In summary, immunologic markers of exposure can be used in several ways to distinguish exposed from nonexposed individuals and to distinguish among those with different levels of exposure within exposed groups. The use of immunologic markers should be considered in the context of other information, such as exposure history or environmental or breathing-zone measurements. The use of biologic markers may need consideration during the design of a study, during implementation, and during the interpretation of results. However, before any study, the investigator should ask what benefit biologic markers might provide over exposure assessment by ambient personal or environmental monitoring. If this question cannot be answered in a convincing way, biologic markers should not be used in environmental-epidemiologic research.

Susceptible Populations

The frequency of many diseases is likely to increase in certain population groups. For example, an increasingly large proportion of the population is over age 65 (Cooper et al., 1991). Older persons have an increased vulnerability to many stresses. Whether and to what extent an elderly person is more susceptible to the toxic effects of potentially hazardous compounds from the environment is largely unknown. Elderly individuals may be at increased risk from toxicant exposure because of age-related changes in the body's protective mechanisms (NRCb, 1989). Older persons have had a long period of exposure to chemicals, more time for latent adverse effects to manifest themselves, and more time for cumulative effects to emerge.

The role of aging in environmental toxicity has been a subject of extensive research. In aged populations, 2 classes of adverse effects may exist: those caused by aging alone and those caused by interactions between aging and toxic agents (Williams et al., 1987). It may not be possible to discern with confidence the health decrements from these. A problem, then, is the identification of experimental approaches that will give insight into the relation between age and toxicity (NRC, 1989b).

A major impediment to understanding the role of environmental agents in the aging process is the paucity of reliable and valid biomarkers of aging per se (Reff and Schneider, 1982; Ingram, 1988; McClearn, 1988). Harrison (1988) suggested 4 criteria for determining whether a particular physiologic assay may be useful as a biomarker of aging in an individual organism: the results should change significantly with age, the changes should be repeatable in the same individual, assays of independent physiologic parameters should give similar estimates of age for the same individual, and the degree of aging as determined by the assays should correlate with subsequent longevity.

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