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INTRODUCTION 65 4 Introduction Exposure to a variety of air contaminants has been shown to produce adverse health and discomfort responses in humans. In another report from the National Academy of Sciences (NRC, 1985), the methodological issues of studying exposures to air pollutants and subsequent health effects are discussed in detail. This part of the report considers issues relevant to assessing exposure to ETS. Ideally, evidence for health effects in humans should be demonstrated in epidemiologic studies that are consistent with a plausible hypothesis across a range of exposures or doses. However, many epidemiologic studies have substantial uncertainties associated with exposure variables. A framework for assessing exposures to environmental tobacco smoke (ETS) is discussed below. A variety of approaches to current and historic exposures to ETS, such as personal monitoring, locational monitoring, questionnaires, and biologic monitoring, are presented. Concentrations of air contaminants exhibit pronounced spatial and temporal variations, regardless of the microenvironments in which they are found (outdoors, residential, industrial, etc.). Ideally, identifying the air contaminant or class of contaminants implicated in producing adverse health or comfort effects is essential in designing an air-monitoring program. In practice, however, it is often necessary to monitor a class of contaminants (for instance, total mass of respirable particles) or a proxy contaminant (for instance, nicotine), when the specific air contaminant producing the adverse impact can not be identified or easily measured. The air contaminants associated with ETS are comprised of a
INTRODUCTION 66 broad range of many vapor- and particle-phase inorganic and organic chemicals noted in Chapter 2, some of which can undergo pronounced physicochemical changes. Assessing impact on human health and comfort requires the identification of proxy air contaminants for ETS that will permit a determination of exposure in a background of contaminants from other sources (see Chapter 5). In epidemiologic studies of air contaminants, it is important to specify exposure to specific particulates or gases on a time scale corresponding to the health or comfort effect sought. The impact of exposure to an air contaminant should, ideally, be evaluated in terms of the biologic dose of the contaminant or its metabolites received by the target tissue. In most cases, this is not practical. The uptake, distribution, metabolism, and site and mode of action of the contaminant in humans is neither well understood nor easily measured. Moreover, dose cannot be directly assessed. Factors affecting the uptake of air contaminants include physical characteristics of the contaminant, as well as physiological characteristics and activity levels of the exposed person (see Chapter 7). In the absence of an ability to measure or specify the dose of a contaminant received, exposures to air contaminant(s) are assessed by either using biological markers, measured in the subject population, or by measuring the air- contaminant concentrations in the physical environment (Figure 4â1). Exposures to airborne contaminants can be assessed by three basic approaches (Figure 4â1): â¢ personal air-contaminant monitoring, â¢ modeling, based on air sampling, time-activity patterns, and questionnaires, and â¢ biological markers. Personal monitoring employs samplers (worn by subjects) that record the integrated concentration individuals are exposed to in the course of their normal activity for time periods of several hours to several days (see Chapter 5). The modeling approach employs the use of stationary monitors to measure the air-contaminant concentrations in a number of microenvironments. These measured concentrations are combined with time activity patterns (time budgets) to determine the average exposure of an individual as the sum of the concentrations in each environment weighed by the time spent in that environment.
INTRODUCTION 67 Questionnaires are employed in two capacities: (1) to provide information on the physical properties of each environment, including source use parameters, in order to model the concentration of air contaminants in the microenvironment, thus permitting a prediction of air-contaminant concentrations in spaces not monitored; and (2) to provide a simple categorization of exposure levels, such as exposed versus unexposed or none versus low versus high. FIGURE 4â1 Flow diagram of components for assessing human exposures to air contaminants from environmental tobacco smoke. Questionnaires have been used to categorize subjects' exposure to ETS in all studies of risk of chronic lung disease reported to date. Chapter 6 discusses the use of questionnaires to categorize ETS exposures. Chapter 7 reviews assumptions required to estimate exposure-dose relationships for ETS and gives an approximation to the dose received under a specific situation.
INTRODUCTION 68 Chapter 8 examines the use of biological markers, such as urinary cotinine, as indices of exposure to ETS. There are several factors (Figure 4â1) that determine the composition and level of ETS air contaminants in the indoor environments. Determining the range of values for each of these factors will lead to an understanding of their impact on ETS exposures. Efforts to modify or eliminate exposures to ETS must focus on the factors that control the concentrations in the physical environment, since these factors result in the exposure that relates to the adverse health or comfort effect. REFERENCE National Research Council, Committee on the Epidemiology of Air Pollutants. Epidemiology and Air Pollution. Washington, D.C.: National Academy Press, 1985. 224 pp.