general population, not to assess health risks associated with the measurements. As will be discussed in later chapters, such studies are laudable in that they contribute to our knowledge about human exposure to environmental chemicals. But the limitations of that type of information are not necessarily recognized by all who need to know and understand them. The committee considered that clarification of the properties of the various biomarkers of exposure would be useful in helping to understand and clarify what can be said about a given measurement. A systematic framework to characterize the properties of biomarkers would help to inform scientists and the general population about biomarkers and their meaning when they are used in biomonitoring studies. It would also allow assessment of potential research gaps that need to be addressed to meet the requirements of specific uses of biomarkers. Detailed information about the interpretation of biomonitoring data is provided in Chapter 5. The present chapter focuses on the properties that characterize biomarkers of exposure in general. These properties are based on a weight-of-evidence approach that takes into account the specific context of a biomonitoring study under consideration.

TYPES OF BIOMARKERS

Chapter 1 describes the relationship between exposure to a toxic chemical and its clinically relevant health effects as a series of steps along a continuum. There often is no clear-cut distinction between some of the steps, and Figure 1-1 can help position three types of biomarkers: biomark-ers of exposure, of effect, and of susceptibility.

As the name implies, biomarkers of exposure allow assessment of exposure to a chemical on the basis of its measurement in a biologic matrix (NRC 1991). Typical examples are the measurement of dioxins in blood or blood lipids, mercury in hair, benzene in exhaled breath, and cadmium in urine. In itself, quantification of such a biomarker in a biologic matrix proves only that the chemical is in the organism. If the substance is not otherwise known to be endogenous, it can be concluded that there has been a transfer from the external environment to the individual organism. Any further interpretation of the concentration of a biomarker of exposure requires additional information about some of the relationships in the continuum. If the biomarker of exposure indicates that a chemical reached a critical target in the organism— for example, if it formed a DNA adduct—there is a greater likelihood of a potential link with a biological perturbation. As discussed in Chapter 1, this report focuses on biomarkers of exposure.

Biomarkers of effect are used to assess changes that have occurred in the biochemical or physiologic makeup of an individual. The further to the right in the continuum the biomarker is, the greater the clinical or health



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