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ENVIRONMENTAL EPIDEMIOLOGY: Volume 1
are among the most commonly used, presumably because of their nonspecificity and ease of analysis. Sometimes these effects are transitory, as was shown in a study of persons with increases in the liver enzyme alkaline phosphatase who had been exposed to chlorinated chemicals in domestic water in Hardeman County, Tennessee.
Other multiphasic tests to find markers of exposure or effect in blood and urine also have been used, but to a lesser extent. For example, serum cholesterol, gamma-glutamyl transpeptidase (an indicator of enzyme induction in the liver), and blood pressure have been studied as markers of effect in residents of Triana, Alabama, who were exposed to PCBs chiefly from subsistence eating of contaminated fish. Eighty to 90 percent of the population in Triana had levels of PCB within the range found in other community groups. For those with elevated levels, results indicated that PCB was positively associated with measures of blood pressure and other indicators, independent of age, sex, body mass, and social class. Similar findings have been reported in studies of workers exposed to PCBs in capacitor manufacturing.
Researchers also have studied markers of neurologic function in persons from Woburn, Massachusetts, some six years after exposure to TCE ceased, and in others with similar exposure. In Woburn, TCE levels in domestic water had been from 30 to 80 times higher than the recommended EPA Maximum Contamination Level of 5 parts per billion (ppb). Exposed and control subjects were studied with a neurobehavioral evaluation protocol that included clinical tests, nerve conduction studies, blink reflex measurements, and extensive neuropsychological testing. The highly significant differences in a variety of neuropsychologic tests indicate that neurotoxic effects occurred in those who had been exposed.
Although they are not commonly thought of as constituting markers, the results of neurobehavioral tests can provide a diverse range of measures of toxic exposures and effects. A battery of neurobehavioral tests has been applied to the study of persons exposed to materials that occur at hazardous-waste sites. A comprehensive review of developing techniques in neurobehavioral assessment found consistent and significant neurobehavioral effects and a range of other subtle neurological alterations in persons exposed to metals, solvents, and insecticides, with some indication of greater effects in those with higher estimated exposures. These findings corroborate studies that reveal that TCE inhalation induces a range of neurotoxic effects in rodents.
Although the risk of cancer provides a central focus for much research on markers, risks to human reproduction offer another focal point for which much shorter time periods between exposure and