Drinking Water and Health, Volume 6 (1986) / Chapter Skim
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7. Data on Humans: Clinical and Epidemiological Studies
7. Data on Humans: Clinical and Epidemiological Studies
Pages 226-249
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From page 226... ...
Thus, the data on humans generally fall into one of two categories: clinical data, which describe the effects of specific agents on certain individuals, and epidemiological data, which reveal patterns of disease or death in groups of humans exposed to single agents or to a variety of substances. CLINICAL OBSERVATIONS: CASE REPORTS Much of the knowledge concerning the human health effects of toxic materials comes from clinical information amassed after exposure to high doses, such as poisonings and industrial accidents.
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For several of these materials, the evidence for carcinogenicity was found first in test animals and later confirmed in humans (Davis and Mandula, 19851. When there is both delayed onset of effects and a lack of distinctive clinical characteristics, risk assessments must be based on data from analytical epidemiological studies, clinical or experimental case reports, and extrapolation from laboratory experiments.
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EPI DEM IOLOG ICAL OBSERVATIONS Most epidemiological studies conducted to date have focused on the evaluation of accidental or workplace exposures. There have been relatively few studies of humans exposed to contaminants in drinking water.
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Among the major contaminants found in the river water were the trihalomethanes. Reviewing 14 epidemiological studies relating cancer mortality to drinking water containing trihalomethanes, Williamson (1981)
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(see Figures 7- 1 through 7-31. Analytical Epidemiology Follow-up epidemiological studies are generally grouped under He heading analytical epidemiology, although all epidemiological studies include anal
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studies. Recognizing that descriptive epidemiological studies do not provide proof of cause-and-effect relationships, Hill (1953)
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. Case-Comparison and Cohort Studies Epidemiological studies can attempt to discover the etiology of a disease by starting from either the exposure or the biological effect.
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The elimination or reduction can be part of a well-designed randomized trial as in the MRFIT (Multiple Risk Factor Intervention Trial) studies of the National Heart, Lung, and Blood Institute (Tillotson and Hulley, 1985)
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In reviewing the possibilities for conducting epidemiological studies on di(2-ethylhexyl) phthalate, the National Institute for Occupational Safety and Health found similar conditions and did not undertake such a study (Roberts, 19831.
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As power increases, the chances of producing a false-negative error decrease. Power is determined jointly by the level of relative risk being assessed, i.e., the magnitude of the effect, and by the number of cases that would be expected in the exposed pop
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The finding of an association between hepatic angiosarcoma and exposures of vinyl chloride polymerization workers illustrates that even small epidemiological studies (in essence, clinical observations of a few people) may have sufficient power to detect a very large relative risk (Creech and Johnson 1974; Heath et al., 19751.
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However, if the relative effect of drinking water is rather small, then even this large study will have a low power of detecting any such effect. Study-ReIated Limitations Additional limitations associated with epidemiological studies derive from the intrinsic characteristics of diseases under study.
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Epidemiological studies are susceptible to certain biases, which must be minimized. Of particular importance to case-comparison studies is response bias or recall bias.
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Moreover, epidemiological studies necessarily include people who have had multiple toxic exposures. But scientists' understanding of the ways multiple toxic exposures affect health is very limited.
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Young women exposed to ionizing radiation at Hiroshima were at higher risk of developing breast cancer than women who were older at the time of exposure (Land et al., 19801. Nature of Populations Studied Many useful epidemiological studies are conducted on industrial populations for which measurement of exposure can sometimes be obtained.
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RISK ASSESSMENT Relative Risk Epidemiological studies are capable of generating several types of data that might be used to assess risk and to develop public health policy. When there is reason to believe that the exposure-response relationship multiplies the background rates, the increase should be reported as relative risk, as with most studies of cancer.
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For example, epidemiological studies indicated that adverse health effects developed in humans exposed to foods or water supplies contaminated with lead, cadmium, and mercury (Calabrese, 19831. Although drinking water constitutes a small percentage of total daily exposure to many substances, it may play a determinative role nonetheless.
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References Sperm count reductions in workers exposed during agricultural application of DBCP. Sperm count reduction, elevated serum follicle- stimulating hormone (FSH)
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Bergner et al., 1965 Robson et al., 1969 Klemmer et al., 1980 Wyllie et al., 1975 Takahashi et al., 1976 Zober et al., 1981 van Bao et al., 1974 This is illustrated by the studies of Calabrese and Tuthill (1977) , who found that blood pressure measurements differed significantly in matched groups of students living in two communities with different levels of sodium in the drinking water.
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Since people who previously consumed large amounts of salt are now towering their intake, additional monitoring of their blood pressure levels and sodium intake, including that from drinking water, could provide useful information regarding the effects of elevated levels of sodium in drinking water on human health. Table 7-3 summarizes the epidemiological studies on the pollutants assessed in Chapter 9.
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1977. Elevated blood pressure and high sodium levels in the public drinking water: Preliminary results of a study of high school students.
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1981. A survey of availability of epidemiologic data on humans exposed to animal carcinogens.
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1980. Mortality experience of 161 employees exposed to ethylene dibromide in two production units.
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1981. Epidemiological studies on cancer and organic compounds in U.S.
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