adhikari Atin; Sivasubramani Satheesh K; Reponen Tiina; Grinshpun Sergey A
American journal of infection control (2006), 34 (2), 51-7 ISSN:0196-6553.
BACKGROUND: Respiratory protection devices are used to protect the wearers from inhaling particles suspended in the air. Filtering face piece respirators are usually tested utilizing nonbiologic particles, whereas their use often aims at reducing exposure to biologic aerosols, including infectious agents such as viruses and bacteria. METHODS: The performance of 2 types of N95 half-mask, filtering face piece respirators and 2 types of surgical masks were determined. The collection efficiency of these respiratory protection devices was investigated using MS2 virus (a nonharmful simulant of several pathogens). The virions were detected in the particle size range of 10 to 80 nm. RESULTS: The results indicate that the penetration of virions through the National Institute for Occupational Safety and Health (NIOSH)-certified N95 respirators can exceed an expected level of 5%. As anticipated, the tested surgical masks showed a much higher particle penetration because they are known to be less efficient than the N95 respirators. The 2 surgical masks, which originated from the same manufacturer, showed tremendously different penetration levels of the MS2 virions: 20.5% and 84.5%, respectively, at an inhalation flow rate of 85 L/min. CONCLUSION: The N95 filtering face piece respirators may not provide the expected protection level against small virions. Some surgical masks may let a significant fraction of airborne viruses penetrate through their filters, providing very low protection against aerosolized infectious agents in the size range of 10 to 80 nm. It should be noted that the surgical masks are primarily designed to protect the environment from the wearer, whereas the respirators are supposed to protect the wearer from the environment.
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Balazy, A.; Toivola, M.; Reponen, T.; Podgorski, A.; Zimmer, A.; Grinshpun, S. A. Manikin-Based Performance Evaluation of N95 Filtering-Facepiece Respirators Challenged With Nanoparticles. Ann. Occup. Hyg. 2005, 50, 259– 269
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Abstract
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Figure 1
Figure 1. Schematic of the experimental setup. A polydisperse NaCl aerosol is introduced into the mixing chamber, where it is mixed and passed through the material being tested (“test specimen”). The test specimen is held in place using a clamp for a better seal. The aerosol is sampled before (upstream, C_u) and after (downstream, C_d) it passes through the specimen. The pressure difference is measured using a manometer, and the aerosol flow velocity is measured using a velocity meter. We use two circular holes with a diameter of 0.635 cm to simulate the effect of gaps on the filtration efficiency. The sampled aerosols are analyzed using particle analyzers (OPS and Nanoscan), and the resultant particle concentrations are used to determine filter efficiencies.
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Figure 2
Figure 2. Particle concentration as a function of particle size at a flow rate of 1.2 CFM. Plots showing the particle concentration (in arbitrary units) upstream and downstream through a single layer of natural silk for particle sizes <300 nm (a,c) and between 300 nm and 6 μm (b,d). Each bin shows the particle concentration for at least six trials. The particle concentrations in panels (b) and (d) are given in log scale for better representation of the data. The y-axis scales are the same for panels "a" and "c"; and for panels "b" and "d".
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Figure 3
Figure 3. Filtration efficiency of individual fabrics at a flow rate of 1.2 CFM (without gap). (a) Plot showing the filtration efficiencies of a cotton quilt consisting of two 120 threads per inch (TPI) cotton sheets enclosing a ∼0.5 cm thick cotton batting, 80 TPI quilters cotton (Q Cotton 80 TPI), and a 600 TPI cotton (cotton 600 TPI). (b) Plot showing the filtration efficiencies of one layer of natural silk (Silk-1L), four layers of natural silk (Silk-4L), one layer of flannel, and one layer of chiffon. The error bars on the <300 nm measurements are higher, particularly for samples with high filtration efficiencies because of the small number of particles generated in this size range, the relatively poorer counting efficiency of the detector at <300 nm particle size, and the very small counts downstream of the sample. The sizes of the error bars for some of the data points (>300 nm) are smaller than the symbol size and hence not clearly visible.
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Figure 4
Figure 4. Filtration efficiency of hybrid fabrics at a flow rate of 1.2 CFM. (a) Plot showing the filtration efficiencies without gap for an N95 respirator and a combination of different fabrics: 1 layer of 600 threads per inch (TPI) cotton and 2 layers of silk (cotton/silk), 1 layer of 600 TPI cotton and 2 layers of chiffon (cotton/chiffon), and 1 layer of 600 TPI cotton and 1 layer of flannel (cotton/flannel). (b) Plot showing the filtration efficiencies of a surgical mask and cotton/silk with (dashed) and without a gap (solid). The gap used is ∼1% of the active mask surface area. The error bars on the <300 nm measurements are higher, particularly for samples with high filtration efficiencies because of the small number of particles generated in this size range, the relatively poorer counting efficiency of the detector at <300 nm particle size, and the very small counts downstream of the sample. The sizes of the error bars for some of the data points (>300 nm) are smaller than the symbol size and hence not clearly visible.
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  Evaluating the efficacy of cloth facemasks in reducing particulate matter exposure
  Shakya, Kabindra M.; Noyes, Alyssa; Kallin, Randa; Peltier, Richard E.
  Journal of Exposure Science & Environmental Epidemiology (2017), 27 (3), 352-357CODEN: JESEBS; ISSN:1559-0631. (Nature Publishing Group)
  Inexpensive cloth masks are widely used in developing countries to protect from particulate pollution albeit limited data on their efficacy exists. This study examd. the efficiency of four types of masks (three types of cloth masks and one type of surgical mask) commonly worn in the developing world. Five monodispersed aerosol sphere size (30, 100, and 500 nm, and 1 and 2.5 μm) and dild. whole diesel exhaust was used to assess facemask performance. Among the three cloth mask types, a cloth mask with an exhaust valve performed best with filtration efficiency of 80-90% for the measured polystyrene latex (PSL) particle sizes. Two styles of com. available fabric masks were the least effective with a filtration efficiency of 39-65% for PSL particles, and they performed better as the particle size increased. When the cloth masks were tested against lab-generated whole diesel particles, the filtration efficiency for three particle sizes (30, 100, and 500 nm) ranged from 15% to 57%. Std. N95 mask performance was used as a control to compare the results with cloth masks, and our results suggest that cloth masks are only marginally beneficial in protecting individuals from particles <2.5 μm. Compared with cloth masks, disposable surgical masks are more effective in reducing particulate exposure.
10. 10
  Rengasamy, S.; Eimer, B.; Shaffer, R. E. Simple Respiratory Protection--Evaluation of the Filtration Performance of Cloth Masks and Common Fabric Materials Against 20–1000 nm Size Particles. Ann. Occup. Hyg. 2010, 54, 789– 798, DOI: 10.1093/annhyg/meq044
  10
  Simple Respiratory Protection-Evaluation of the Filtration Performance of Cloth Masks and Common Fabric Materials Against 20-1000 nm Size Particles
  Rengasamy, Samy; Eimer, Benjamin; Shaffer, Ronald E.
  Annals of Occupational Hygiene (2010), 54 (7), 789-798CODEN: AOHYA3; ISSN:0003-4878. (Oxford University Press)
  A shortage of disposable filtering facepiece respirators can be expected during a pandemic respiratory infection such as influenza A. Some individuals may want to use common fabric materials for respiratory protection because of shortage or affordability reasons. To address the filtration performance of common fabric materials against nano-size particles including viruses, five major categories of fabric materials including sweatshirts, T-shirts, towels, scarves, and cloth masks were tested for polydisperse and monodisperse aerosols (20-1000 nm) at two different face velocities (5.5 and 16.5 cm s-1) and compared with the penetration levels for N95 respirator filter media. The results showed that cloth masks and other fabric materials tested in the study had 40-90% instantaneous penetration levels against polydisperse NaCl aerosols employed in the National Institute for Occupational Safety and Health particulate respirator test protocol at 5.5 cm s-1. Similarly, varying levels of penetrations (9-98%) were obtained for different size monodisperse NaCl aerosol particles in the 20-1000 nm range. The penetration levels of these fabric materials against both polydisperse and monodisperse aerosols were much higher than the penetrations for the control N95 respirator filter media. At 16.5 cm s-1 face velocity, monodisperse aerosol penetrations slightly increased, while polydisperse aerosol penetrations showed no significant effect except one fabric mask with an increase. Results obtained in the study show that common fabric materials may provide marginal protection against nanoparticles including those in the size ranges of virus-contg. particles in exhaled breath.
11. 11
  Davies, A.; Thompson, K. A.; Giri, K.; Kafatos, G.; Walker, J.; Bennett, A. Testing the Efficacy of Homemade Masks: Would They Protect in an Influenza Pandemic?. Disaster Med. Public Health Prep. 2013, 7, 413– 418, DOI: 10.1017/dmp.2013.43
  11
  Testing the efficacy of homemade masks: would they protect in an influenza pandemic?
  Davies Anna; Thompson Katy-Anne; Giri Karthika; Kafatos George; Walker Jimmy; Bennett Allan
  Disaster medicine and public health preparedness (2013), 7 (4), 413-8 ISSN:.
  OBJECTIVE: This study examined homemade masks as an alternative to commercial face masks. METHODS: Several household materials were evaluated for the capacity to block bacterial and viral aerosols. Twenty-one healthy volunteers made their own face masks from cotton t-shirts; the masks were then tested for fit. The number of microorganisms isolated from coughs of healthy volunteers wearing their homemade mask, a surgical mask, or no mask was compared using several air-sampling techniques. RESULTS: The median-fit factor of the homemade masks was one-half that of the surgical masks. Both masks significantly reduced the number of microorganisms expelled by volunteers, although the surgical mask was 3 times more effective in blocking transmission than the homemade mask. CONCLUSION: Our findings suggest that a homemade mask should only be considered as a last resort to prevent droplet transmission from infected individuals, but it would be better than no protection.
12. 12
  van der Sande, M.; Teunis, P.; Sabel, R. Professional and Home-Made Face Masks Reduce Exposure to Respiratory Infections Among the General Population. PLoS One 2008, 3, e2618 DOI: 10.1371/journal.pone.0002618
  There is no corresponding record for this reference.
13. 13
  van Doremalen, N.; Bushmaker, T.; Morris, D. H.; Holbrook, M. G.; Gamble, A.; Williamson, B. N.; Tamin, A.; Harcourt, J. L.; Thornburg, N. J.; Gerber, S. I.; Lloyd-Smith, J. O.; de Wit, E.; Munster, V. J. Aerosol and Surface Stability of SARS-CoV-2 as Compared With SARS-CoV-1. N. Engl. J. Med. 2020, 382, 1564, DOI: 10.1056/NEJMc2004973
  13
  Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1
  van Doremalen Neeltje; Bushmaker Trenton; Holbrook Myndi G; Williamson Brandi N; de Wit Emmie; Munster Vincent J; Morris Dylan H; Gamble Amandine; Tamin Azaibi; Harcourt Jennifer L; Thornburg Natalie J; Gerber Susan I; Lloyd-Smith James O
  The New England journal of medicine (2020), 382 (16), 1564-1567 ISSN:.
  There is no expanded citation for this reference.
14. 14
  Morawska, L.; Cao, J. Airborne Transmission of SARS-Cov-2: The World Should Face the Reality. Environ. Int. 2020, 139, 105730, DOI: 10.1016/j.envint.2020.105730
  14
  Airborne transmission of SARS-CoV-2: The world should face the reality
  Morawska, Lidia; Cao, Junji
  Environment International (2020), 139 (), 105730CODEN: ENVIDV; ISSN:0160-4120. (Elsevier Ltd.)
  Hand washing and maintaining social distance are the main measures recommended by the World Health Organization (WHO) to avoid contracting COVID-19. Unfortunately, these measured do not prevent infection by inhalation of small droplets exhaled by an infected person that can travel distance of meters or tens of meters in the air and carry their viral content. Science explains the mechanisms of such transport and there is evidence that this is a significant route of infection in indoor environments. Despite this, no countries or authorities consider airborne spread of COVID-19 in their regulations to prevent infections transmission indoors. It is therefore extremely important, that the national authorities acknowledge the reality that the virus spreads through air, and recommend that adequate control measures be implemented to prevent further spread of the SARS-CoV-2 virus, in particularly removal of the virus-laden droplets from indoor air by ventilation.
15. 15
  Wang, J.; Du, G. COVID-19 May Transmit Through Aerosol. Ir. J. Med. Sci. 2020, 1– 2, DOI: 10.1007/s11845-020-02218-2
  There is no corresponding record for this reference.
16. 16
  Santarpia, J. L.; Rivera, D. N.; Herrera, V.; Morwitzer, M. J.; Creager, H.; Santarpia, G. W.; Crown, K. K.; Brett-Major, D.; Schnaubelt, E.; Broadhurst, M. J.; Lawler, J. V.; Reid, S. P.; Lowe, J. J. Transmission Potential of SARS-CoV-2 in Viral Shedding Observed at the University of Nebraska Medical Center. 2020, medRxiv; https://10.1101/2020.03.23.20039446 (accessed 2020-04-04).
  There is no corresponding record for this reference.
17. 17
  Zhang, H.; Li, D.; Xie, L.; Xiao, Y. Documentary Research of Human Respiratory Droplet Characteristics. Procedia Eng. 2015, 121, 1365– 1374, DOI: 10.1016/j.proeng.2015.09.023
  17
  Documentary Research of Human Respiratory Droplet Characteristics
  Zhang Hualing; Li Dan; Xie Ling; Xiao Yimin; Zhang Hualing; Xiao Yimin; Li Dan
  Procedia engineering (2015), 121 (), 1365-1374 ISSN:1877-7058.
  Respiratory droplet characteristics are key to determine the droplet-borne pathogen transmission, which provide scientific basis for formulating the disease prevention from droplet transmission and control measures. Through studying the data information from existing documents, this paper gives the respiratory droplet characteristics, like size, concentration, velocity, etc. Meanwhile, droplet evaporation, droplet-borne pathogen activity and their transmission are discussed. The droplet size is no significant difference with human health level, gender and age. The size of droplets produced by health people is between 0.1 and 10 μm, it produced by patients is between 0.05 and 10 μm, and the patients' droplet concentration is higher. The coughed droplet concentrations change with the size into a peak rule. The velocity of the cough droplets is the biggest, the range of 10 to 25m/s, the transmission distance is more than 2m.
18. 18
  World Health Organization. Annex C - Respiratory droplets. In Natural Ventilation for Infection Control in Health-Care Settings; Atkinson, J., Chartier, Y., Pessoa-Silva, C. L., Jensen, P., Li, Y., Seto, W. H., Eds.; World Health Organization: Geneva, 2009; pp 77– 82.
  There is no corresponding record for this reference.
19. 19
  Morawska, L. Droplet Fate in Indoor Environments, or Can We Prevent the Spread of Infection?. Indoor Air 2006, 16, 335– 347, DOI: 10.1111/j.1600-0668.2006.00432.x
  19
  Droplet fate in indoor environments, or can we prevent the spread of infection?
  Morawska L
  Indoor air (2006), 16 (5), 335-47 ISSN:0905-6947.
  UNLABELLED: When considering how people are infected and what can be done to prevent the infections, answers from many disciplines are sought: microbiology, epidemiology, medicine, engineering, and physics. There are many pathways to infection spread, and among the most significant from the epidemiological point of view is airborne transport. Microorganisms can become airborne when droplets are generated during speech, coughing, sneezing, vomiting, or atomization of feces during sewage removal. The fate of the droplets is governed by the physical principles of transport, with droplet size being the most important factor affecting their dispersion, deposition on surfaces and determining the survival of microorganisms within the droplets. In addition, physical characteristics of the indoor environment as well as the design and operation of building ventilation systems are of critical importance. Do we understand the mechanisms of infection spread and can we quantify the droplet dynamics under various indoor conditions? Unfortunately no, as this aspect of infection spread has attracted surprisingly little scientific interest. However, investigations of numerous cases in which a large number of people were infected show how critical the physics of microorganism spread can be. This paper reviews the state of knowledge regarding mechanisms of droplet spread and solutions available to minimize the spread and prevent infections. PRACTICAL IMPLICATIONS: Every day tens of millions of people worldwide suffer from viral infections of different severity at immense economic cost. There is, however, only minimal understanding of the dynamics of virus-laden aerosols, and so the ability to control and prevent virus spread is severely reduced, as was clearly demonstrated during the recent severe acute respiratory syndrome epidemic. This paper proposes the direction to significantly advance fundamental and applied knowledge of the pathways of viral infection spread in indoor atmospheric systems, through a comprehensive multidisciplinary approach and application of state-of-the-art scientific methods. Knowledge gained will have the potential to bring unprecedented economical gains worldwide by minimizing/reducing the spread of disease.
20. 20
  Ching, W.-H.; Leung, M. K. H.; Leung, D. Y. C.; Li, Y.; Yuen, P. L. Reducing Risk of Airborne Transmitted Infection in Hospitals by Use of Hospital Curtains. Indoor Built Environ. 2008, 17, 252– 259, DOI: 10.1177/1420326X08091957
  There is no corresponding record for this reference.
21. 21
  Lai, A. C. K.; Poon, C. K. M.; Cheung, A. C. T. Effectiveness of Facemasks to Reduce Exposure Hazards for Airborne Infections Among General Populations. J. R. Soc., Interface 2012, 9, 938– 948, DOI: 10.1098/rsif.2011.0537
  21
  Effectiveness of facemasks to reduce exposure hazards for airborne infections among general populations
  Lai A C K; Poon C K M; Cheung A C T
  Journal of the Royal Society, Interface (2012), 9 (70), 938-48 ISSN:.
  Facemasks are widely used as a protective measure by general public to prevent inhalation of airborne pathogens including seasonal, swine and other forms of influenza and severe acute respiratory syndrome (SARS), etc. However, scientific data on effectiveness of facemasks in reducing infections in the community are extremely limited and even inconsistent. In this work, two manikins labelled as 'source' and 'susceptible' were used to measure the protection provided by facemasks under various emission scenarios. The source was modified to generate polydisperse ultrafine particles, whereas the susceptible was modified to mimic a realistic breathing pattern. The facemask was challenged by both pseudo-steady and highly transient emissions generated by an expiratory process where parameters, such as separation distance between manikins, emission velocity and expiratory duration, were controlled and measured systematically. Performances of four different types of facemask fits, varying from ideal to normal wearing practice, were also investigated. Under the pseudo-steady concentration environment, facemask protection was found to be 45 per cent, while under expiratory emissions, protection varied from 33 to 100 per cent. It was also observed that the separation between the source and the manikin was the most influential parameter affecting facemask protection.
22. 22
  Leung, N. H. L.; Chu, D. K. W.; Shiu, E. Y. C.; Chan, K.-H.; McDevitt, J. J.; Hau, B. J. P.; Yen, H.-L.; Li, Y.; Ip, D. K. M.; Peiris, J. S. M.; Seto, W.-H.; Leung, G. M.; Milton, D. K.; Cowling, B. J. Respiratory Virus Shedding in Exhaled Breath and Efficacy of Face Masks. Nat. Med. 2020, DOI: 10.1038/s41591-020-0843-2
  There is no corresponding record for this reference.
23. 23
  Hinds, W. C. 9 - Filtration. In Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles, 2nd ed.; John Wiley & Sons: New York, 1999; pp 182– 205.
  There is no corresponding record for this reference.
24. 24
  Vincent, J. H. 21 - Aerosol Sample Applications and Field Studies. In Aerosol Sampling. Science, Standards, Instrumentation and Applications; Vincent, J. H., Ed.; John Wiley & Sons: New York, 2007; pp 528– 529.
  There is no corresponding record for this reference.
25. 25
  Colbeck, I.; Lazaridis, M. 5 - Filtration Mechanisms. In Aerosol Science: Technology and Applications, 1st ed.; Colbeck, I., Lazaridis, M., Eds.; John Wiley & Sons: New York, 2014; pp 89– 118.
  There is no corresponding record for this reference.
26. 26
  Jung, H.; Kim, J.; Lee, S.; Lee, J.; Kim, J.; Tsai, P.; Yoon, C. Comparison of Filtration Efficiency and Pressure Drop in Anti-Yellow Sand Masks, Quarantine Masks, Medical Masks, General Masks, and Handkerchiefs. Aerosol Air Qual. Res. 2014, 14, 991– 1002, DOI: 10.4209/aaqr.2013.06.0201
  There is no corresponding record for this reference.
27. 27
  Holton, P. M.; Tackett, D. L.; Willeke, K. Particle Size-Dependent Leakage and Losses of Aerosols in Respirators. Am. Ind. Hyg. Assoc. J. 1987, 48, 848– 854, DOI: 10.1080/15298668791385697
  27
  Particle size-dependent leakage and losses of aerosols in respirators
  Holton, Patricia M.; Tackett, Denise Lynne; Willeke, Klaus
  American Industrial Hygiene Association Journal (1958-1999) (1987), 48 (10), 848-54CODEN: AIHAAP; ISSN:0002-8894.
  Measuring particle size-dependent leakage into and losses inside a respirator shows the deposition mechanisms occurring at the leak site and the flow dynamics inside the respirator. This study investigated particle size-dependent leakage and deposition within the mask by examg. the leakage into the mask for different hole locations, probe locations, hole shapes, hole lengths, and hole sizes. The shape of the leak has an effect on particle size-dependent leakage. Probe and leak location tests indicated that not only does the total measured leakage change but that the size-dependence of the leakage also changes, depending on the leak and probe locations. When the leak site is in the chin area, the clean air entering through the filters at the chin helps to carry the inward leakage into the breathing zone. Particle size-dependent leakage does occur and is due to both inertial entry losses at the leak site and within the mask, and diffusional losses within the mask and leak site. Particle size-dependent curves change shape as the hole size changes, with relatively more larger particles entering through the small hole size.
28. 28
  Rengasamy, S.; Eimer, B. C. Nanoparticle Penetration Through Filter Media and Leakage Through Face Seal Interface of N95 Filtering Facepiece Respirators. Ann. Occup. Hyg. 2012, 56, 568– 580, DOI: 10.1093/annhyg/mer122
  28
  Nanoparticle penetration through filter media and leakage through face seal interface of N95 filtering facepiece respirators
  Rengasamy Samy; Eimer Benjamin C
  The Annals of occupational hygiene (2012), 56 (5), 568-80 ISSN:.
  National Institute for Occupational Safety and Health recommends the use of particulate respirators for protection against nanoparticles (<100 nm size). Protection afforded by a filtering facepiece particulate respirator is a function of the filter efficiency and the leakage through the face-to-facepiece seal. The combination of particle penetration through filter media and particle leakage through face seal and any component interfaces is considered as total inward leakage (TIL). Although the mechanisms and extent of nanoparticle penetration through filter media have been well documented, information concerning nanoparticle leakage through face seal is lacking. A previous study in our laboratory measured filter penetration and TIL for specific size particles. The results showed higher filter penetration and TIL for 50 nm size particles, i.e. the most penetrating particle size (MPPS) than for 8 and 400 nm size particles. To better understand the significance of particle penetration through filter media and through face seal leakage, this study was expanded to measure filter penetration at sealed condition and TIL with artificially introduced leaks for 20-800 nm particles at 8-40 l minute volumes for four N95 models of filtering facepiece respirators (FFRs) using a breathing manikin. Results showed that the MPPS was ~45 nm for all four respirator models. Filter penetration for 45 nm size particles was significantly (P < 0.05) higher than the values for 400 nm size particles. A consistent increase in filter penetrations for 45 and 400 nm size particles was obtained with increasing breathing minute volumes. Artificial leakage of test aerosols (mode size ~75 nm) through increasing size holes near the sealing area of FFRs showed higher TIL values for 45 nm size particles at different minute volumes, indicating that the induced leakage allows the test aerosols, regardless of particle size, inside the FFR, while filter penetration determines the TIL for different size particles. TIL values obtained for 45 nm size particles were significantly (P < 0.05) higher than the values obtained for 400 nm size particles for all four models. Models with relatively small filter penetration values showed lower TIL values than the models with higher filter penetrations at smaller leak sizes indicating the dependence of TIL values on filter penetration. When the electrostatic charge was removed, the FFRs showed a shift in the MPPS to ~150 nm with the same test aerosols (mode size ~75 nm) at different hole sizes and breathing minute volumes, confirming the interaction between filter penetration and face seal leakage processes. The shift in the MPPS from 45 to 150 nm for the charge removed filters indicates that mechanical filters may perform better against nanoparticles than electrostatic filters rated for the same filter efficiency. The results suggest that among the different size particles that enter inside the N95 respirators, relatively high concentration of the MPPS particles in the breathing zone of respirators can be expected in workplaces with high concentration of nanoparticles. Overall, the data obtained in the study suggest that good fitting respirators with lower filter penetration values would provide better protection against nanoparticles.
29. 29
  Rengasamy, S.; Zhuang, Z.; Niezgoda, G.; Walbert, G.; Lawrence, R.; Boutin, B.; Hudnall, J.; Monaghan, W. P.; Bergman, M.; Miller, C.; Harris, J.; Coffey, C. A Comparison of Total Inward Leakage Measured Using Sodium Chloride (NaCl) and Corn Oil Aerosol Methods for Air-Purifying Respirators. J. Occup. Environ. Hyg. 2018, 15, 616– 627, DOI: 10.1080/15459624.2018.1479064
  29
  A comparison of total inward leakage measured using sodium chloride (NaCl) and corn oil aerosol methods for air-purifying respirators
  Rengasamy, Samy; Zhuang, Ziqing; Niezgoda, George; Walbert, Gary; Lawrence, Robert; Boutin, Brenda; Hudnall, Judith; Monaghan, William P.; Bergman, Michael; Miller, Colleen; Harris, James; Coffey, Christopher
  Journal of Occupational and Environmental Hygiene (2018), 15 (8), 616-627CODEN: JOEHA2; ISSN:1545-9624. (Taylor & Francis, Inc.)
  The International Organization for Standardization (ISO) std. 16900-1:2014 specifies the use of sodium chloride (NaCl) and corn oil aerosols and sulfur hexafluoride gas for measuring total inward leakage (TIL). However, a comparison of TIL between different agents is lacking. The objective of this study was to measure and compare TIL for respirators using corn oil and NaCl aerosols. TIL was measured with 10 subjects donning two models of filtering facepiece respirators (FFRs) including FFP1, N95, P100, and elastomeric half-mask respirators (ERs) in NaCl and corn oil aerosol test chambers, using continuous sampling methods. After fit testing with a PortaCount (TSI, Inc., St. Paul, MN) using the Occupational Safety and Health Administration (OSHA) protocol, five subjects were tested in the NaCl chamber first and then in the corn oil chamber, while other subjects tested in the reverse order. TIL was measured as a ratio of mass-based aerosol concns. in-mask to the test chamber, while the subjects performed ISO 16900-1-defined exercises. The concn. of NaCl aerosol was measured using two flame photometers, and corn oil aerosol was measured with one light scattering photometer. The same instruments were used to measure filter penetration in both chambers using a Plexiglas setup. The size distribution of aerosols was detd. using a scanning mobility particle sizer and charge was measured with an electrometer. Filter efficiency was measured using an 8130 Automated Filter Tester (TSI). Results showed the geometric mean TIL for corn oil aerosol for one model each of all respirator categories, except P100, were significantly (p < 0.05) greater than for NaCl aerosol. Filter penetration in the two test chambers showed a trend similar to TIL. The count median diam. was ∼82 nm for NaCl and ∼200 nm for corn oil aerosols. The net pos. charge for NaCl aerosol was relatively larger. Both fit factor and filter efficiency influence TIL measurement. Overall, TIL detn. with aerosols of different size distributions and charges using different methodologies may produce dissimilar results.
30. 30
  Electronic Code of Federal Regulations (eCFR), Title 42: Public Health, Part 84—Approval of Respiratory Protective Devices. Code of Federal Regulations , April 2020.
  There is no corresponding record for this reference.
31. 31
  Lord, J. 35—The Determination of the Air Permeability of Fabrics. J. Text. I. 1959, 50, T569– T582, DOI: 10.1080/19447025908659937
  There is no corresponding record for this reference.
32. 32
  Silverman, L.; Lee, G.; Plotkin, T.; Sawyers, L. A.; Yancey, A. R. Air Flow Measurements on Human Subjects With and Without Respiratory Resistance at Several Work Rates. AMA Arch. Ind. Hyg. Occup. Med. 1951, 3, 461– 478
  32
  Air flow measurements on human subjects with and without respiratory resistance at several work rates
  SILVERMAN L; LEE G; PLOTKIN T; SAWYERS L A; YANCEY A R
  A.M.A. archives of industrial hygiene and occupational medicine (1951), 3 (5), 461-78 ISSN:0096-6703.
  There is no expanded citation for this reference.
33. 33
  Grinshpun, S. A.; Haruta, H.; Eninger, R. M.; Reponen, T.; McKay, R. T.; Lee, S.-A. Performance of an N95 Filtering Facepiece Particulate Respirator and a Surgical Mask During Human Breathing: Two Pathways for Particle Penetration. J. Occup. Environ. Hyg. 2009, 6, 593– 603, DOI: 10.1080/15459620903120086
  33
  Performance of an N95 filtering facepiece particulate respirator and a surgical mask during human breathing: two pathways for particle penetration
  Grinshpun Sergey A; Haruta Hiroki; Eninger Robert M; Reponen Tiina; McKay Roy T; Lee Shu-An
  Journal of occupational and environmental hygiene (2009), 6 (10), 593-603 ISSN:.
  The protection level offered by filtering facepiece particulate respirators and face masks is defined by the percentage of ambient particles penetrating inside the protection device. There are two penetration pathways: (1) through the faceseal leakage, and the (2) filter medium. This study aimed at differentiating the contributions of these two pathways for particles in the size range of 0.03-1 microm under actual breathing conditions. One N95 filtering facepiece respirator and one surgical mask commonly used in health care environments were tested on 25 subjects (matching the latest National Institute for Occupational Safety and Health fit testing panel) as the subjects performed conventional fit test exercises. The respirator and the mask were also tested with breathing manikins that precisely mimicked the prerecorded breathing patterns of the tested subjects. The penetration data obtained in the human subject- and manikin-based tests were compared for different particle sizes and breathing patterns. Overall, 5250 particle size- and exercise-specific penetration values were determined. For each value, the faceseal leakage-to-filter ratio was calculated to quantify the relative contributions of the two penetration pathways. The number of particles penetrating through the faceseal leakage of the tested respirator/mask far exceeded the number of those penetrating through the filter medium. For the N95 respirator, the excess was (on average) by an order of magnitude and significantly increased with an increase in particle size (p < 0.001): approximately 7-fold greater for 0.04 microm, approximately 10-fold for 0.1 microm, and approximately 20-fold for 1 microm. For the surgical mask, the faceseal leakage-to-filter ratio ranged from 4.8 to 5.8 and was not significantly affected by the particle size for the tested submicrometer fraction. Facial/body movement had a pronounced effect on the relative contribution of the two penetration pathways. Breathing intensity and facial dimensions showed some (although limited) influence. Because most of the penetrated particles entered through the faceseal, the priority in respirator/mask development should be shifted from improving the efficiency of the filter medium to establishing a better fit that would eliminate or minimize faceseal leakage.
34. 34
  Wells, W. F. Airborne Contagion and Air Hygiene: An Ecological Study of Droplet Infections. J. Am. Med. Assoc. 1955, 159, 90, DOI: 10.1001/jama.1955.02960180092033
  There is no corresponding record for this reference.
35. 35
  Huang, H.; Fan, C.; Li, M.; Nie, H.-L.; Wang, F.-B.; Wang, H.; Wang, R.; Xia, J.; Zheng, X.; Zuo, X.; Huang, J. COVID-19: A Call for Physical Scientists and Engineers. ACS Nano 2020, DOI: 10.1021/acsnano.0c02618
  There is no corresponding record for this reference.
36. 36
  Perumalraj, R. Characterization of Electrostatic Discharge Properties of Woven Fabrics. J. Textile Sci. Eng. 2015, 06, 1000235, DOI: 10.4172/2165-8064.1000235
  There is no corresponding record for this reference.
37. 37
  Frederick, E. R. Fibers, Filtration and Electrostatics - A Review of the New Technology. J. Air Pollut. Control Assoc. 1986, 36, 205– 209, DOI: 10.1080/00022470.1986.10466060
  There is no corresponding record for this reference.
38. 38
  Sanchez, A. L.; Hubbard, J. A.; Dellinger, J. G.; Servantes, B. L. Experimental Study of Electrostatic Aerosol Filtration at Moderate Filter Face Velocity. Aerosol Sci. Technol. 2013, 47, 606– 615, DOI: 10.1080/02786826.2013.778384
  38
  Experimental study of electrostatic aerosol filtration at moderate filter face velocity
  Sanchez, Andres L.; Hubbard, Joshua A.; Dellinger, Jennifer G.; Servantes, Brandon L.
  Aerosol Science and Technology (2013), 47 (6), 606-615CODEN: ASTYDQ; ISSN:0278-6826. (Taylor & Francis, Inc.)
  Aerosol collection efficiency was studied for electrostatically charged fibrous filters (3M Filtrete, BMF-20F). In this study, collection efficiencies at moderate filter face velocities (0.5-2.5 m/s) representative of some high vol. sampling applications was characterized. Exptl. data and anal. theories of filter performance are less common in this flow regime since the viscous flow field assumption may not be representative of actual flow through the filter mat. Addnl., electrostatic fiber charge d. is difficult to quantify, and measurements of aerosol collection efficiency are often used to calc. this fundamental parameter. The purpose of this study was to assess the relative influence of diffusion, inertial impaction, interception, and electrostatic filtration on overall filter performance. The effects of fiber charge d. were quantified by comparing efficiency data for charged and uncharged filter media, where an isopropanol bath was used to eliminate electrostatic charge. The effects of particle charge were also quantified by test aerosols brought into the equil. Boltzmann charge distribution, and then using an electrostatic precipitator to sep. out only those test particles with a charge of zero. Electrostatically charged filter media had collection efficiencies as high as 70-85% at 30 nm. Filter performance was reduced significantly (40-50% collection efficiency) when the electrostatic filtration component was eliminated. Expts. performed with zero charged NaCl particles showed that a significant increase in filter performance is attributable to an induction effect, where electrostatic fiber charge polarizes aerosol particles without charge. As filter face velocity increased the electrostatic filtration efficiency decreased since aerosol particles had less time to drift toward electrostatically charged fibers. Finally, exptl. data at 0.5 m/s were compared to theor. predictions and good agreement was found for both electrostatic and nonelectrostatic effects.
39. 39
  Bałazy, A.; Toivola, M.; Adhikari, A.; Sivasubramani, S. K.; Reponen, T.; Grinshpun, S. A. Do N95 Respirators Provide 95% Protection Level Against Airborne Viruses, and How Adequate are Surgical Masks?. Am. J. Infect. Control 2006, 34, 51– 57, DOI: 10.1016/j.ajic.2005.08.018
  39
  Do N95 respirators provide 95% protection level against airborne viruses, and how adequate are surgical masks?
  Balazy Anna; Toivola Mika; Adhikari Atin; Sivasubramani Satheesh K; Reponen Tiina; Grinshpun Sergey A
  American journal of infection control (2006), 34 (2), 51-7 ISSN:0196-6553.
  BACKGROUND: Respiratory protection devices are used to protect the wearers from inhaling particles suspended in the air. Filtering face piece respirators are usually tested utilizing nonbiologic particles, whereas their use often aims at reducing exposure to biologic aerosols, including infectious agents such as viruses and bacteria. METHODS: The performance of 2 types of N95 half-mask, filtering face piece respirators and 2 types of surgical masks were determined. The collection efficiency of these respiratory protection devices was investigated using MS2 virus (a nonharmful simulant of several pathogens). The virions were detected in the particle size range of 10 to 80 nm. RESULTS: The results indicate that the penetration of virions through the National Institute for Occupational Safety and Health (NIOSH)-certified N95 respirators can exceed an expected level of 5%. As anticipated, the tested surgical masks showed a much higher particle penetration because they are known to be less efficient than the N95 respirators. The 2 surgical masks, which originated from the same manufacturer, showed tremendously different penetration levels of the MS2 virions: 20.5% and 84.5%, respectively, at an inhalation flow rate of 85 L/min. CONCLUSION: The N95 filtering face piece respirators may not provide the expected protection level against small virions. Some surgical masks may let a significant fraction of airborne viruses penetrate through their filters, providing very low protection against aerosolized infectious agents in the size range of 10 to 80 nm. It should be noted that the surgical masks are primarily designed to protect the environment from the wearer, whereas the respirators are supposed to protect the wearer from the environment.
40. 40
  Balazy, A.; Toivola, M.; Reponen, T.; Podgorski, A.; Zimmer, A.; Grinshpun, S. A. Manikin-Based Performance Evaluation of N95 Filtering-Facepiece Respirators Challenged With Nanoparticles. Ann. Occup. Hyg. 2005, 50, 259– 269
  There is no corresponding record for this reference.
41. 41
  National Academies of Sciences. Medicine. Reusable Elastomeric Respirators in Health Care: Considerations for Routine and Surge Use; The National Academies Press: Washington, DC, 2019; p 226.
  There is no corresponding record for this reference.
42. 42
  Bullock, W. H.; Ignacio, J. S. A Strategy for Assessing and Managing Occupational Exposures; AIHA Press, American Industrial Hygiene Association, 2006.
  There is no corresponding record for this reference.
Supporting Information
Supporting Information
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsnano.0c03252.
- Filtration efficiencies for various fabrics tested at two different flow rates and the effect of layering on the filtration efficiencies of chiffon, silk, and 600 TPI cotton; detailed information on various fabrics used (PDF)
- nn0c03252_si_001.pdf (866.43 kb)
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Polydisperse Aerosol Generation

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Aerosol Filtration Efficiency of Common Fabrics Used in Respiratory Cloth Masks
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