Environmental Epidemiology, Volume 2 Use of the Gray Literature and Other Data in Environmental Epidemiology (1997) / Chapter Skim
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3 Exposure Assessment in Environmental Epidemiology
3 Exposure Assessment in Environmental Epidemiology
Pages 26-67
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Exposure assessment for purposes of environmental epidemiology may differ from exposure assessment for site remediation, mitigation, control, and risk assessment. The differences are sometimes subtle but may have substantial impact on the conduct of studies and associated allocation of resources.
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described some limitations and problems in the quantitative estimation of exposure when the focus of a study is possible adverse consequences of chemical exposure from hazardous-waste sites. This chapter reviews some aspects of exposure assessment or analysis that are important in environmental epidemiology and that illustrate the central role of exposure assessment.
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This chapter is not intended to repeat the contents of those documents, but will focus on certain specific issues in exposure assessment for use in environmental epidemiology. PRINCIPAL CONCEPTS THAT UNDERLIE THE CONTENT OF THIS CHAPTER It is relevant to state the assumptions that the committee used as the basis and context for this chapter.
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and EPA (1992) have also developed definitions of potential dose, applied dose, internal dose, and biologically effective dose for purposes of exposure assessment.
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; that include mathematical modeling, questionnaire/diaries, or spatial factors, e.g., residence in a country or region or distance from a source of chemical contamination (figure 3-2~.
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Validation of the specific applicability of indirect monitoring is an important requirement for the successful use of this method. Some important epidemiologic studies have emphasized indirect measures of exposure as the primary linkage to health outcome.
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32 ENVIRONMENTAL EPIDEMIOLOGY TABLE 3-1 Parameters Required to Calculate Potential and Internal Dose Airborne contaminant Water contaminant I Concentrations (,ug/m3,ppb)
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Waste dumps F Water supply G
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Diaries tonicity raise questions about causality." Therefore, a key purpose of exposure assessment is often to support evaluation of dose-response relations. DESCRIPTIVE EPIDEMIOLOGIC STUDIES Exposure data for descriptive epidemiologic studies must fairly apply to the population from which the disease is arising.
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Analytic methods to cope with these problems are discussed in chapter 6. ANALYTIC EPIDEMIOLOGIC STUDIES The types of analytic studies used in environmental epidemiology were summarized in chapter 2.
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Concentrations of pollutants in food supply. Concentrations of pollutants in drinking water supply.
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Ground Water Supply Survey,4 Community Water Supply Survey,e tap water. Population location and activities relative to monitoring locations; fate of pollutants over distance between monitoring and point of exposure; time variation of pollutant concentration at point of exposure.
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38 TABLE 3-2 Continued ENVIRONMENTAL EPIDEMIOLOGY Type of measurement (sample) Usually attempts to characterize (whole)
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Special studies of indoor air, house dust, contaminated surfaces, radon measurements, office building studies. Soil sampling at hazardous waste sites.
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Urine Total internal dose for individuals or population (usually indicative of relatively recent exposures)
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Same as above; relationship between urine content and body burden.
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ISSUES IN EXPOSURE ASSESSMENT RISK ASSESSMENT AND ENVIRONMENTAL EPIDEMIOLOGY Better measures of exposure are clearly desirable in epidemiology. Exposure data exist on a continuum ranging from measures of emission, to measures of ambient concentration, to microenvironmental measures weighted by time spent in each environment, to personal monitoring, to measures of internal dose and biomarkers.
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Small errors in exposure assignment may have dramatic results on estimation of effect. Because of the limited scope of exposure assessments in most environmental epidemiology, misclassification is likely to be a substantial problem.
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Landrigan (1983) has commented on the advantages of improved exposure characterization by using individual versus grouped data in a study of the health effects of arsenic in drinking water.
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(1991) have developed a location and activity log for assessing personal exposure to air pollutants and conducted pilot studies to validate the approach.
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has conducted an epidemiologic investigation to identify the chronic effects of ambient air pollutants in southern California. The design for this 10-year study emphasizes exposure assessment.
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Carefully designed studies have provided a wealth of important information about relations between ambient exposures and adverse health consequences. A brief review of some of these studies illustrates the importance of microenvironmental monitoring and indirect measures of exposure characterization.
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and the report Human Exposure Assessmentfor Airborne Pollutants (NRC, l991b, pp.
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reviewed the exposure assessments conducted in environmental-epidemiologic investigations of hazardous wastes. The report concluded that repositories of potentially dangerous substances can be found at many hazardous-waste sites but that future risks to public health could be determined only with more-detailed information about human exposure.
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However, for the most part, epidemiologic studies of the environment must address the issue of complex mixtures.
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However, there has been little effort in environmental epidemiology to relate the levels and effects of exposure to biologic mechanisms. This may be because information on biologic mechanisms is limited or absent.
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53 00 0 - .go' L o ~ ~ ~ ~ ~ ~ r)
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Checkoway and Rice (1992) considered cumulative exposure to silica and concluded that "disentanglement of peak and cumulative exposure effects should be accomplished most effectively in investigations of relatively short-term sequelae of substances with short retention times in the body." SUBJECTIVE SYMPTOMS AND EXPOSURE ASSESSMENT An important issue in epidemiologic studies of hazardous-waste sites is the relation between positive findings as evidenced by increased subjective symptoms in a defined population and the general lack of exposure assessment that would link the findings to specific exposures.
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(1991) examined the results of 5 epidemiologic studies of symptom rates observed around hazardous-waste sites.
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. Three types of biologic markers may be used to provide information on exposure (Hulka and Wilcosky, 1988; Schulte 1989; Stevens et al., 1991~: markers of internal dose, such as blood lead; markers of biologically effective dose, such as blood DNA and protein adducts; and markers of biologic effects, such as chromosomal micronuclei.
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evaluate the relation between exogenous exposure and internal or biologically effective dose. Biologic markers may also be useful to identify the effect of an intervention.
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Markers of the biologically effective dose require additional research to assess the role of host factors, particularly genetic susceptibilities, as effect modifiers. DOSIMETRIC MODELING More attention is being devoted to characterizing the quantity and timing of toxic chemical agents' reaching target tissue, because concentrations measured by microenvironmental monitoring or even personal dosimetry may not accurately reflect target-tissue dose.
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These tenets result in a 2-phase approach to epidemiologic investigation: first, is the development of a mathematical model to estimate individual doses; and second, the use of epidemiologic models to estimate the risk of disease associated with these estimated doses, with appropriate control for confounding. The advantage of this approach for environmental epidemiology is that the dosimetric model can quantify a hypothesis about uptake processes and metabolism of the chemicals in question and may provide insight into the biologic mechanisms of effect.
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Given the costs, resource requirements, and political sensitivity of many environmental-epidemiologic studies, the failure to provide training for environmental assessment will need to be addressed by policymakers and educators if we are to have substantial improvement in environmental epidemiology and risk assessment. CONCLUSIONS Exposure assessment is important in all environmental-epidemiologic studies.
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The exposure-dose relation should be examined for nonlinearity before cumulative estimates are calculated. The relation between cumulative exposure and peak exposure is unknown in many environmental-epidemiologic studies, particularly those involving hazardous-waste site exposures or community exposures to episodic pollution.
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Because of the difficulties of conducting epidemiologic studies, both descriptive and analytic, it is rare for any one study to be definitive, and this is especially true in environmental epidemiology. Every public presentation, written or oral, including reports to scientific colleagues, should contain prominent caveats about overinterpretation.
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1987. Chronic obstructive pulmonary disease symptom effects of long-term cumulative exposure to ambient levels of total suspended particulates and sulfur dioxide in California Seventh-Day Adventist residents.
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1988. Chronic obstructive pulmonary disease symptom effects of long-term cumulative exposure to ambient levels of total oxidants and nitrogen dioxide in California Seventh-Day Adventist residents.
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1991. Epidemiologic approaches for assessing health risks from complex mixtures in indoor air.
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1991. National Human Exposure Assessment Survey.
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Pregnancy outcomes in women potentially exposed to solvent-contaminated drinking water in San Jose, California.
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