Richard Denison, Ph.D., is a Senior Scientist.
When the chemical industry talks about prioritization – a central question in the debate over TSCA reform – more often than not it quickly reduces the question down to the argument that we should focus only on those chemicals, however hazardous or untested they may be, to which we know people are exposed. In a perfect world, that might suffice. But, as this post will explore, the world of exposure assessment is anything but perfect.
While both hazard and exposure are clearly relevant in determining chemical risks, there are critical differences between our ability to assess hazard and exposure that have implications for the development and application of chemicals policies. And real-world experience in chemical assessment programs that have attempted to rely on exposure information to prioritize chemicals also offers lessons for exposure assessment.
Critical differences between assessing hazard and exposure
Approaches that seek to rely on exposure to prioritize or assess chemicals need to acknowledge and account for a number of critical differences between the nature of hazard and exposure information and their relative extent of availability. Certain characteristics of exposure information pose serious challenges to sound decision-making. Here are several reasons why.
1. Hazard is largely inherent to a chemical and (aside from degradation or metabolism) doesn’t fundamentally change over space or time, whereas any exposure information necessarily represents only a “snapshot” in both space and time.
A chemical’s hazard is an intrinsic property, one that is directly related to the chemical’s composition. While manifestation of a hazard can of course vary by route of exposure, it otherwise exists largely independent of how the chemical is used, where or how it enters the environment, or other factors that vary with time and place. Hazard data are therefore relevant and needed regardless of how the chemical is used. That is, such data are useful in understanding any actual or potential use or release of a chemical – and in deciding what kind of exposure-reducing efforts may need to be taken.
Just the opposite is true for exposure, which can change dramatically depending on how a chemical is produced, used, transported and discarded or released. The consequences of exposure depend on who or what might be exposed, and the level, frequency and duration of the exposure.
Conditions that determine exposure can and often do differ enormously for every setting and point in time that a chemical is present. Basic physiological differences (including those associated with age and life stage) as well as cultural factors (e.g., extent of fish consumption) and activity patterns also amplify the variability in exposure to a chemical. And even if a “snapshot” of current exposure is able to be assembled, the next new use or activity leading to a release alters the exposure picture.
The highly variable nature of exposure poses a major challenge to exposure (and risk) assessment: It means that exposure assessment must be an ongoing activity, with the scope and frequency of its measurement sufficient to characterize the variation (spatial and temporal) in, as well as magnitude of, exposure.
That’s but one reason why exposure assessment is often called the “weakest link” in risk assessment.
2. Mechanisms for generating and collecting exposure information are undeveloped relative to those for hazard information.
An extensive body of methods developed through international consensus specifies how to test a chemical for most hazardous properties. And while new hazard concerns and new test methods emerge over time and must be incorporated, the infrastructure for doing so is largely in place: detailed government-sanctioned procedures, guidelines, criteria and standards have been specified for conducting hazard tests, for assuring the quality and reliability of the results, and for determining whether the results constitute evidence of a particular hazard. Moreover, these measures allow that results are reproducible and can be independently verified.
In contrast, virtually none of these mechanisms are in place or established to assure that exposure information is complete and accurate. Debates over what constitutes adequate exposure assessment and how to address the “moving target” nature of such information are far from resolved. Government-sanctioned procedures for generating, evaluating the adequacy of and interpreting exposure data have yet to be developed or validated, including testing and measurement standards, guidance, methods and tools.
Use and exposure information is rarely systematically collected and even less often made public in any useful form. For the first time, beginning in 2006, the Environmental Protection Agency (EPA) began to require the reporting of basic information relevant to understanding uses of and exposure to chemicals. But that program is fraught with limitations, as I’ve described in detail in earlier posts here and here.
Because of these and other factors, estimates of chemical exposures typically depend on severely limited sources of information. In many instances, little or no information is publicly available on how chemicals are manufactured, processed, used, or discarded; the numbers of workers and consumers exposed; quantities released to the environment; how chemicals are distributed and transformed in the environment; and other parameters necessary for estimating exposures.
3. Assumptions and modeling used to characterize exposure are notoriously inaccurate.
As a result of the information constraints just discussed, exposure-driven prioritization initiatives and exposure modeling exercises may well utilize incorrect exposure assumptions. Biomonitoring has revealed such instances by “ground-truthing” exposure assumptions – providing objective, incontrovertible evidence of the extent to which chemicals end up in people. Recent biomonitoring data on both phthalates and poly-brominated diphenyl ethers amply illustrate this point.
Phthalates are very widely used in products ranging from plastics to cosmetics and other personal care products. They exhibit a range of toxicity, including to the liver, kidney, and male reproductive system. The first CDC National Report demonstrated surprisingly high levels of di-butyl phthalate (DBP) and di-ethyl phthalate (DEP) in U.S. residents in general, and for DBP, in women of child-bearing age in particular (see the first two letters here). Indeed, these data demonstrated high-end levels of DBP that were an order of magnitude higher than a prior estimate that had been developed based on industry-provided use data and expert judgment.
Polybrominated diphenyl ethers (PBDEs) are widely used flame retardants. Different species of PBDEs are used in products ranging from plastics (such as computer cases) to upholstery foam. Toxicological studies indicate that they can disrupt thyroid metabolism and may have effects on other organs, including the liver. Because PBDEs are not very volatile or water soluble, they were assumed to more or less stay in place in products, and were not believed to have a high potential for exposure. However, biomonitoring studies from around the world have demonstrated that levels of PBDEs in peoples’ bodies have been dramatically increasing over the past two decades, with the highest levels currently reported in the United States; see, for example, this paper).
4. Differential access to both exposure data and the means to generate them severely limit the “reproducibility” of such data.
In addition to the variability and absence of agreed-upon procedures noted above, other factors limit “reproducibility,” that is, the ability to readily and independently measure or verify exposure data. Most exposure data and the means to generate them reside virtually exclusively with industry. It simply must be acknowledged that industry has a strong interest in maintaining that exposure to its chemicals is low, so the ability to independently measure and verify exposure data is critical. Yet physical access to many exposure “settings” (e.g., workplaces) is very limited and infrequent at best, even for government officials.
Broader access to exposure-relevant information is even more restricted: Wide latitude is typically provided to claim chemical use and exposure information as CBI, preventing even its review outside government; this situation is often in contrast to that applying to hazard data, which is more likely to be deemed ineligible from designation as CBI.
Finally, even chemical manufacturers have incomplete access to and information on their customers and how their chemicals are used. Intermediaries (vendors, brokers, distributors) are a formidable information flow bottleneck, as is the often-proprietary nature of information concerning downstream use and competition among suppliers. These factors serve to impede information-sharing even within supply chains, which in turn affects the extent and accuracy of exposure-relevant information that any one entity in a supply chain can provide if asked or required to do so; see this paper and Modules 1 and 2 of this report for more discussion of these limitations.
Conclusion
For all of these reasons, we cannot continue to rely on assumptions about chemical exposure or on the industry-supplied or otherwise-limited exposure information that is currently available. That is simply too uncertain and unreliable a basis on which to set aside hazardous or untested chemicals as low-priority, or to decide for which chemicals hazard data should be developed.
Of course, where affirmative evidence indicates exposure to a chemical is occurring, for example as revealed through biomonitoring, this should clearly suffice to prioritize such a chemical at a minimum for further testing, assessment or control. But the converse cannot be said: that in the absence of reliable information about exposure we can simply drop a chemical from further consideration.
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There is actually considerable real-world experience that documents the adverse consequences of an over-reliance on exposure to prioritize chemicals. For more detail, see our analyses of:
- EPA’s recently replaced ChAMP Initiative;
- Canada’s Chemical Categorization; and
- the Existing Chemicals Program of the Organization for Economic Cooperation and Development (OECD) – see the paper included as pages 109-114 of this OECD report, which I wrote several years ago and from which this post is derived.