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Framework for Assessing Exposures to Air Contaminants . INTRODUCTION This chapter presents the mathematical relationships of the components of human exposure to airborne contaminants. It also presents an overview of the methods and their applications for assessing exposure. Each of the methods presented in this overview is discussed in detail in the following chapters in the context of the techniques currently available and the critical needs to advance exposure assessment. Human expos~e to air contaminants is associated with a wide variety of health and nuisance effects. Acute biological effects encompass outcomes, such as aggravation of existing disease (e.g., increase in frequency and severity of asthmatic attacks), acute respiratory infections (e.g., increase in the res- piratory illness rates in children), transient deficits in lung function, and aller- genic reactions. Chronic health outcomes include long-term decrements in lung growth, chronic obstructive lung disease (e.g., bronchitis), cancer, neuro- behavioral alterations, and heart disease. The most common effects encoun- tered by large segments of the population, however, are nuisance effects. These often are acute and include noxious odors; eye, nose, and throat irrita- tion; and coughing, which is a symptomatic respiratory response. Air contaminants found in various industrial, occupational, residential, outdoor, and public access and transportation environments consist of a broad and complex spectrum of chemicals in gaseous and particle-associated forms, as well as particles of biological origin. Ideally, the air contaminants impli- cated in producing an adverse health or nuisance effect would be identified and an exposure assessment protocol would be designed. Frequently, the identification of the causes of an effect is confounded by air-quality factors other than traditional air contaminants, including temperature, humidity, noise, and lighting. In practice, however, exposures to a class of contaminants, 37

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decay. 38 ASSESSING HUMAN EXPOSURE source category, or a prosy contaminant often must be addressed when a specific contaminant cannot be identified or easily measured. The a~r-contaminant concentrations present In any environment are usually the result of several interrelated factors. In the indoor nonindustrial env~- ronment, these factors Include the following: Number and location of sources, type of sources, and generation rate of the contaminants. Source use characteristics. Building or vehicle characteristics. Infiltration and ventilation rates. Air mixing. Removal rates by surfaces, chemical transformation, or radioactive Existence and effectiveness of air-contaminant removal systems. Penetration of outdoor contaminants. Meteorological conditions. Activities of humans and their pets. In the outdoor environment, these factors include variables such as those below: Meteorological conditions. Atmospheric transport of contaminants. Atmospheric chemical reactions and physical removal processes. Source types. Source emission rates and emission density. Location and activities of the individual. Air concentrations in the industrial environment are controlled by the same factors with the addition of material handling, local process exhaust systems, worker habits, and the use and effectiveness of personal protective equipment. Development of accurate models to predict air-contaminant levels requires information on the above factors. Questionnaires or environmental measure- ments used in assessing exposures should gather information on these factors as well. Such information also will aid in establishing effective risk-manage- ment mitigation measures to eliminate or reduce air-contaminant exposures.

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FR4MEWORK FOR ASSESSING EXPOSURES 39 MATHEMATICAL RELATIONSHIPS The mathematical relationship for air-contaminant exposure for a person can be represented by the following equation: AE = C At, (Eq. 2.1) where AE is the exposure of a person to the air-contaminant concentration (C) during a specific time period Alp. The units of exposure are concentra- tion multiplied by time (e.g., (llg/m ~ fur). This can be defined in the integral form as E = i6C(t)dt, tl (Eq. 2.2) where C(tJ represents the functional relationship of concentration with time for an interval to through t2. This time interval can be instantaneous as well as representing longer contact periods. An operational form of the above equation that delineates the exposures of an individual for different microenvironments in which that individual spends time is given by AEi,k = Cj'(~t3 ~t3~, (Eq. 2.3) where AEjk = the exposure of person k to a given pollutant during time interval At as a result of that person's activities in microenvironmentj; Cjk(~) = the average concentration to which person k is exposed during the time interval At while in microenvironment j; and Atjk = the time spent by person k ire microenvironment j. For acute health effects, t must have a short enough interval to ensure the average reflects the peak concentration that can cause an effect. This equation can be written to include exposures of multiple contaminants to various persons in diverse microenvironments:

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40 ASSESSING HUMAN EXPOSURE Eij, = ~ CijA{jI' i=1 (Eq. 2.4) where the kth person is exposed to a concentration Ci, of the ith chemical contaminant in the jth microenvironment. The concentration, C,j, is consider- ed to be constant for that location during the interval 6tjk The integrated exposure, ET, then can be calculated for many different situations. For the exposure of the kth person to the ith contaminant, the time-integrated expo- sure for the kth person is the sum of the individual exposures to the ith con- taminant over all of the possible microenvironments: T~ = ~ Cij~tj'. j=1 (Eq. 2.5) Alternatively, population exposure can be calculated for a group of persons in contact with the ith contaminant in a single microenvironment, such as for a person at a workstation: = K ~ Cij~tj,. k=1 (Eq. 2.6) A time-integrated population exposure to a single contaminant, as shown below, also could be defined for a population of persons for a series of micro- environments: = J ~ j~tjI. j=1 k=1 (Eq. 2.7) Total airborne exposure to all contaminants can be defined by summing this

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FR4AffEWORK FOR ASSESSING EXPOSURES 41 equation for all contaminants, i = 1 to I. Thus, a quantitative exposure as- sessment requires accounting of the time spent by each person in the presence of each different concentration of every different contaminant of biological d slgmi~cance. Because a human must be In an ad environment at all times, some re- searchers use the concept of ~t~me-weighted average erosive." The equation below is the same as that for integrated exposure, except that wok refers to the proportion of T (time-weighted average associated with exposure) that a person k spends in microenvironment j, with the ith contaminant: T' = ~ (At~ij,'. j=1 (Eq. 2.8) In the summation over exposures, At was not explicitly written, because the person k "brings" the time to the microenvironment. The microenvironment and the time interval are defined by the person being studied; an exposure occurs in a microenvironment only if person k is in that microenvironment at a specific time. Exposure to a contaminant is defined as contact at a boundary between a human and the environment at a specific contaminant concentration for a specific interval of time; it is measured in units of concentrations multiplied by time (or time interval). Exposure has, however, often been defined and used differently. For example, exposure is defined in the 1988 EPA Proposed Guidelines for Exposure Measureme'~fs (EPA, 1988b) as concentration multi- plied by contact rate multiplied by time.) In this mathematical relationship, volume and time cancel out leaving only the mass term, which is more appro- priately considered potential dose, assuming 100% bioavailability and absorp- tion, as defined in Chapter 1. Defining exposure as mass will lead to misinter- pretations of exposure by those who conduct exposure assessments, because understanding of concentrations and actual time periods of exposure is critical for analysis and mitigation of adverse exposures. The field of exposure assess- ment should use standard definitions and practices. The scientific and regula- tory communities, including those responsible for reviewing articles for scien 1The 1988 EPA Proposed Guidelines for Exposure Measurement are being modi- fied to incorporate new and improved approaches to understanding exposure.

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42 ASSESSING HUMAN EXPOSURE tific journals, should use consistently the definitions recommended in this document. MEASUREMENT AND ESTIMATION TECHNIQUES EMPLOYED IN EXPOSURE ASSESSMENT Exposure to individual air contaminants, categories of air contaminants, or sources of air contaminants can be assessed by using personal monitoring of the concentration in the breathing zone of individuals (that can be directly inhaled) arid In some cases by biological markers of exposure or by coupling models with measurement of air-contaminant concentrations in microenv~ron- ments. Biological markers and personal monitoring are called direct measures of exposure while microenvironment monitoring is known as an indirect meas- ure. A schematic of approaches for determining exposure is shown in Fig. 2.1. Direct Measures of Exposure Personal Monitoring Personal monitoring provides direct measurements of the concentrations of air contaminants in the breathing zone of an individual. Samplers worn by subjects directly record the concentration or collect time-integrated samples of specific contaminants with which individuals come into contact for specific intervals. For specific compounds, samplers can be used for several hours to several days, thus including all time-concentration patterns during a specific period. Samplers can be active or passive. Active samplers use small pumps either to draw air through a collection medium (e.g., filter or vapor trap) and collect the air contaminants or to draw air through a direct-reading detector. Passive gas samplers use diffusion or permeation to concentrate gases on a collection medium. They usually must be carried for several days to obtain contaminant masses greater than the detectable limit of the analysis method employed. The samples then are returned to a laboratory for analysis. Personal monitoring can be a useful measure of an individual's exposure to an air contaminant or class of contaminants and has been used extensively by industrial hygienists in occupational settings. When combined with biological markers, personal-exposure data can link air concentrations with internal dose. For example, air CO concentrations can be linked with blood carboxyhemo- globin levels, and air nicotine concentrations can be linked with blood, urinary, or salivary levels of cotinine.

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43 - .. ~ o C C .' o on x IU _ ~ o "C~ _ fi _~ e e _ ]] _ 119 ' . ~- 0 ~ 0 If L ~ ~ Li 1 .= o o . ~ . C! ~ } E Y C o E IL o Cot ._ U. Cot A: Cot V) en o Cot D ._

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44 ASSESSING HUMAN EXPOSURE Personal monitoring also provides a measure of the exposure across the various microenvironments where individuals spend their time. It usually does not supply information on the physical environment in which exposures occur or on the factors (e.g., emission rates) controlling concentrations In those environments. Hence, if effective mitigation measures are to be developed and instituted, measures of personal exposure need to be supplemented with measurements of the factors in the physical environment (e.g., temperature, humidity, and ventilation) that control the exposure, and information on hu- man activity from questionnaires. In large field studies, it is difficult to obtain sufficient numbers of subjects willing to carry the samplers; furthermore, distribution and retrieval of samplers is manpower intensive, time consuming, and expensive. Therefore, personal sampling has practical limitations in its usefulness for assessing duration, intensity of exposures, and variability of periodic exposures. One important application is the development of proto- cols for long-term sampling of individuals, which ultimately would assist in demonstrating environmental improvements and reductions in exposures by regulatory agencies. Several recent technological developments have made personal monitoring more widely useful. Passive samplers (e.g., badges) for air contaminants such as volatile organics (Lewis et al., 1985), formaldehyde (Geisling et al., 1982), nicotine (Hammond and Leaderer, 1987), nitrogen dioxide (Palmes et al., 1976), and other gases have been developed. These monitors provide the sensitivity and specificity necessary to conduct personal air-monitoring expo- sure assessments at reasonable cost. Recent advances also have been made in active personal monitors; for example, miniature denuder monitors for assessing personal exposures to acid particles and gases have recently become available (Koutrakis et al., 1989~. Advances in the use of electrochemical sensors for active personal monitoring have recently been reported for NO2 (Penrose et al., in press), CO (Penrose et al., 1990a) and ozone (Penrose et al., 1990b). Biological Markers Biological markers refer to cellular, biochemical, or molecular measures that are obtained from biological media such as human tissues, cells, or fluids and are indicative of exposure to environmental chemicals. A biological mark- er of exposure is an exogenous substance or its metabolite or the product of an interaction between an environmental contaminant [xenobiotic agent] and some target molecule or cell that is measured in a compartment within and organism (NRC, 1989~. They are measures of dose when appropriate meta

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FRAMEWORK FOR ASSESSING EXPOSURES 45 boric data are available and the relationships between times of exposure and sample collection are adequately defined. This information can be used as a surrogate for exposure by using pharmacokinetic or pharmacodynamic models. Biological markers include: (a) unchanged exogenous agents (e.g., heavy mama organic vapors' nicotine, asbestos fibers, and PCBs); (b) metabolized exogenous agents (e.g., phenol and cotinine); (c) endogenously produced molecules (e.g., alpha-1-antitrypsin and porphyrin ratio); (d) molecular changes (e.g., DNA adJucts and hydroxyproline); (e) cellular or tissue changes (e.g., cell histologr and sperm mobility); and (f) measurements of pulmonary response to an agent. 13;ologi~ markers can be used as indicators of e~o- sure (e.g., PCBs, cotinine, DNA albums, and carbo~hemogIob~n), disease susceptibility (e.g., alpha-1-antitrypsin), or disease state (e.g., cell histology and red blood cell counts). Some biological markers can indicate the integrated intake into the body of an air contaminant across all microenvironments and sources. They also can aid in elucidating relationships among exposure, dose, and health or nuisance effects. However, biological markers by themselves do not provide information on the microenvironment in which the exposures take place and hence on the factors that control exposure such as contaminant emission rates and fate. Without information on the microenvironment or the person's activities, effective mitigation measures cannot be developed and instituted. Use of biological markers alone as a measure of exposure is usual- ly insufficient for air-pollution epidemiology, intervention, risk assessment, or risk management. Biological markers of exposure have been used to study some environmen- tal contaminants, such as urinary cotinine for tobacco smoke (Wald et al., 1984~; carbo~hemoglobin levels in blood for exposure to CO (Redford and Drizd, 1982~; and lead levels in blood, teeth, and hair for inhalation and inges- tion of lead (Harlan et al, 1985~; and benzoapyrene-DNA adducts (Perera et al., 1988~. Biological markers can be used to measure a specific contaminant, or they may be proxies for a number of contaminants. The usefulness of a biological marker in a specific study depends on many factors: potential applications, type of testing (e.g., animal testing and human clinical testing), properties of the marker (e.g., specificity, sensitivity, metabolic characteristics, and invasiveness of sample collection), laboratory protocols (e.g., collection' handling, and cost), and experimental design issues (e.g., sample size and confounding variables). Use of biological markers as measures of exposure is often limited for the following reasons: Difficulty in obtaining physiological samples from subjects. Poorly understood relationships among biological marker concentrations

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46 ASSESSING HUMAN EXPOSURE and air concentration of a contaminant, specific sources of contaminants, duration of contact, and the adverse effect under study (e.g., relation between urinary cotinine concentrations and exposure to respirable suspended particu- late matter or vapor-phase organics from environmental tobacco smoke (ETS)~. Cost and available resources. Person-to-person variability. indirect Measures of Exposure Indirect methods of assessing an individual's or population's exposure to air contaminants combine microenvironmental monitoring or modeling with questionnaires or with other information on human activities. Indirect meth- ods supply information on contaminant concentrations in microenvironments and the physical and chemical processes that control those concentrations. Models that predict spatial and temporal concentration distributions of air contaminants in various microenvironments are an important component of an overall human exposure model. These models take three general forms: physical/chemical, empirical/statistical, and hybrid. Indirect methods gen- erally provide exposure information at a lower cost than direct approaches. However, indirect approaches do not link air concentrations with internal contaminant dose or metabolites. In addition, the models generally used in exposure assessment have large uncertainties associated with their estimates, and few have been validated. Questionnaires in exposure assessment have been used extensively in the past and will continue to be relied upon heavily in the future. Microenvironmental Measurements Microenvironmental measurements involve monitoring air-contaminant concentrations in the locations where exposures take place. Often, the physi- cal and chemical factors that control air-contaminant concentrations in those microenvironments are measured, although this is not always necessary to determine exposure. Monitoring studies can use long-term sampling at one location or spot or grab samples in several locations. A wide range of active and passive samplers with excellent specificity and sensitivity is available to assess the spatial distribution of the contaminants and the frequency distribu- tion of peak and average concentrations. Monitoring physical and chemical processes (e.g., meteorological conditions, concentrations of precursor contam

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FRAMEWORK FOR ASSESSING EXPOSURES 47 inants in outdoor air, ventilation and deposition rates In indoor air, and hood ventilation rates in industrial settings) that control the spatial and temporal contaminant concentration distribution is critical to develop exposure models for contaminants As with any type of continuous or integrated environmental monitoring for individual; contaminants, ~ s-amplir~g protocol me be woefully designed within the framework of the use of the data gathered and resources available. Care must be taken to specify what samples should be taken, where they should be taken, how many should be taken, and with what frequency. Questionnaires Different types or classes of questionnaires are used as indirect measures of exposure: meet. Those that provide information on the physical properties of an environ Those that provide a simple categorization of potential exposure. Those that obtain information on the activity patterns of individuals. The first category provides information on the existence of sources, source use, and other characteristics of each microenvironment in a community or occupational setting. When combined with the results of fixed-site air moni- toring of the environment, this information permits a model to be developed to eslim ate air-contaminant levels in similar environments. This type of ques- tionnaire is used extensively in characterizing sources, source use, and building factors in studies of residential air quality and of buildings with occupancy complaints (sometimes referred to as Sick-building syndromes. Efforts are under way to standardize a questionnaire for residential indoor air-quality studies (Lebowitz et al., 1989a). Recently a similar effort has been made for investigations of buildings with occupancy complaints. The development of such questionnaires and their use in field studies might make possible inter- comparison of data acquired from different studies. Categorization of exposure has been used extensively in epidemiological studies of environmental air contaminants for many years. For example, several epidemiological studies of environmental tobacco smoke and cancer determined exposure only by asking the subjects or household members whether they ever were exposed to environmental tobacco smoke. In occupa- tional epidemiology, exposure categories often are determined by a worker's job classification. Categorical estimates of exposure are crude; however, if

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48 ASSESSING HUMAN EXPOSURE carefully designed, questionnaires can be an inexpensive way to obtain general information on exposure categories for large populations. The third category of questionnaire activity patterns- is desired to take into account that an individual spends varying amounts of time conducting specific activities at various locations in the course of a day. To accurately model or estimate total exposures, microenvironments and time spent in them must be identified as a function of air contaminant concentrations. A recent review of adult time-activity pattern studies (Ott, 1988) found that people spend 65-70% of their time inside homes and more than 90% of their time indoors (home, transit, and work combined). Demographic and personal (e.g., smoking history) variables can affect time-activity patterns. These studies clearly highlight the need to consider concentrations in the indoor environ- ment when assessing exposures. Of course, for some air contaminants, such as ozone, knowledge of time spent outdoors is essential to assessing exposure. Recognition of the importance of assessing the indoor environment has result- ed in studies that focus on the spatial and temporal aspects of people's activi- ties and the concentrations of contaminants present in specific environments. These efforts are essential in the modeling of total exposure to air contami- nants, but present significant challenges in study design. Models In many situations, it is either impossible or impractical to measure directly the exposures of individuals or populations (e.g., predicting exposure for pro- spective industrial processes or sources or retrospectively estimating exposures in an epidemiological study). In such cases, models are used to estimate exposures. Models present a conceptual framework or a mathematical formu- lation of individual or population exposures based upon scientific principles. Exposure models generally include synthesized microenvironmental concentra- tions (measured or modeled) and estimates of the time spent in various mi- croenvironments. However, the term "model" also can be used to refer to the conceptual fra;ncwork that provides the basis for formulating a mathematical model or for planning exposure measurements. When sufficient measurements cannot be made, microenvironmental con- centrations are estimated using concentration models. These models are based upon the physics and chemistry of the environment. Concentration models have been developed for emissions from sources, atmospheric disper- sion, ventilation, infiltration, transport, deposition, and atmospheric chemistry. Stochastic models describe the transport of contaminants by examining the

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FRAMEWORK FOR ASSESSING EXPOSURES 49 motion of large numbers of small parcels of air moving due to advection and diffusion (Sexton and Ryan, 1988~. Statistical/empirical models attempt to relate either the measured exposure or the concentration to variables associated with emissions, transformation, and accumulation in specific microenvironments. The results are used for hypothesis generation. Statistical/empincal models often use questionnaire responses as independent variables, e.g., source use or house volume to pre- dict concentration or to identify the major factors controlling concentration or exposure. The time spent in a microenvironment with a concentration is another important input to an exposure model. Time spent by individuals in various environments is typically obtained through observation, diaries, or activity logs. Time-activity patterns measured for a statistical sample of a population can be used to generate distributions of time-activity patterns for larger popula- tions. These distributions are then combined in an exposure model with concentrations to yield population-exposure estimates. Population-exposure models are a recent advance in exposure modeling and there has been very limited world on their validation. The principal advantage of models is their ability to estimate concentrations in different microenvironments or exposures of individuals or populations with little direct information, which may be difficult to obtain. The processes of formulating, testing, and refining models contribute to the fundamental under- standing of exposure; such understanding is critical for designing exposure- assessment studies. Models provide a conceptual and scientific framework for considering factors that control exposure and afford a means for designing and testing cost-effective mitigation measures. Model outputs are, however, only as good as the degree to which factors are identified and specified and depend on the quality of the input data. The assumptions employed in any model need to be clearly specified, and their applicability to a given exposure assessment needs to be considered. Furthermore, it is essential to test and validate models before they are used in exposure and risk assessments to better characterize the uncertainty in the model output. Mitigation Measures The choice of mitigation measures to be applied and the success of those measures in reducing or eliminating exposures to reduce health or nuisance effects are dependent on the sensitivity and accuracy of the model employed. As many of the relevant factors are accurate!,' incorporated into an exposure model, the risk-management effort should become more efficient and cost

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50 ASSESSING HUMAN EXPOSURE effective. The choice of measures to be employed can range from source removal (the most effective) to source substitution, source-emission reduc- tions, altered ventilation, air filtration, and human-behavior modification. Air- quality standards directed at controlling concentrations of a contaminant In different air environments, e.g., ambient air standards, also are employecl, but are of no value unless followed by effective control strategies. Formulation and institution of a successful mitigation strategy depend strongly on the accuracy of the exposure model employed. INTEGRATION OF EXPOSURE-ASSESSMENT TECHl!lIQUES Studies to assess exposures to environmental contaminants, whether to complement environmental epidemiology, disease diagnosis and intervention, risk assessment, or risk management, must consider the three principal meth- ods of exposure assessment: personal monitoring, biological markers, and indirect estimates. These studies should incorporate into their study design several methods (as many as practical) to accurately define exposure and estimate dose. Such studies need to determine the physical and chemical factors in the environment responsible for environmental concentrations, the multimedia routes of exposure and the number of microenvironments in which exposures take place so that cost-effective mitigation measures to reduce exposure can be identified and evaluated. Toward the development of consis- tent exposure-assessment practices, the use of personal monitoring as well as microenvironmental monitoring should be considered in long-term studies that examine or determine changes in population exposures to airborne contami- nants. Exposure-assessment studies need to explore the use of nested designs (Leaderer et al., 1986, 1990a). A nested exposure-assessment strategy refers to obtaining an easily measured indicator of exposure (e.g., questionnaire) for all or a large segment of the study population, while simultaneously obtaining ever-increasing detail on the exposure measures for ever-decreasing numbers by using personal monitoring, monitoring of microenvironments, biological markers, etc. The former will provide data with a higher degree of uncer- tainty, while progressive application of the latter should incrementally reduce the uncertainty in the analysis of exposure. These different types of studies can be conducted simultaneously or in separate investigations for specific contaminants or classes of contaminants. The detailed measures of exposure can then be used to model the exposure of the entire population. Easily obtained measures of exposure can be incorporated or developed to validate the models. Such exposure-assessment designs typically use all three methods

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FRAMEWORK FOR ASSESSING EXPOSURES 51 to a varying degree, based upon the applicability and practicality of available exposure-assessment techniques. These efforts will require a fundamental change in simple approaches that have been used to assess exposure to air- borne contaminants and will improve the quality of assessments by providing information on the extent and major sources of uncertainty in exposure assessments as part of the portrayal of e~sure-assessment results. SUMMARY Exposure to a contaminant occurs when there is contact at a boundary between a human and the environment with a contaminant of a specific concentration for an interval of time; the units of exposure are concentration multiplied by time. The occurrence of the concentration of a pollutant, a person s exposure, and a dose to target organs and tissues are different points in a continuum between emission of a contaminant and any resultant health effects. Integrated air exposure is calculated from individual exposures by summing over time (time-integrated exposure), over persons (population-integrated exposure), or over airborne contaminants (contaminant-integrated exposure). The field of exposure assessment should use standard definitions and prac- tices. The scientific and regulatory communities, including those responsible for reviewing articles for scientific journals, should use consistently the def~ni- tions recommended in this document. Toward the development of consistent exposure-assessment practices, the use of personal monitoring as well as microenvironmental monitoring should be considered in long-term studies that examine or determine changes in population exposures to airborne contaminants.

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