A quantitative definition of exposure and related concepts

This paper develops a unified theoretical framework for understanding exposure to environmental pollutants and other agents. It reviews the scientific literature to describe the many diverse and often confusing ways in which the term "exposure" is being used. Using six criteria proposed for a useful framework, a set of quantitative definitions, which encompass and expand upon existing definitions, is developed. After "agent" (e.g., a pollutant) and "target" (e.g., a person's hand) are defined, "exposure" is defined as the contact between an agent and a target. An "instantaneous point exposure" is defined as the joint occurrence of two events: 1) point i of a target is located at (xi, yi, zi) at time t, and 2) an agent of concentration Ci is present at location (xi, yi, zi) at time t. It is shown that the definition of instantaneous point exposure is fundamental in that all other functions of exposure with respect to space or time-such as the average exposure and the integrated exposure-can be derived from it. Because exposure and dose are closely related and often confused, our framework also includes a general definition of dose that is consistent with common usage. Finally, the definitions in this unified theoretical framework are shown to apply to inhalation exposure, dermal exposure, and ingestion exposure. In addition to the literature review and the quantitative definitions of exposure, this paper includes a glossary of terms that are proposed to help establish a common language for the exposure sciences.