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An exposure assessment is a comprehensive evaluation of how an individual or a population comes into contact with one or more potentially hazardous agents or stressors of concern. The agent or stressor can be any biological, chemical, or physical entity that can cause or induce an adverse response and will hereafter be referred to as an “agent.” The exposure is thus the concentration or amount of the agent that comes into contact with a host over a relevant time scale. When defining an exposure, it is important to define three dimensions of exposure: (1) the exposure level (i.e., the concentration or intensity); (2) the duration of the exposure (i.e., how long someone is exposed to the agent); and (3) the frequency of exposure (i.e., how often the exposure occurs over some time scale of interest). It is common to think of “exposure” only in terms of the exposure level or concentration, but in order to appropriately characterize an agent’s potential health effects, all three dimensions must be considered over an appropriate time scale. The appropriate time scale depends on the pathogenesis and natural history of the health outcome of concern. For example, for an acute response (e.g., allergic response such as dermal irritation or asthma exacerbation), the time scale of exposure may be short (minutes to days), but for chronic health effects (e.g., cancer), the time scale may be much longer (years to decades). Additionally, for some agents, the timing of when exposures occur should be considered. Exposures during critical windows of development, including during the preconception, prenatal, and childhood or adolescence periods may merit particular attention as these could result in long-lasting health impacts. For example, exposures to organophosphate (OP) pesticides during the prenatal period, have been linked to poorer cognition and behavior problems in children. Gestational age and dose are important determinants for potential adverse effects in the offspring.1

To appropriately characterize total exposure, it is necessary to consider all possible routes of exposure: inhalation, direct (e.g., contaminated foods, breastmilk, beverages) and indirect (e.g., hand-to-mouth activity) ingestion, skin or mucosal absorption, injection, and the transplacental route (in the case of a fetus). It is common to consider these routes separately, due to different sources of the agent or different activities undertaken by an individual that result in exposure to a given agent. A complete estimation of exposure would include all routes (i.e., aggregate exposure); however, methods of exposure assessment are most advanced for inhalation exposures and will be the focus of much of this chapter.

Domains of Exposure

Once the dimensions and possible routes of exposure have been determined, an exposure assessment can be planned. Exposures can be assessed in two domains: ecological or population level, or personal, that is, for an individual. Each approach has strengths and weaknesses, depending on the context and the purpose. Ecological estimates involve exposure assessment that is done ...

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