Pathways and Measuring Exposure to Toxic Substances

For toxic substances in the environment to exert adverse effects on humans, they must deposit on and/or penetrate through a body surface and reach target sites where they can alter normal functions and/or structures. The critical pathways and target sites can vary greatly from substance to substance and, for a given substance, can vary with its chemical and physical form. A further complication arises from the fact that chemical and/or metabolic transformations can take place between deposition on a body surface and the eventual arrival of a toxic substance or metabolite of that substance at a critical target site. A critical target site is where the toxic effect of first or greatest concern takes place. This chapter reviews and summarizes current knowledge concerning the generic aspects of the environmental pathways and processes leading to: (1) deposition of toxicants on body surfaces (skin, respiratory tract, and gastrointestinal tract); (2) uptake of toxicants by epithelial cells from environmental media (air, water, waste, and food); (3) translocation and clearance pathways within the body for toxicants that penetrate a surface epithelium; and (4) the influence of chemical and physical form of the toxicant on the metabolism and pathways of the chemical of concern. Where the physical attributes of the toxicant such as the length and biopersistence of airborne fibers are of generic concern, these are also discussed in this chapter. Other aspects of the pathways and the fates of toxicants that are specific to the chemical species that are the subject of the following chapters of this volume are discussed, as appropriate, in those chapters. This chapter also summarizes and discusses techniques for measuring personal and population exposures to environmental toxicants and their temporal and spatial distributions. Quantitative exposure assessment, as a component of risk assessment, involves consideration of: (1) the nature and properties of chemicals in environmental media; (2) the presence in environmental media of the specific chemicals that are expected to exert toxic effects; (3) the temporal and spatial distributions of the exposures of interest; and (4) the ways that ambient or workplace exposure measurements or models can be used to draw exposure inferences. In this context, the knowledge of deposition, fate, pathways, and rates of metabolism and transport within the body, to be reviewed later in this chapter, provide appropriate rationales for size-selective aerosol sampling approaches and/or usage of biomarkers of exposure. Finally, this chapter discusses the choices of sampling times, intervals, rates, durations, and schedules that are most appropriate for exposure measurements and/or modeling and are most relevant to risk assessment strategies that reflect data needs for: (1) documenting compliance with exposure standards; (2) performing epidemiological studies of exposure–response relationships; (3) developing improved exposure models; and (4) facilitating secondary uses of exposure data for epidemiological research, studies of the efficacy of exposure controls, and analyses of trends. Keywords: aerosols; air; air contaminants; airways; biomonitoring; databases; dosimetry; food; human exposure pathways; liquids; mass transfer; measurement modeling; particles; physical properties; retention; skin; solids; translocation; water