Human biomarkers serve as indicators of actual or potential events in biologic systems or specimen. There are three types of biomarkers: biomarkers of exposure, biomarkers of effect, and biomarkers of susceptibility. Biomarkers of chemical exposure are measured as the unchanged, parent chemical substance, its metabolite, or a product of its interaction with a target within the body. The ability to detect and measure the parent substance, or a substance-specific metabolite, in an easily accessed tissue, fluid, gas, or excretion product at a sub-toxic concentration makes an ideal biomarker. Biomarkers of exposure to physical and biological agents (also called stressors) also exist. These types of biomarkers are measured as the agent in its initial form of contact or exposure, or as direct indicators of its effect on, or in, the body. It must be noted that the use of “agent” in this context is not restricted to radiologic, biologic, or chemical weapons of military application, but rather, the term is used in the more general occupational and environmental context that applies to all physical and biologic entities, as well as, chemical substances. Biomarkers of exposure can be affected by several conditions. The method for detecting and measuring the biomarker must be sufficiently sensitive and specific. Ideally, a biomarker should be measurable at a level below which there are no significant or permanent untoward effects. The measured biomarker may not be specific to exposure to one specific agent, or exposure to only one source of the agent. The period of time in which the parent or initial agent, or its directly-related metabolite or effect, can be detected in the body determines when specimens for these indicators must be obtained; a biomarker with a short half-life may be eliminated before realization that an exposure had occurred. In order for a biomarker of exposure or effect to be a valuable tool in medical surveillance, the...
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