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Epidemiological and toxicological methods produce similar assessments
Michael Gough Resources for the Future Wahington, DC 20036 One of the major justifications for environmental regulation is concern about the carcinogenic potential of such factors as air and water pollutants and pesticide residues on foodstuffs. During the 1970% it was common to hear claims that the “environment” was responsible for 80-90% of all cancers. “Environment,” however, has two quite different meanings. As used by epidemiologists, it refers to everything that humans encounter: everything that is eaten, drunk, and smoked; drugs, medicines, and occupational exposures; and air, water, and soil. In this context, it means everything outside the body as distinct from a person’s genetics. Thus, when an epidemiologist says that 80001393BW89/0923w25$01.SO10
90% of cancer is associated with the environment, he or she means, primarily, that factors other than a person’s own genetics are almost always involved in cancer causation. The sources of those factors are not pinpointed. In contrast, “environment” is more commonly and colloquially taken to mean air, water, and soil in the sense conveyed by the name Environmental Protection Agency. Many people hear environment used in that sense, and interpret the claim concerning cancer causation to mean that 80-9056 of cancer is caused by exposures to substances in air, water, and soil. In 1981 Doll and Pet0 ( I ) clarified the two uses of the word with a comprehensive study based on an analysis of US. Mtional cancer mortality records from 1933 to 1978. In addition to concluding that age-adjusted cancer mortality rates
0 1989 American Chemical Society
had remained almost constant during that period, they attributed only about 2% of total cancer mortality to environmental exposures (or “pollution,” as they referred to it) and another 3% to “geophysical factors,’’ which include sunlight and other kinds of “natural” radiation. The Doll-F‘eto report had a much larger impact than those of other scientists (2, 3) who had come to similar conclusions about the role of pollutants in cancer causation. It was more detailed and comprehensive than the other studies and it contained more data, analysis, and documentation because Doll and Pet0 computerized national cancer mortality records. These records enabled Doll and Pet0 to evaluate information on cancers at a, sites and in all age groups. Additionally, the distinguished reputations of Sir Richard Doll (epidemiology) and Richard Pet0 Environ. Sci. Technol.. Vol. 23, No. 8. 1989 925
such an approach would compare with that of Doll and Peto. EPA recently completed a landmark study, Unfinished Business: A Comparative Assessmenr of Environmental Problems (7,8), which examines the risks presented by 31 different environmental problems (Table 1). For many of those 31 problem areas, EPA relied on toxicologybased risk assessments. In the balance of this article, I compare those risks with the risks estimated by Doll and Pet0 from epidemiologic data and point out how similar the findings are.
(statistics) no doubt added credence to their findings. And finally, the DollPet0 report was published soon after the Reagan administration came to Washington, and its conclusion that the environment contributed very little to cancer risk fit into the antiregulatory bent of the new administration. The Doll-Pet0 estimates, which have come to be regarded as conventional wisdom concerning environmental carcinogenesis (4. 3, were based exclusively on epidemiological data. Some of their estimates have come into unqualified use. It is possible, nevertheless, to use other methods. For instance, cancer risk assessments can be based on knowledge of which chemicals have been shown to cause cancer in animals, estimates of human exposure to those chemicals, and the use of extrapolation procedures that relate exposures to human risks. In this article, those methods are referred to as “toxicology-based risk assessments.” Doll and Pet0 dismissed these types of as926 Envimn. Sci. Technol., Vol. 23, No. 8. 1989
sessments because they believed that extrapolation from animal studies to humans is so uncertain that the estimates have little value. Other scientists, however, place more faith in toxicology-based risk assessment, and a recent paper supports the idea that there are quantitative similarities between cancer potencies in humans and laboratory animals. Allen et al. (6) found 23 substances for which data were sufficient to examine cancer potencies in both humans and animals, and they used several methods to investigate the possible correlations between the potencies. They concluded that the “correlations were highly statistically significant . . .” and that “These findings provide support for the general use of animal data to evaluate carcinogenic potency in humans and also for the use of animal data to quantify human risk.” The common use of toxicology-based risk assessments in public policy makes it important to know how an estimate of environmental carcinogenesis based on
Doll and PetoS estimates Doll and Pet0 (1) divided all exposures to carcinogens into 12 distinct categories (Table 2). TWO of those categories, “pollution” and “geophysical factors,” encompass the exposures that are subject to regulation by EPA and that are discussed in Unfinished Business (7, 8). TWO more of Doll and Peto’s exposure categories, “occupation” and “industrial products” (the latter would be called “consumer pmducts” in the United States), also are discussed in Unfinished Business. Although their estimates were precise in the sense that specific percentages of cancer were associated with each of the exposure categories they considered, Doll and Peto pointed out the uncertainties inherent in the data and methods they used. Furthermore, because of differences in the quality and quantity of data available, Doll and Pet0 relied on somewhat different methods to estimate the cancer mortality associated with each of their exposure categories. The methods they used for the four categories discussed in this Paper are “mm ‘ dbelow: Pollution. Doll and Pet0 considered air and water pollution and pesticide residues on food separately. For air pollution, they referred to others’ work that compared the concentrations of known carcinogenic substances in urban air with concentrations of the same materials that had been associated with cancer in workplace atmospheres. Assuming a direct relationship between concentrations and cancer risk, Doll and Pet0 estimated that urban air pollution might cause 1% of cancer mortality (4ooo deaths annually in 1981). Arsenic and asbestos in outside air were estimated to make “minute” contributions to lung cancer rates, and radiation from industrial activities was associated with an estimated 590 cancer deaths a Year. Doll and Pet0 also considered the possible depletion of stratospheric ozone by chlorofluorocarbons (CFCs) or other chemicals and the possible increase in skin cancer rates that may result from an increased flux of ultravi-
olet (UV) radiation reaching the earth. They estimated that chemical attack on the ozone layer was associated with
