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Although the toxic effects of metals, such as arsenic, cadmium, lead, and mercury, have been well established for higher level exposures, the demonstration of population-wide harmful effects at lower concentrations has been more difficult. One possible reason for this is that at comparatively lower levels of exposure, only some people are at risk. Whitfield et al. (p. 776) hypothesized that studies of the causes of individual variation could improve the power of epidemiologic studies to detect harm to susceptible people. These investigators examined associations of personal and socioeconomic characteristics with concentrations of arsenic, cadmium, copper, mercury, lead, selenium, and zinc in erythrocytes, and tested for genetic effects using data from twin pairs. The authors conclude that genetic variation plays a significant role in determining concentrations of the measured elements. They also concluded that, although environmental exposure is a precondition for accumulation of toxic elements, individual characteristics and genetic factors are also important.

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