Chemical screening—Faster, cheaper, better?
A new approach could help regulators break through the monumental logjam of existing chemicals that have never been assessed for their potential to harm humans or the environment.
As the EU and Canada gear up to evaluate the tens of thousands of chemicals currently in use that have never before been screened for risk to humans or the environment, questions about the right strategy and practical issues about the duration and cost of such an undertaking are being raised. An approach recently published in ES&T (Environ. Sci. Technol. 2006, 40, 2316–2323) could fit the bill, say several nongovernmental scientists familiar with these activities.
)
)
)
)
)Governments began crafting laws to evaluate the health risks of existing chemicals in the wake of the 2001 Stockholm Treaty that banned the “dirty dozen”: 12 persistent, bioaccumulative, and toxic (PBT) organic chemicals. Before that, the vast majority of chemicals used in commerce were not subject to such evaluations, because the environmental laws put in place some two decades earlier in Europe, Canada, and the U.S. exempted substances already on the market. This group of “grandfathered” chemicals comprises more than 95% by volume of the chemicals used in production processes and consumer products.
Canada, the first country to prioritize chemicals, started in 2000 and aims to finish screening some 23,000 chemicals on its Domestic Substances List—the official list of chemicals used in the country—for their PBT properties by September 2006. Chemicals that fail the PBT screen will be subject to further assessment. The European Union is currently moving toward legislation that would require reviews of some 100,000 chemicals.
Assessments like Canada’s effort are welcome, says chemical fate and transport pioneer Don Mackay at Trent University (Canada). But splitting up the assessment job into two parts—an initial PBT screen and then a more detailed risk assessment—makes extra work and ignores important information that could make this a better, faster, and easier one-step job, he says. Moreover, schemes that rely only on PBT properties could miss chemicals that are marginally persistent or bioaccumulative, such as phthalate esters and silicones, but are still problematic because they are used in such high volumes. Or they can focus unnecessary effort on very toxic chemicals that are emitted in such low volumes that they pose little risk.
“These hazard assessment schemes are fundamentally inadequate, and they are not what the public wants,” Mackay says. “People rightly want to know if they are at risk.” PBT schemes only assess the hazard posed by a chemical, he says.
Regulators can go straight to a risk analysis by combining estimates of chemical properties, including toxicity and environmental fate, with a rough estimate of emissions rate to conduct a simple, transparent screening-level risk assessment. In their ES&T paper, Mackay and his student Jon Arnot demonstrate such a scheme by evaluating a set of 70 chemicals from Canada’s Domestic Substances List.
They show that the relative risk associated with releasing these 70 to the environment varies by over 14 orders of magnitude. Those that came up as relatively risky include the herbicide atrazine, the antibacterial agent triclosan, the flame retardant tetrabromobisphenol A, the antibacterial agent hexachlorophene, and the fungicide chlorothalonil. Screening programs based solely on PBT properties could miss these chemicals, says Arnot. Among those near the bottom in relative risk are chemical intermediates such as toxic DCE (1,2-dichloroethane), and camphene, an ingredient in flavors and fragrances. Chloroform also ranks low, but typical exposure is primarily from indoor evaporation, which the model does not address.
This kind of data should make it very easy for regulators to prioritize their efforts, Mackay says. “Including emissions is an obvious way to evaluate risk,” says modeler Thomas McKone at Lawrence Berkeley National Laboratory.
Such a screening effort could also catch problem chemicals early, says Mackay. Perfluorinated chemicals, which have been emerging as contaminants of concern over the past 5 years, might have come under closer scrutiny earlier had this model been used for screening, agree University of Toronto chemist Scott Mabury and former 3M environmental toxicologist Rich Purdy. “This model applied to precursor fluorinated alcohols would have likely indicated the atmosphere as the dominant sphere for contamination,” Mabury says. “Perhaps scientists would have wondered about the fate of these reactive fluorochemicals and thereby helped to avoid a global contamination problem we are now just understanding,” he adds.
One anonymous regulator agrees that the model performs well. “It handles assessments in a clever way by including emissions. [With the model,] it is possible to screen a larger number of chemicals more quickly and, as a result, to more efficiently rule in, or rule out, chemicals.”
Actual emissions estimates are often very uncertain, says Arnot, so they are not included directly as a model input. Instead, the model calculates a critical level of emissions that would cause concern. Regulators can then compare emissions estimates with this critical value to assess the risk.
“However, [in] the real world, assessment and regulation is more complex and has many factors, as much economic and political as scientific, that render a model like this inadequate,” the regulator warns.
But Mackay asks, “Should we continue on the present course of using PBT screening to slowly evaluate some thousands of chemicals for hazard, which is interesting but misses the point, [and] some hundreds for risk, and leave tens of thousands totally unassessed? Or do we assess the tens of thousands for approximate risk in a timely manner with a view to identifying the potentially risky chemicals for closer subsequent scrutiny? I suspect the public wants the latter.”
Toxicologist Tim Kroop of the Environmental Working Group, an advocacy organization, agrees: “This is the kind of thing that needs to be done, something that looks at all the factors that determine risk. The current chemical screening program in the United States, where there is no effort to evaluate all chemicals in commerce, is based on timing: Is a chemical grandfathered in or not? What people want is chemical screening based on risk.”
