Biomarkers of exposure can be used as part of an exposure assessment to assess potential health risks when exposure measurements and health effects occur during the same period of time. Exposure assessment is the determination of the concentration of a chemical in an environmental medium coupled with the presence of the human in that environment. A biomarker of exposure gives an assessment of absorbed dose in the worker. A biomarker of effect, often referred to as a biomarker, is used in the assessment of health risk. A biomarker of effect gives an assessment of the effect of a chemical on a physiological process and is an indicator of a possible adverse health effect. For example, biomarkers of exposure give an assessment of a worker's current exposure to solvents and some metals. Biomarkers of exposure for chronic health effects with long latent periods, such as carcinogens, are more problematical. These exposure indicators, however, can be used to help reconstruct past exposures. Selected biomarkers of exposure can also be predictors of adverse health effects. Examples include the use of 2,5-hexanedione and 2-ethoxyacetic acid in urine as indicators of exposure to n-hexane and 2-ethoxyethanol, respectively. These two urinary metabolites are 'active' metabolites responsible for observed neurotoxic and reproductive effects. Biomarkers of exposure are particularly useful for the objective assessment of current systemic exposure to chemicals that are readily absorbed through the skin. Exposure assessments, including the use of biomarkers of exposure, are necessary to postulate a cause of observed adverse health effects seen in epidemiological investigations and to determine the bioavailability of the chemicals present in the workplace.