measurable perchlorate. Thus, it is likely that everyone has some exposure to perchlorate, depending on how much of their water intake comes from the community’s water supply. Individual variation in water exposure undoubtedly still occurs, however, because of the use of wellwater or bottled water or because of nonuniform distributions of contaminants in a geographic area. Some degree of individual variability undoubtedly exists, but it may not be as great as for other types of exposures that are not part of a communitywide, common-source exposure.

Another limitation of ecologic studies is that their design cannot control for many confounding factors, because such data are not usually available at the population level. Results of ecologic studies can be useful in providing supporting data on a possible causal relation, but they cannot themselves provide direct evidence of causation.

The available pertinent occupational and epidemiologic studies are summarized in Table 3-1 and discussed in the following sections.

STUDIES IN OCCUPATIONAL COHORTS AND ADULTS

An early study of an occupational cohort by Rockette and Arena (1983) reported mortality patterns for 59 selected causes of death among workers at the Niagara Plant of Hooker Chemical. It included people with at least 1 year of employment from January 1, 1949, to December 31, 1978. The cohort consisted of 3,963 workers (3,715 men and 248 women) at the plant in Niagara Falls, New York. Only 13 deaths were recorded among women, and detailed analyses of the female data were not conducted because the sample was so small. The following results are related to men.

Results showed statistically significant excess mortality from stomach cancer (standardized mortality ratio [SMR], 178.9; p < 0.05) and respiratory cancers (SMR, 145.9; p < 0.01). In the respiratory-cancer category, the SMR was significantly increased for cancer of the lung (SMR, 142.4; p < 0.01). A review of work areas and production indicated that exposure to magnesium perchlorate occurred in 1970-1976 with simultaneous exposure to dozens of other chemicals at the plant. Of the work areas in the plant, 12 departments were identified as having exposure to “groups of chemicals.” The department referred to as “area 4” included exposure to magnesium perchlorate and 23 other chemicals. Job titles in the department were not provided, so specific jobs were not assessed with regard to exposure to magnesium perchlorate. A comparison of department-specific



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