Exposure assessments can be simple or complex, depending on the needs of a particular risk-management question. They are based on measurements, models, and assumptions, and generally focus on individual chemicals, media, and sources. Often, unvalidated mathematical models are used to make predictions about a population's exposure on the basis of limited information on chemical contamination and assumptions about the population. The results oversimplify actual exposure magnitudes and conditions, in part to allow for population variability. And the methods generally do not consider other sources of exposure to the same or similar chemicals and their interdependence, which the Commission's risk-management framework will stimulate. This section recommends ways to generate credible and understandable exposure information for informed decisions by risk managers and the public about the need for risk reduction. The Commission also recommends that agencies exhibit an active preference for actual exposure data for communities and populations at risk.

FINDING 3.2.1: Because of statutory requirements and the desire not to underestimate maximal chemical exposures, many risk assessments have estimated risks for a hypothetical, nonexistent "maximally exposed individual" (MEI) and have neglected information about the frequency, duration, and magnitude of actual population exposures. More recent assessments have used less extreme exposure scenarios. Congress specified in the 1990 amendments to the Clean Air Act that, after maximum available control technology is implemented for stationary sources, further controls must be considered if the lifetime excess cancer risk to the "individual most exposed to emissions from a source" in a category exceeds 10^-6. The criteria for the "individual most exposed" were not stated; in fact, Congress mandated this Commission to advise what exposure scenarios should be used.

RECOMMENDATION: Exposure assessments should not be based on a hypothetical maximally exposed individual (MEI). Screening risk assessments should rely on more representative estimates, such as EPA's high-end exposure estimate (HEEE) or a maximally exposed actual person and estimates of the total number of potentially exposed people in the geographical areas of interest. Risk-management decisions should be based on refined exposure assessments that evaluate the distribution of a population's varied exposures and should address explicitly for any segments of the population that have unusually high exposures. Exposure assessments should emphasize the characteristics of actual or potential future populations in relation to specific sources of exposure and should reflect multiple sources of exposure, as appropriate in each case.

RATIONALE

With the intention of protecting public health, past exposure-assessment and health risk-assessment practices have relied on exposure estimates derived from a hypothetical maximally exposed individual (MEI). An MEI is a person who might spend a 70-year lifetime living at the point of greatest deposition from a plume of contaminant emissions from an industrial facility or a person who might spend a 70-year lifetime drinking only groundwater with the highest concentrations of contaminants detected. The MEI was often so unrealistic that its use impaired the scientific credibility of health risk assessment.

Federal agencies have generally moved away from exposure assessments relying on such MEIs. For example, EPA's exposure assessment guidelines have adopted the use of distributions of individual exposures and high-end exposure estimates (HEEEs) chosen from values in the upper tail of those distributions (EPA 1992a). EPA's risk characterization guidelines provide guidance on the use of exposure descriptors to characterize risk (EPA 1995a). At this time, implementation of those guidelines among EPA regional offices is uneven; some continue to use point estimates, while others use probability distributions of exposure estimates.

The Commission supports distributional approaches to exposure characterization that are based on knowledge of the characteristics of a population's variability. Where possible, the entire distribution of the variability associated with exposure should be used in a risk characterization (see section 5.1 and the discussion of variability and uncertainty in section 3.5). That distribution should be based on the characteristics of the entire exposed population and not solely on a highly exposed subpopulation; any highly exposed subpopulations known to exist should be considered separately. If a single value representing a population's or subpopulation's exposure is required, such as for priority-setting, a point in the upper end of the distribution should be used, such as the 90th percentile. Agencies should develop standard distributions to use in exposure assessments as defaults when population-specific information is unavailable. If data limitations do not permit the development of a defensible exposure distribution, a value representing a hypothetical highly exposed individual should be used. Such point exposure estimates might be useful for simple screening-level risk assessments. Probabilistic exposure estimates should be considered when standard default methods are expected to yield unrealistically conservative exposure estimates, when population estimates of exposure are desired, or when the exposure assessment is complex. Mark Van Putten, of the National Wildlife Federation, testified before the Commission that the environmental-justice movement has provided some impetus for considering distributions instead of point estimates, on the grounds that populations with disproportionate exposures can be more explicitly identified and considered in risk assessments. We agree.

One advantage of using distributions to describe a population's exposure is that it focusses attention on the characteristics of the population. Exposure estimates derived primarily from the emission or other characteristics of a particular source of contamination are incomplete. Exposure is experienced by individual members of populations and should be assessed accordingly. A population-based approach can be source-specific but should include information on the variables that affect exposure characteristics, such as activity patterns that influence the mode, frequency, and duration of exposures. A complementary community-based approach would begin by determining a population's exposures and moving from that information to identify sources of exposure. The total exposure assessment methodology (TEAM) study conducted by EPA, in which representative members of several urban populations used small personal samplers to measure individual exposure to airborne chemicals (EPA 1987a), is an example of a community-based approach to exposure assessment. Monitoring blood lead in a community's children and tracing the sources of lead is another example.

Many exposure assessments are based on source characteristics, not population characteristics. For example, air pollution sources typically have been licensed on the basis of modeled projections of their stack emissions. Few data (if any) on actual population exposures exist. Such data deficiencies create problems, as emphasized by Ellen Silbergeld, of the Environmental Defense Fund, in testimony before the Commission: there is no direct way to estimate the actual health risks experienced by an exposed population; there is no way to assess the relative contribution of multiple sources to risk; and there are no baseline data with which to evaluate the effects of new sources or of pollution-reduction activities on existing sources. Resistance to collecting data on populations' actual exposures arises from the substantial time and expense associated with monitoring efforts, especially given the large variations in local climate and the problems associated with accurate detection of small pollutant exposures. Environmental monitoring is needed, however, to generate actual data that are consistent with a public-health approach to risk assessment and with the Commission's framework for risk management. Exposure assessment must begin to address aggregate exposure. Stimulated in part by Toxics Release Inventory reports, communities are interested not just in what a particular industrial facility exposes them to, but in how that facility adds to the burden of exposures that they are already experiencing. Focusing on real populations is essential to identifying multiple-exposure situations. We expect biomarkers of exposure to become useful in validating exposure estimates and in relating exposures to specific subgroups and even to individuals.

FINDING 3.2.2: Some population groups are at increased risk for toxic effects of chemical exposures because their exposures are greater than those of other population groups. Cultural practices, occupational exposures, behavior patterns, eating habits, and effects of related chemicals can be responsible. The high-risk subpopulations might be of special concern when risk assessments are conducted and risk-management decisions are made. Risk assessors often have not sought information from knowledgeable citizens and as a result, have not explicitly considered specific exposure conditions that might be present in minority-group communities, particular occupational settings, or areas of low socioeconomic status.

RECOMMENDATION: Risk assessments should be conducted so as to identify increased risk to particular groups of people who are likely to have higher exposures to the chemicals of interest. Affected parties should be consulted in the early stages of an assessment to obtain information on all known sources of exposure to a particular chemical and to related chemicals and to characterize exposure factors peculiar to particular subpopulations and link them with host susceptibility factors (see section 3.1).

RATIONALE

Increased risks of adverse health effects of contaminant exposures can result from increased doses and from increased susceptibility, which was discussed in section 3.1. Dose is a function of the concentration of a substance in the environment and the extent of exposure that a person has with the substance. Advances in the use of biologic markers will help to define relationships between exposure and dose. The following is a list of some factors that can increase risk as a result of increased exposure.

The Clinton Administration, the 103rd and 104th Congresses, several interest groups, and the scientific community have attempted to address the issue of high-risk populations in several ways. For example, Executive Order 12898 on Environmental Justice requires that federal programs protect minority-group and low-income populations from disproportionately high exposures and adverse human health and environmental effects. EPA addressed the potentially greater susceptibility of children to pesticides and pesticide residues by requiring that assessments of environmental risks explicitly take into account health risks to children and infants associated with environmental hazards in the air, in food, and in water (EPA 1995b). That policy followed a National Research Council report that variations in dietary exposure to pesticides related to differences in food and beverage intake, age, geographic region, and ethnicity were not addressed adequately by current regulatory practice (NRC 1993). Infants and children might be more heavily exposed to pesticides than adults because of their relatively high intake of fruit juices, and they are more susceptible to the toxic effects of pesticides because of the sensitivity of their still-developing nervous systems and probably because of their greater concomitant exposures to lead and other environmental hazards.

Community assistance in characterizing exposure factors peculiar to particular segments of the population can focus a risk assessment and broaden risk-management options. The Commission heard testimony from Asians and Pacific Islanders about their fish-consumption patterns and about the role that education can play in risk management. Not only do they consume more fish, but they consume parts of seafood that are usually discarded by others and in which pollutants are often concentrated, placing themselves at higher risk than the general population for the effects of contaminants in fish. They reported that educational brochures, signs around contaminated bodies of water, and community involvement led to voluntary reduction in exposure through modest changes in fish-eating in the Seattle area. In contrast, Mark Van Putten, of the National Wildlife Federation, testified that in the Great Lakes region it was difficult to convince risk managers that subsistence fishers, such as Native Americans, should be considered in risk assessments.

Using specific information gathered from the community and stakeholders could reduce the need for default assumptions and improve the quality of risk assessments in communities with multiple polluting operations, such as a municipal incinerator, a chemical plant, a dry-cleaning establishment, and an abandoned hazardous-waste site. Involving the community and other stakeholders in the planning stages of a risk assessment can help to engage individuals, families, schools, businesses, and municipalities in targeted pollution-prevention and pollution-reduction actions that reduce exposures. The Commission's framework for risk management (section 2), calls for stakeholders to be involved in every step of the process, including evaluation of the actions taken.

FINDING 3.2.3: Exposure assessments vary greatly in design and content. Complex risk-management decisions often are based on simplistic, deterministic estimates of exposure derived from few data, many assumptions, and inadequately validated models. In contrast, some exposure assessments are more complex than is needed for straightforward risk-management decisions.

RECOMMENDATION: Exposure assessments should be designed to be commensurate with the needs of the risk-management decisions at issue. The design of an appropriate exposure assessment should take place at the problem-definition stage of the risk-management process.

RATIONALE

Several measurement tools, statistical methods, and other procedures and considerations can be used to design and conduct an exposure assessment. No method or group of methods should be used in all cases. Selection of appropriate methods should be discussed and evaluated during the planning stages of a risk-management process (the problem/context stage of the Commission's risk-management framework) to ensure that they meet the needs and expectations of risk managers and other stakeholders. The following general principles are suggested as the basis of the planning of an exposure assessment.

  • Simple methods should be considered before more-complex methods. Such a tiered assessment strategy is increasingly used in risk assessment and can be cost-effective.

  • Chemicals are more biologically available in some media than in others; that is, the matrix within which chemicals occur (such as air, water, food, or soil) can greatly affect the extent of human exposure. The effect of the matrix should be considered in assessing exposure before assuming as a default that contaminants are 100% bioavailable.

  • Whenever possible, measurements should be obtained to support or validate any generic values used in exposure assessments, to check modeling results, or to provide more-realistic estimates of exposure than can be obtained with models. Such measurements might include collecting data at locations where exposures are anticipated, monitoring the exposures experienced by individuals, collecting data on the physical and chemical conditions that affect the movement and availability of chemicals, and providing information that relates exposure to effects, possibly using biologic markers. Measurements of exposure can be very different from estimated exposures based on source characteristics.