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until it makes contact with humans in its original form or after conversion to other substances. For most airborne substances, inhalation is assumed to be the primary route of entry into the body. There has recently been an extensive review of advances in assessing human exposure to airborne constituents (NRC, 1991a). That review attempted to define exposure carefully as a part of the overall continuum that leads to illness brought about by environmental contaminants. The definition of exposure as a part of this continuum has been incorporated into the 1992 revised guidelines for exposure assessment developed by EPA (1992a).

Human exposure to a contaminant is an event consisting of contact with a specific contaminant concentration at a boundary between a human and the environment (e.g., skin or lung) for a specified interval; total exposure is determined by the integrated product of concentration and time. The amount of a substance that is absorbed or deposited in the body of an exposed person in a given period is the administered dose. Calculating the dose from the exposure depends on a number of factors, including the mode of entry into the body. For substances that move into the body through an opening—such as the mouth or nose via breathing, eating, or drinking—the dose depends on the amount of the carrier medium that enters the body. For airborne substances, the potential dose is the product of breathing rate (volume of air inhaled per unit of time), exposure concentration, and fractional deposition of the substance throughout the respiratory tract. However, an inhalation exposure will not lead to a dose if none of the substance is absorbed through the lung or deposited on the surface of the lung or other sections of the respiratory tract.

A pollutant can also enter the body through the skin or other exposed tissues, such as the eyes. The substance is then directly absorbed from the carrier medium into the tissue, often at a rate that is different from the rate of absorption of the carrier. The pollutant uptake rate is the amount of the pollutant absorbed per unit of time, and the dose is the product of exposure concentration and uptake rate at that concentration. The NRC report on exposure assessment (NRC, 1991a) provides a scientific framework to identify routes of entry and degree of contact and indicates how exposure assessment integrates data on emitted pollutants with biological effects.

Exposure assessment involves numerous techniques to identify a pollutant, pollutant sources, environmental media of exposure, transport through each medium, chemical and physical transformations, routes of entry to the body, intensity and frequency of contact, and spatial and temporal concentration patterns of the pollutant. Mathematical models that can be used to describe the relationships among emissions, exposures, and doses are shown in Appendix C.

Exposure to a contaminant can be estimated in three ways. It can be evaluated directly by having a person wear a device that measures the concentration of a pollutant when it comes into contact with the body. Environmental monitoring is an indirect method of determining exposure, in which a chemical's concentration is measured in an environmental medium at a particular site, and the extent