of effect is a measurable biochemical, physiologic, or other alteration within an organism that, depending on magnitude, can be recognized as an established or potential health impairment or disease. A biologic marker of susceptibility is an indicator of an inherent or acquired limitation of an organism's ability to respond to the challenge of exposure to a specific xenobiotic substance (NRC,1987).

Biologic markers have been discussed extensively in the scientific literature in the past ten years but rarely with regard to hazardous-waste research (Perera and Weinstein 1982; Fowle, 1984; CEQ, 1985; Underhill and Radford, 1986; Harris et al., 1987; Perera, 1987a,b; Hatch and Stein, 1987; NRC, 1987; Schulte, 1987, 1989; Hulka and Wilcosky, 1988; Hulka et al., 1990).

Biologic markers are not new. Markers such as blood lead, urinary phenol levels in benzene exposure, and liver function assays after solvent exposure have long been used in occupational and public health research and practice to indicate recent exposures to these compounds. What distinguishes the current generation of research on markers from previous markers is the greater degree of analytical sensitivity available to detect markers and the ability these markers offer researchers to describe events that occur all along the continuum between exposure and clinical disease. There are domains of biologic response and levels of resolution that were unknown 20 years ago (Schulte, 1990). For instance, within the past few years more than 400 proteins have been identified on sperm. In theory, chemical adducts to these can form and they have already been detected in protamine, hemoglobin, and other vital human proteins (NRC, 1987).

Accompanying these advances in sensitivity is the requirement to consider that numerous factors can influence the appearance of biological markers. All people with similar exposures do not develop disease or markers indicative of exposure or disease. Various acquired and hereditary host factors are responsible for this variation in responses.

Biologic markers may represent signals in a continuum or progression of events between a causal environmental exposure and resultant disease (NRC, 1987). Current technological advances and developments in basic sciences allow for detection of smaller signals at diverse points in the continuum. These markers are generally biochemical, molecular, genetic, immunologic, or physiologic signals of an event. The current method for estimating risks by relating exposure to clinical disease (morbidity and mortality) can now be supplemented by a fuller method, one that identifies intervening relationships more precisely or with greater detail than in the past. As a result, health events are less likely to be viewed as binary phenom-

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