Interfacial Chemistry in Indoor Environments

Although many researchers have studied ozone chemistry on indoor surfaces, little is known about the consequences of higher levels of ozone reaction products in indoor air.

Surface chemistry greatly influences human exposure to reactants and products in indoor environments. To illustrate one of the more dramatic instances, ozone reactions on indoor surfaces result in a 2–10-fold reduction in indoor ozone concentrations while simultaneously increasing levels of the products of ozone reactions. Chemistry that occurs at interfaces is remarkably important because compared with outdoor settings, the available surface area is extremely large relative to the building volume; surface sorption extends the average residence time of reactants and increases the probability that conversions will occur; and unique compositions and morphologies at indoor surfaces can promote some reactions or promote selectivity in reaction pathways. Because many people spend 80–90% of their time indoors, this chemistry can have a dramatic impact on personal exposure.

To assess the current state of knowledge in this area, researchers and other experts recently participated in the Workshop on Interfacial Chemistry in Indoor Environments sponsored by the National Science Foundation (NSF), the California Air Resources Board (CARB), and the Environmental Research Center of the Missouri University of Science and Technology. Workshop participants identified research needs and ranked gaps in the existing knowledge base of indoor interfacial chemistry, particularly as they relate to human exposure to air pollutants. This article, which is based on discussions held at the workshop, provides an overview of indoor surface chemistry, primarily due to smog reactions, with an eye toward how we might reduce occupant exposure to air pollution. The full NSF report is available online (1).

Indoor chemistry

An active research area has been the study of ozone reactions with various indoor surfaces and with compounds on these surfaces. Smog enters buildings by ventilation and infiltration, bringing with it energy in the form of ozone and other photochemically generated oxidants. Ozone readily reacts with an ample supply of unsaturated compounds that are present in typical buildings at concentrations many times greater than those observed outdoors. Products of this chemistry include carcinogens (formaldehyde, acrolein), irritants (carbonyls, dicarbonyls, acids), free radicals, secondary organic aerosols, and other oxidation products. The resulting concentrations are large enough to have health and comfort consequences at typical indoor ozone levels (2).

Ozone chemistry has been studied on a wide range of surfaces and coatings relevant to indoor environments. Ozone uptake rates on surfaces, without regard to the chemistry, have been quantified on carpets, painted drywall, tile, concrete, wood, countertops, and glass (3). Ozone oxidation of organic matter on surfaces generates volatile carbonyl compounds from carpet, paint, ventilation duct materials (4), soiled surfaces in homes (5), and surfaces in a simulated aircraft cabin (6, 7). Product yields are generally in the range of 0.1–0.5, meaning that 10–50% of the ozone consumed by the surface generates a detected gas-phase product. In a typical urban home, this means that reaction products will likely be present at tens-of-parts-per-billion levels during high-ozone periods, levels comparable to those for 10 carbonyls measured in the Relationships of Indoor, Outdoor, and Personal Air (RIOPA) study (8) of air pollutants in 300 homes.

Ozone also reacts with chemicals intentionally or unintentionally applied to indoor surfaces. Known ozone-reactive chemicals include nicotine from cigarette smoking (9), pesticides such as cypermethrin (10), and terpenes that are found in cleaning or other scented products (11). Recently, researchers showed that cleaning products and air fresheners enhance ozone uptake at surfaces (11); sorbed compounds accounted for half of ozone reactions in some cases. In studies of the reaction of single terpenoid species (squalene and α-terpineol) attached to real surfaces, Flemmer et al. (12) detected dicarbonyl species generated from both terpenoids, including glyoxal, methylglyoxal, and 4-oxopentanal. These dicarbonyl compounds are thought to be particularly irritating to eyes and mucous membrane tissues (13).

Chemistry taking place at the “human surface” is emerging as an important factor in our understanding of ozone exposure and reaction products. This became apparent in experiments with simulated aircraft cabins, densely occupied with human subjects (6, 7). The researchers concluded that aircraft occupants are major ozone sinks—larger than the carpet, seats, and a soiled HEPA filter combined. On the basis of questionnaires completed by the aircraft occupants, they concluded that ozone and its oxidation products had adverse effects on 12 of 29 self-reported symptoms. Evaluation of individual aircraft surfaces, in small-chamber experiments, confirmed the reactivity findings (14). Thus, the collective reactivity of humans in densely occupied settings, such as a classroom, will reduce ozone exposure and increase exposure to skin-oil reaction products. In studies of ozone reaction with human hair, researchers found that the ozone reaction probability for hair is large (>10^–4) relative to other indoor surfaces (15). Model analysis suggests that the boundary layer of air surrounding the body, including the breathing zone, may become significantly ozone-depleted and oxidation-product-rich.

Indoor-surface studies have also focused on other components of smog, such as nitrogen oxides (NO_x). Early work on NO_x surface chemistry showed that NO₂ will react with water on smog chamber surfaces to generate nitric acid (HNO₃) and volatile HONO (16). Pitts et al. (17) showed that this chemistry also occurs on indoor surfaces and can generate HONO levels that exceed outdoor levels when NO_x is released from gas burners. Recently, Ramazan et al. (18) showed that nitric acid may further participate in chemistry and photolysis that releases NO and HONO. Thus indoor surfaces can act as a sink for NO₂, a reservoir for HNO₃, and a source of HONO and possibly NO. Nitrate radicals (19) may also be important oxidants in the low-light environment unique to indoor spaces.

Also of emerging importance is hydrolysis, which can generate toxins and odors. The plasticizer di-2-ethylhexylphthalate (DEHP) can be hydrolyzed, generating monoethylhexylphthalate (20), and hydrolysis products may be associated with asthma (21). These and other plasticizers are commonly found in vinyl flooring and adhesives. Highly basic concrete flooring and gypsum board can help catalyze this hydrolysis. In an intensive study of paint components, researchers identified hydrolysis products of Texanol isomers, including isobutyric acid (strong odor), emitted in the first few days after application to gypsum board (22). Hydrolysis reactions tend to be slower than the ozone and NO_x reactions and are mediated by local pH and moisture conditions.

These examples of indoor chemistry just scratch the surface. Much of the research to date has been phenomenological in nature but now requires a more fundamental approach, especially since generalizing in the face of highly variable indoor microenvironments is problematic. Although many researchers have studied ozone chemistry with indoor surfaces, little is known about the influence of environmental conditions on this chemistry. A small fraction of the predicted volatile products have been identified, and few studies have attempted to identify the resulting low-volatility products of this chemistry that remain on surfaces.

Indoor chemistry and occupant health

Very few of the pollutants associated with indoor chemistry have been scrutinized for health outcomes, as is the case for all indoor air pollutants as a class. In a facile analysis, this chemistry may be seen as beneficial, in that it reduces ozone exposure. But what are the consequences of higher levels of ozone reaction products? How do modest increases in formaldehyde, nitrous acid, secondary organic aerosols, or hydrolyzed phthalates affect the most vulnerable occupants? These and many other questions have yet to be answered, but circumstantial evidence points to substantial problems.

Weschler (23) suggested that epidemiological correlations between outdoor levels of ozone and morbidity or mortality are due, in large part, to indoor exposures to ozone and the byproducts of its reaction with other species indoors. He estimated, conservatively, that indoor exposure to ozone (that originates outdoors and is transported indoors) is from two-thirds to three times greater than outdoor exposure to ozone, while indoor exposure to ozone oxidation products is commonly about one-third to six times greater, on a molar basis, than outdoor exposure to ozone itself. In workshop discussions, Nazaroff (24) concurred and showed that the intake fraction (the fraction of source emissions that reach a person) of ozone reaction products is high (~4000 per million) compared with intake fractions for typical outdoor sources (~10 per million). Although indoor levels of reaction products may correlate with outdoor levels of ozone, this hypothesis has not been fully evaluated in field settings. In a recent effort to evaluate the toxicological potential of species generated as a result of indoor chemistry, Anderson et al. (13) showed with quantitative structure-activity relationships (QSARs) and animal models that most dicarbonyl products are irritants and are predicted to be sensitizers. In addition, a raft of human and animal experiments have shown adverse reactions to homogeneous ozone chemistry (see references in 25). Thus, toxins, irritants, and sensitizers are generated at levels of apparent concern.

Estimating incremental increases (or decreases) in exposure to reactants and products is challenging. At present, we rely on the results of laboratory chamber studies and a handful of field experiments. Many studies to date have been performed under typical conditions and may be trusted, in principle. For example, ozone uptake and product yields in a field study of five homes (5) were consistent with laboratory results. A comparison of these parameters for aircraft cabin materials with results from a real aircraft cabin showed remarkable consistency (6, 14). Yet, questions remain about the relevance of chamber studies in predicting actual ozone conversion rates in real homes (26), and whether the impact on indoor levels can be discerned in homes with highly variable levels of compounds. Little is known about the effects of relative humidity, competitive adsorption of compounds, or acidity on the chemistry.

Recent fundamental studies of chemistry on model surfaces point to nonintuitive outcomes that will affect our ability to extrapolate from experiment to exposure. For ozone reactions with building materials, the combination of surface-reaction probabilities, mass-transfer coefficients, and product yields is thought to be sufficient for estimating ozone loss rates and product emission rates. But in studies of organic monolayers, the Finlayson-Pitts (27, 28) and Geiger (29) groups have begun to question the “reaction probability” as a fundamental metric for characterizing ozone rates on surfaces because the value depends on ozone concentration. One unusual outcome of these and other studies is that the second-order interfacial reaction rate is roughly the same regardless of the organic reactant. Thus, a better molecular-level understanding is needed to discern the chemical mechanisms that are so important in understanding how this chemistry influences human exposure.

Controlling indoor chemistry

Controlling indoor chemistry means controlling sources, reactants, and conditions that promote that chemistry. Ozone is a clear target, and its removal from buildings is anticipated to lower indoor concentrations of aldehydes, ketones, organic acids, free radicals, and secondary organic aerosols. Activated carbon (AC) filtration is available for commercial buildings and is effective at removing ozone and some volatile organic compounds from supply air. Standard 62.1 from the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) requires AC filtration for high-ozone areas, but the standard is rarely implemented. Some members of ASHRAE have been working to broaden ASHRAE Standard 62.1 to make ozone removal more widespread in commercial buildings. No standards exist for AC filtration in residential buildings, and its use is negligible (30).

Direct control of other reactants indoors may be preferable in some cases and impractical in others. Use of effective venting to reduce NO_x emissions from indoor combustion sources has the additional benefit of reducing exposure to combustion-generated formaldehyde and aerosols. Reducing moisture helps keep mold in check, as well as controlling hydrolysis. Many reactants are unnecessary components of consumer products (highly reactive fragrances) that do not improve the products’ efficacy and could be removed or replaced with low-reactivity species. In other products, such as cleaners, reactive terpenoids act as solvents and are not easily replaceable. It would be particularly impractical to attempt to prevent cooking oils, skin oils, and soap residues from coating indoor surfaces. Removal of volatile organic species by filtration is possible, but less effective than removal of sources. For example, UV photocatalytic reactors can be effective at reducing the gas-phase concentrations of reactants and products of indoor chemistry. But many compounds are only partially oxidized, resulting in undesirable aldehydes or organic acids (31). A mechanistic understanding of UV photocatalysis (32) may help overcome this problem of incomplete oxidation. In general, control of inorganic reactants (O₃, NO_x, H₂O) by source control, filtration, or other means appears to be more practical than eliminating all organic reactants.

Undoubtedly, the homeowner, architect, and builder bear some responsibility for controlling indoor air pollution. Buyers looking for safer products often rely on environmental certification programs. But these programs evaluate only primary emissions, often by diverse criteria, and do not consider chemical transformations. Further, much of the chemistry and many of the outcomes described here are the direct result of ambient smog and are therefore a societal, state, and national responsibility. Other than radon testing, bans on cigarette smoking in public areas, and various public school initiatives, few states take actions to improve indoor air quality through means other than information dissemination. CARB recently approved a regulation that will limit ozone emissions from indoor-air cleaning devices, such that the steady-state level is no greater than 50 ppb in a well-defined chamber test. Some ozone-generating air cleaners can increase indoor ozone to levels above the California (70 ppb) and the National Ambient Air Quality Standard (75 ppb) 8 hour limits for ambient air (33).

Individuals who commented on the CARB rule have suggested that future consideration be given to making the device standard more stringent in light of indoor ozone chemistry. Intense secondary aerosol generation has been observed when these devices are used in the presence of air fresheners (34), and chemistry at surfaces is likely to significantly increase aerosol and oxidized-gas levels.

Recommendations

During the workshop, participants widely agreed that a molecular-level understanding of physical and chemical processes on real surfaces is necessary to fully understand macroscopic phenomena such as exposure. Specifically, insights need to be gained into important transformative processes such as ozone and free-radical oxidation, hydrolysis, dissociation, and ozonide decomposition. Sorption; desorption; aqueous film chemistry; the role of water on surfaces; timescales of processes; and the influence of interfaces on nucleation, condensation, and other properties of aerosols also need a more fundamental approach. For many common surfaces, fundamental parameters, such as surface area, pH, and water-uptake isotherms, are at best poorly characterized. In a similar vein, emphasis was placed on composition and morphology. The composition of substrates (and their engineered coatings), such as polyvinyl chloride flooring, painted drywall, carpet, and upholstery, can vary widely even within substrate types; yet compositional information is rarely investigated in detail or reported. The composition of the material overlaying substrates, such as dirt, dust, oils, water, reaction residues, and salts, is also poorly understood.

Researchers have made some efforts to analyze these films, but workshop participants expressed concern that traditional methods will not capture the true composition. For example, moderately stable ozonides or epoxides present on real surfaces may be transformed to other species during extraction and analysis. Morphology, including surface area and pore size distribution, will also influence interfacial phenomena. Yet traditional methods of measuring morphology are difficult to apply to indoor materials, in part because many outgas volatiles. Further, traditional vacuum analyses inevitably change the surface composition of indoor materials, and perhaps even the morphology.

To reach these goals, the community will have to develop and apply newer analytical techniques to characterize the chemistry and physics of surfaces. Extraordinary tools already exist and have been applied to understand ambient aerosol chemistry, but rarely have these been used to study indoor surfaces. Methods identified as being particularly promising include proton transfer reaction mass spectrometry (MS), desorption electrospray ionization MS, attenuated total reflection Fourier transform IR spectroscopy, and hyperspectral sensors.

Health and indoor-air chemistry. Pollutants and potential pollutants observed or predicted to be found in indoor environments vastly outnumber the chemical species that have been evaluated for toxicity or other health outcomes. A concerted effort needs to be made to rank species on the basis of existing toxicological data or potential toxicity, which can be estimated from QSARs. Further, collaborations among chemists, engineers, and toxicologists are necessary to identify species for future toxicological assays. This idea was promoted at the Workshop on Indoor Chemistry and Human Health, held in Santa Cruz, Calif., July 12–15, 2004 (35). Collaborative identification of chemical mechanisms, exposure pathways and intensity, and health outcomes should provide an efficient route to rapid improvement of indoor air quality.

Indoor and outdoor air are part of the same continuum. Yet scientific, legislative, and philosophical separation of these domains has adversely affected our ability to target efficient solutions for reducing exposure to smog and its consequences. Study and control of indoor air pollution should be considered an integral part of the larger campaign to improve air quality embodied by the Clean Air Act. Smog chemistry does not stop at the door, but churns away in the indoor spaces where we spend most of our time. Thus, indoor air deserves the same attention given to ambient air for the past 50 years.

Glenn Morrison is an associate professor of civil, architectural, and environmental engineering at the Missouri University of Science and Technology. Address correspondence to Morrison at gcm@mst.edu.

References

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