Environmental Epidemiology, Volume 1 Public Health and Hazardous Wastes (1991) / Chapter Skim
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1. Introduction
1. Introduction
Pages 25-59
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, that have become more common in industrial societies than before. Questions have come to be raised about the possible relationship of industrial waste and other aspects of modern life to chronic diseases.
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convened the Committee on Environmental Epidemiology to review current knowledge of the human health effects caused by exposure to hazardous-waste sites and to suggest how to improve the scientific bases for evaluating the effects of environmental pollution on public health, including specifically the conduct of health assessments at Superfund sites. With additional support from the Environmental Protection Agency (EPA)
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The first section of this chapter defines environmental epidemiology. The second section discusses conventional views of statistical significance and principles for inferring causation based on epidemiologic evidence.
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In using this focal point, the committee has not restricted itself to sites officially listed under various state and federal laws, but has undertaken a broad review of available evidence on the human health effects that could be linked to exposures from materials at sites where disposal of hazardous wastes has taken place. The committee's members acknowledge that the published literature regarding toxic chemical waste disposal sites is limited and uneven and that profound methodological and practical problems attend the field, as others have noted (Grisham, 1986~.
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For instance, persons living near most hazardous-waste sites have in common a measured or estimated exposure to toxic substances in the area. Researchers cannot both verify this exposure and expect people to remain near the sites and continue to be exposed.
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Accordingly, the committee must rely on a combination of evidence from different sources to reach any conclusion in accordance with its mandate to estimate health effects associated with hazardous wastes. Figure 1-1 illustrates the types of information on which the committee has relied.
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:-~'' -- ':~1 Knowledge on health risks from similar exposures in other circumstances ~~'~.~ ~ ~~:~ ~:~ :~ 1 Studies that reveal symptomatology or disease in those exposed to hazardous waste sites similar to the health risks seen in other circum~t~nc~n : :~ ::~ ~ .....
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Consequently, the committee does not adhere strictly to conventional approaches to establishing causality only after a finding of statistical significance has been made. Before detailing the committee's reasons for relying on an inferential approach in developing an understanding of causation in environmental epidemiology, it is useful to consider the function and limits of statistically significant findings in studies of the health effects of hazardous wastes.
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A better indication of the statistically plausible range of values can be provided by identifying the estimated confidence interval, that is, the range within which there is a 90 percent chance that the true value is contained. The confidence interval brackets the interval or range of values that may occur and provides a clearer indication of the significance of a study than does strict application of p values and other measures of statistical significance.
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A fundamental dilemma for epidemiologic research on hazardous-waste sites, or any other topic involving multiple causes and results, derives from the fact that the statistical correlation of variables does not necessarily indicate any causal relationship among them, even where tests of statistical significance may be met. Mere coincident occurrence of variables says nothing about their essential connection.
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Thus, evidence linking lung cancer and cigarette smoking is strong; active smokers have a tenfold or greater risk of contracting lung cancer than non-smokers do. In contrast, evidence linking lung cancer and passive smoking is less firmly established; a variety of studies (NRC, 1986a)
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However, to the extent that multiple causes are involved, as they are with most chronic diseases, larger populations are generally required in order to obtain significant results in studies of more common diseases as well. Refining the measures of diseases and the assessment of exposure can improve the power of a study to detect an association.
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Finally, for many hazardous-waste sites, there usually are no data on relevant exposures that could have occurred several decades earlier, given the long and indefinite period for development of many forms of cancer. This and the other considerations described in this section explain why the observed strength of an association between pathology and exposure to hazardous wastes can be weak, even though the link may be causal.
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Thus, vinyl chloride emissions have also been tied to clusters of birth defects, but the study lacked sufficient power and the findings were not significant (Rosenman et al., 1989~. In general, hazardous wastes have been linked in toxicological studies to a wide range of diseases, some of which have long latencies and many of which have multiple causes.
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Assessing the consistency of an association in the arena of hazardous wastes is also hampered by the diversity of exposures. In principle, studies can be conducted in several communities where there have been varying levels of exposure after those exposures are reasonably well defined.
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reported that the peak in low birth weight in children born to women who were residents of Love Canal, New York, occurred during the time of greatest estimated exposure to contaminants at that site. Biologic Gradient or Relationship Between Estimated Exposure and Disease In general, the greater the exposure, the stronger the effect.
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For studies of hazardous-waste sites where common diseases with multiple causes are found, the finding of a dose-response relationship can be obscured by the operation of multiple causal factors and the absence of valid estimates of exposure. As Neutra (1990)
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The underlying mechanisms for many modern diseases are not readily apparent. Thus, the precise mechanisms by which cigarette smoking induces lung cancer are unknown, although the causal relationship is clear (Doll and Peto, 1978~.
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Because courts have sealed disputes on these matters that have been resolved, some potentially relevant information is not routinely available to the scientific community regarding the health effects associated with exposures from hazardous-waste sites. As a number of legal theorists have noted, the requirements for inferring causation in law and for inferring them in science differ in several important ways (Henderson, 1990~.
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Because the basic mechanisms of most modern chronic diseases are not well understood, analysts are forced to interpret observational data to find clues about etiology. Despite the immense public interest in the effect of hazardous wastes on public health, rather few empirical data are available.
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The precise number of these sites is unknown, although estimates go as high as the tens of thousands—an issue discussed in more detail in Chapter 2. Reauthorizing amendments in 1986 further strengthened the provisions of the Superfund law to address the issue of assessing health effects of persons exposed to hazardous wastes.
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As its charter indicates, ATSDR can rely on a broad spectrum of evidence in conducting health assessments at a hazardous-waste site. This spectrum encompasses experimental models of chemical structure and activity patterns, in vitro test systems, whole-animal longterm and short-term studies, and clinical studies and epidemiologic investigations of potentially exposed persons.
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OVERVIEW OF THIS VOLUME Because the basic mechanisms of many modern chronic diseases are not well understood, analysts must interpret observational data to find clues about etiology and also must rely on experimental observations. Table 1-1 summarizes published studies that the Committee reviews for this report on the health effects linked with exposures from hazardous-waste disposal sites.
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1987. Toxic Wastes and Race in the United States: A National Report on the Racial and Socio-Economic Characteristics of Communities with Hazardous Waste Sites.
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1985. Low birth weight, prematurity and birth defects in children living near the hazardous waste site, Love Canal.
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1986. Residential health study of families living near the Drake Chemical Superfund site in Lock Haven, Pennsylvania.
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1987. Growth of children living near the hazardous waste site, Love Canal.
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Environmental Health Criteria 27. Ceneva: World Health Organization.
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