Estimating Separately Personal Exposure to Ambient and Nonambient Particulate Matter for Epidemiology and Risk Assessment: Why and How

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1 Journal of the Air & Waste Management Association ISSN: (Print) (Online) Journal homepage: Estimating Separately Personal Exposure to Ambient and Nonambient Particulate Matter for Epidemiology and Risk Assessment: Why and How William E. Wilson, David T. Mage & Lester D. Grant To cite this article: William E. Wilson, David T. Mage & Lester D. Grant (2000) Estimating Separately Personal Exposure to Ambient and Nonambient Particulate Matter for Epidemiology and Risk Assessment: Why and How, Journal of the Air & Waste Management Association, 50:7, , DOI: / To link to this article: Published online: 27 Dec Submit your article to this journal Article views: 416 View related articles Citing articles: 85 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 10 December 2017, At: 00:14

2 TECHNICAL PAPER Wilson, Mage, and Grant ISSN J. Air & Waste Manage. Assoc. 50: Copyright 2000 Air & Waste Management Association Estimating Separately Personal Exposure to Ambient and Nonambient Particulate Matter for Epidemiology and Risk Assessment: Why and How William E. Wilson, David T. Mage, and Lester D. Grant U.S. Environmental Protection Agency, Research Triangle Park, North Carolina ABSTRACT This paper discusses the legal and scientific reasons for separating personal exposure to PM into ambient and nonambient components. It then demonstrates by several examples how well-established models and data typically obtained in exposure field studies can be used to estimate both individual and community average exposure to ambient-generated PM (ambient PM outdoors plus ambient PM that has infiltrated indoors), indoor-gener- IMPLICATIONS Exposure analysts historically have sought to determine the total personal exposure to PM of all types in all environments. The lack of correlation between this parameter and ambient PM concentration has been considered an impediment to epidemiologic studies seeking to find an association between ambient PM concentrations and health outcomes. For community, time-series epidemiology, it is necessary only that the community average personal exposure to ambient-generated PM be correlated with the ambient PM concentration. If everyone spent the same amount of time outside and in each microenvironment each day, and the air exchange rate and any forced-air ventilation that resulted in particle removal was a constant, and PM concentrations were uniform across the community, a high correlation would be expected between the PM concentration measured by a community-based PM monitor and the personal exposure of each individual to ambient-generated PM. Also, a high correlation would be expected between ambient concentration and the exposure surrogate of interest in epidemiology, the community average personal exposure to ambient-generated PM. For short-term panel studies, time-series should be determined for all classes of PM. For cohort studies of long-term effects, consideration must be given to the influence of possible variations in exposures to nonambient-generated PM because of differences among cities in time-location patterns (fractions of time spent outdoors), average air exchange rates, and average concentrations of indoor-generated and personal activity PM. ated PM, and personal activity PM. Ambient concentrations are not highly correlated with personal exposure to nonambient PM or total PM but are highly correlated with personal exposure to ambient-generated PM. Therefore, ambient concentrations may be used in epidemiology as an appropriate surrogate for personal exposure to ambient-generated PM. Suggestions are offered as to how exposure to ambient-generated PM may be obtained and used in epidemiology and risk assessment. INTRODUCTION Epidemiologic studies have found statistical associations between concentrations of ambient PM and mortality, morbidity, exacerbation of preexisting illness, and physiologic changes. 1 However, there is controversy regarding the appropriate measure of PM exposure. 2,3 Personal exposure to PM usually is thought of as the exposure to all PM in a microenvironment summed over the various microenvironments in which people spend time. Thus, personal exposure equals the time-weighted average of exposure indoors and outdoors, including time spent commuting and at work. A personal cloud or personal activity term accounts for the difference between personal exposure measured by a monitor carried by a person and the time-weighted average of concentrations measured in the various microenvironments while the subject is in them. However, for some purposes it is more useful to describe personal exposure to PM in terms of exposures to various classes of PM, summed over the various classes of PM to which a person can be exposed. PM can be divided in many ways. This paper discusses the division of PM into ambient and nonambient PM. (A subsequent paper 4 will discuss the disaggregation of PM by size, composition, and source category.) This division is useful because of the differences in sources, composition, toxicity, temporal variability, and person-to-person variability between ambient and nonambient PM. A key problem in Volume 50 July 2000 Journal of the Air & Waste Management Association 1167

3 this division will be to separate indoor PM into PM that is generated from indoor sources and ambient PM that infiltrates the indoor environment. Several techniques for estimating exposure to ambient and nonambient PM and the usefulness of this division for epidemiology and risk assessment will be discussed. The U.S. Environmental Protection Agency s (EPA) mandate from the U.S. Congress creates a need for disaggregation of PM into ambient and nonambient categories. The EPA is required by Congress to study the health effects of both indoor-generated pollution and ambientgenerated pollution. In the case of indoor-generated pollution, EPA has no regulatory authority but is required to educate the public regarding the hazards. 5 In the case of ambient-generated pollution, EPA is required to protect the public with a margin of safety. In the case of certain widespread pollutants with multiple sources (the so-called criteria pollutants [e.g., PM, O 3, NO 2, CO, SO 2, and Pb]), EPA is required to set National Ambient Air Quality Standards (NAAQS). 6 Therefore, although not neglecting interactive, additive, or synergistic effects of indoor-generated with ambient-generated pollutants, EPA must understand and treat each separately. There are also sound scientific reasons for separating exposures to ambient and nonambient PM. They follow different temporal patterns. Concentrations of ambient PM are driven by meteorology and by changes in the emission rates and locations of emission sources, while concentrations of nonambient PM are driven by the daily activities of people. Because of the difficulty in separating indoor PM into ambient and nonambient PM, there is little direct experimental information on the composition differences between the two. There are certainly similarities; however, there are also important differences. Some inferences can be made based on the knowledge of the composition of PM from ambient and nonambient sources. 7-9 Photochemistry is significantly reduced indoors; therefore, most secondary sulfate [H 2 SO 4, NH 4 HSO 4, and (NH 4 ) 2 SO 4 ] and nitrate (NH 4 NO 3 ) found indoors come from ambient sources. Primary organic emissions from incomplete combustion may be similar, regardless of the source. However, atmospheric reactions of polyaromatic hydrocarbons and other organic compounds produce highly oxygenated and nitrated products, so these species are also of ambient origin. Gasoline, diesel fuel, and vehicle lubricating oil all contain naturally present metals or metal additives. Coal and heavy fuel oil also contain more metals and nonmetals, such as selenium and arsenic, than do materials such as wood or kerosene burned inside homes. Environmental tobacco smoke (ETS), however, with its many toxic components, is primarily an indoor-generated pollutant. Both indoor and outdoor sources contribute biogenic PM (mold spores; insect debris; endotoxins; and ragweed, grass, and tree pollen, etc.) that may aggravate allergy or asthma. However, there are differences in the type and amount of biogenic PM from each source. Thus, ambient-generated and indoor-generated PM may produce different types of health effects. However, even if ambient and nonambient PM were identical, it would be desirable to know exposure to each separately in order to design cost-effective methods for reducing exposure. CHANGING VIEWS OF AMBIENT AND INDOOR PM The emphasis on ambient and indoor PM has varied as the understanding of human exposure to PM has changed over the years. The 1969 PM Air Quality Criteria Document (AQCD) prepared by the National Air Pollution Control Administration 10 expressed the prevailing opinions that the indoor environment was protective, that an ambient concentration measurement could serve as a surrogate for community exposure, and that ambient total suspended particulate matter (TSP) was an acceptable indicator. The 1982 PM AQCD 11 concluded that ambient PM 10 was a better indicator of the toxic fraction than was ambient TSP. It also recognized that particles readily penetrated into buildings (fine particles more so than coarse particles), that indoor activity and smoking add to indoor particle concentrations, and that particles generated indoors could lead to greater variations in exposures than would be indicated by community monitors. In 1991, a National Academy of Sciences (NAS) report 12 emphasized the importance of total exposure and formalized the concept of personal exposure as the sum of exposures in various environments and by various routes of exposure. The opinion expressed in the NAS report, that the indoors was polluted by indoor sources, helped lead to the idea that the correct index of PM exposure was total personal exposure (i.e., the sum of exposure to all types of PM in all environments). THE EXPOSURE PARADOX A paradox confronted the authors of the 1996 PM AQCD. 13 Evidence had accumulated that personal exposure to total PM was uncorrelated with ambient PM concentration. The 1996 PM AQCD 13 reported results of 21 cross-sectional studies (studies involving many subjects but only one or two subjects measured on the same day) of the correlation between ambient concentration and exposure to total PM. Of these 21 studies, 11 reported r < 0.2, and only two reported r > 0.6. Wallace 2 also reports small crosssectional correlations. Nevertheless, statistical associations were found between health outcomes and ambient PM concentrations at concentrations below the then-current PM standards. A different emphasis on PM exposure emerged from efforts to resolve this paradox Journal of the Air & Waste Management Association Volume 50 July 2000

4 The 1996 PM AQCD 13 began to make a distinction between exposure to ambient-generated PM and exposure to total PM (i.e., ambient plus nonambient PM). For example, it states, For the morbidity/mortality studies... that use SAM [concentration from a stationary ambient monitor] as the independent variable, that SAM can be interpreted to stand as a surrogate for the average community exposure to PM from sources that influence the SAM data (Volume I, p 7-119). If exposures to ambient-generated PM were highly correlated with ambient PM concentrations, a correlation of ambient PM concentration with community health effects would be more plausible. A firmer position was expressed by Wilson and Suh. 14 In a section of their paper on indoor/outdoor relationships (p 1245), they state (specifically for community, time-series epidemiology, [i.e., studies in which daily ambient concentrations of a pollutant are related statistically to daily health outcomes]): The indoor-outdoor ratios of total PM mass, however, are not the relevant parameters for community epidemiologic studies because, as would be expected, the emissions of PM from indoor sources are poorly correlated with the PM measured at a community monitoring site.... The relevant epidemiologic parameter is the concentration of the ambient (or outdoor) particles that have penetrated into the indoor microenvironment and remained suspended.... The average personal exposure to ambient particles will be the [time] weighted sum of the outdoor exposure to ambient particles and of the indoor exposure to ambient particles that have infiltrated indoors.... From the point of view of exposure assessment for epidemiology studies, the important question is the independence of variations in concentrations of particles generated indoors from particles generated outdoors. If indoor-generated PM concentrations are statistically independent of outdoor-generated PM concentrations, then variations in health endpoints or health effects indicators associated with variations in ambient PM cannot be confounded by indoor-generated PM.... Addressing these issues experimentally and statistically will require indoor-outdoor-personal exposure studies with separate measurements of fine and coarse particles that can differentiate indoor-generated particles from outdoor-generated particles. EPA expressed a similar point of view in response to public comments on the 1996 proposed rule on the NAAQS for PM in which the exposure paradox issue was raised 15 (Appendix D, p 3): It is true that personal exposure to total PM (PM from indoor sources and outdoor sources) is not well characterized by outdoor concentrations, however, this statement is of limited relevance here. What is true and relevant, as stated clearly in Chapter 7 of PM AQCD (U.S. Environmental Protection Agency, 1996 [13] ), is that personal exposure to PM of ambient origin (PM from only outdoor sources, experienced indoors as well as outdoors) is indeed well characterized by outdoor concentrations. In the above report, the term personal exposure to PM of ambient origin was used to refer to the sum of exposure to ambient PM while outdoors plus exposure while indoorsto ambient PM that had infiltrated indoors. In this paper, ambient-generated PM will be used instead of PM of ambient origin. DOES THE CONCEPT OF EXPOSURE TO CLASSES OF PM WARRANT DISCUSSION? Exposure analysts are fully aware that ambient PM infiltrates into homes and that indoor PM is a mixture of ambient-indoor PM (ambient PM that has infiltrated indoors) and indoor-generated PM. Indeed, they developed the models that describe these relationships. However, ambient-indoor PM is only one of many sources of indoor PM. The interest of exposure analysts in ambientindoor PM generally has been to estimate a total exposure, not to determine exposure to ambient-generated PM and exposure to indoor-generated PM as separate and distinct classes. However, this viewpoint may lead to inadequate consideration of the possible differences in size, source, composition, variability, and toxicity between ambientgenerated and indoor-generated PM. An example from a recently published paper demonstrates the need to continue this discussion. Linn et al. 16 state, If the health/pm association is causal, the following hypotheses must be verifiable:... Personal PM exposures must track background ambient PM; that is, there must be significant cross-sectional (spatial, betweenpersons) associations and significant longitudinal (temporal, within-persons) associations between PM concentrations in the ambient background and in the personal environment. An alternate hypothesis is that it is only necessary that personal exposures to ambient-generated PM correlate with background ambient PM. The authors do not consider the possibility of separating personal exposure into an ambient-generated component and a nonambient-generated component. Personal PM exposure includes exposure to ambient and nonambient PM. Nonambient PM may have health effects similar to or different from health effects resulting from ambient PM. Epidemiology that correlates ambient PM with health outcomes will provide insight into possible health effects of ambient PM. It will not provide any information on the possible effects of exposure to other types of PM unless the concentration and composition of Volume 50 July 2000 Journal of the Air & Waste Management Association 1169

5 such PM is highly correlated with the concentration and composition of ambient PM. Smoking or exposure to ETS may cause health effects and could cause a person to be more susceptible to ambient PM. However, unless smoking or exposure to ETS is correlated with ambient PM concentrations, health effects of such exposures would not be observed in epidemiologic studies using ambient PM concentrations as an exposure surrogate. Epidemiologic studies using exposures to indoor-generated PM or toxicologic studies using indoor-generated PM will be required to determine its health effects. The need to consider personal exposure to ambient-generated particles was recognized in the National Research Council report Research Priorities for Airborne Particulate Matter. 17 In the definition of personal exposure, this report states (p 36), The regulatory objective is to measure the amount or intensity of such exposures [to PM] that are attributable to outdoor sources. It asks in Research Topic 1 (p 46), What are the quantitative relationships between concentrations of particulate matter and gaseous copollutants measured at stationary outdoor air-monitoring sites, and the contributions of these concentrations to actual personal exposures, especially for potentially susceptible subpopulations and individuals? However, the authors are not aware of any published reports with measurements of personal exposure to ambient-generated PM or of any studies undertaken with that as the primary purpose. ANALYSIS OF SERIAL STUDIES Personal exposure to total PM 10 measured for a single individual over several weeks (serial exposure) has been found to be highly correlated with ambient PM 10 concentrations for certain individuals. 181 This is postulated to occur because the variation in nonambient PM exposures (same home and workplace from day to day) was small compared to the variation in ambient PM concentrations. Mage et al. 3 demonstrate how pooling of the personal exposure measurements of two individuals, each with a high correlation between the ambient concentration and their personal exposure to ambient-generated PM, but with different levels of exposure to nonambient PM, can lead to very low cross-sectional correlations (many individuals, but only one or two individuals measured per day). Janssen 18,19 found a higher correlation coefficient (r) for serial as opposed to cross-sectional analyses (see Table 1). Table 1 also shows the increase in r for subjects not exposed to ETS relative to subjects exposed to ETS. Thus, the use of cross-sectional correlations can obscure the higher serial correlation for individuals. For community, time-series epidemiology (CTSE), the mean personal exposure for the community is the exposure parameter of interest. 224 Statistical theory predicts Table 1. Serial and cross-sectional correlations of personal exposure to total PM 10 with ambient concentrations of PM 10 using data from Janssen. 18 Serial a r Cross-Sectional b ETS Exposed Plus Non-ETS Exposed Adults, PM Children, PM Non-ETS exposed Adults, PM Children, PM Notes: a Median of individual correlations (r); b Mean value, estimated by randomly selecting one measurement per subject, 1000 times. that, as the number or measurements of personal exposure made on the same day increase, the correlation between the ambient concentration and the average personal exposure also will increase. 25 This statistical effect has been demonstrated using the Particle Total Exposure Assessment Methodology (PTEAM) database. For the cross-sectional correlation, r = Using statistical theory and their Random Component Superposition model, Ott et al. 26 show that the correlation of daily average personal exposure with daily ambient concentration increases as the number of subjects measured daily increases. For 25 subjects, they predict r = Thus, cross-sectional studies, in which only one or two subjects are measured per day, also obscure the higher correlation between ambient concentrations and the community mean personal exposure. Tamura et al. 20 reported a serial study of seven elderly Japanese individuals; r (for the correlation of the personal exposure to total PM of each individual with the ambient concentration) ranged from 0.78 to 0.96, with an average value of Lioy et al. 21 reported a study in which the personal exposure to total PM was measured for 14 days for 14 active adults. For six subjects, the coefficient of determination (R 2 ) for the relationship between personal exposure to total PM and ambient concentration was high (R 2 > 0.63) and statistically significant. For the other eight, it was not statistically significant. These studies suggest that for subjects with modest or regular nonambient sources, the statistical relationship between ambient concentrations and personal exposure to total PM will be stronger than for subjects with larger and more variable nonambient sources. Further information can be obtained from regression analysis. 268 For ambient and indoor concentrations, the slope of the regression equation gives an indication of the average fraction of ambient PM that has infiltrated indoors. The intercept gives an indication of the average concentration of indoor-generated PM. For the regression of ambient concentrations with personal exposure, the slope gives an indication of the average fraction of ambient PM that is found in personal exposure. 26 The intercept gives the average of the sum of exposure to nonambient PM (i.e., indoor-generated PM plus personal-activity PM). The 1170 Journal of the Air & Waste Management Association Volume 50 July 2000

6 difference between these two intercepts gives the average personal-activity PM. As indicated in Table 2, the average concentrations of indoor-generated PM and especially personal-activity PM are higher for the more active New Jersey cohort 21 than for the elderly Japanese, 20 who had minimal indoor sources and a more sedentary lifestyle. The importance of the analysis of serial exposure studies, given here and in more detail in Mage et al., 3 is twofold. First, when examined on a serial basis, there is a high correlation between ambient concentrations and personal exposure to total PM for some subjects. Second, as nonambient PM approaches zero or becomes relatively constant, the correlation between ambient concentrations and personal exposure to total PM will increase. These observations lead to the postulate that a reasonably high correlation may exist between the ambient PM concentration and both an individual s personal exposure to ambient-generated PM (i.e., the sum of exposure to ambient PM while outdoors and exposure, while indoors, to ambient PM that has infiltrated indoors) and the mean personal exposure for the community. The remainder of this paper will discuss techniques for determining exposure to ambient-generated PM and how to use this information in epidemiology and risk assessment. CLASSES OF PM AND EXPOSURE An analysis of personal exposure requires definition and discussion of four classes of particles and five classes of exposure. The symbol C will be used to refer to PM concentrations, and E will be used to refer to personal exposure. In order to make it easier to differentiate the various classes of PM, the C and E will be given subscripts that are the first letter of the appropriate adjective. Directly measurable quantities have one-letter subscripts; quantities that must be estimated from other quantities have subscripts with two or more letters. Ambient PM (C a ) is PM that is emitted into or formed in the ambient atmosphere. The ambient atmosphere is defined as that portion of the atmosphere, external to buildings, to which the public has access. The discussion in this paper applies primarily to exposure assessment for community-wide epidemiology or risk assessment. Therefore, ambient PM is assumed to be well-mixed in the outdoor air, so that all people in the community are exposed, over time, to a mixture of PM of similar composition at Table 2. Estimates of exposure to various PM 10 classes using the intercept from regression analyses (expressed in µg/m 3 ). Study A. Indoor-Generated PM 10 B. Nonambient PM 10 B. A. Personal Activity PM 10 Lioy, NJ, USA (R 2 = 0.54) 42.0 (R 2 = 0.05) 20.5 Tamura, Tokyo, Japan (R 2 = 0.72) 11.3 (R 2 = 0.69) 1.8 approximately the same mean concentration. PM is not always so evenly distributed across a community. 14 In this case, more complicated models, which estimate the distribution of PM concentrations throughout the community, 29 must be used. Hot spots (i.e., locations significantly impacted by an identifiable local source) have to be treated as separate microenvironments. Indoor PM (C i ) includes several types of PM that are considered to be evenly distributed in an indoor microenvironment. There are two major components of C i. Indoor-generated PM (C ig ) is PM emitted or formed indoors by indoor sources, such as tobacco smoking or cooking. PM also can be formed by nucleation and condensation of organic vapors emitted from heated ovens or produced in indoor combustion. Human activities in the home (walking, cleaning, etc.) can either resuspend previously deposited PM or suspend PM from bulk materials, including soil tracked into the home. PM emitted indoors at work will vary with the type of occupation. Aerosol formation also may occur indoors from the dark (no sunlight) reaction of gaseous terpenes, such as limonene and α-pinene (often found in household deodorizers and cleaning solutions), with ozone, which infiltrates indoors with ambient air. Aerosol formation also may occur from the reaction of outdoor inorganic or organic acids with indoor ammonia. Little is known about the concentrations or toxicity of this type of PM, which will be considered a subset of indoor-generated PM. Ambient-indoor PM (C ai ) is ambient PM that has infiltrated indoors (i.e., ambient PM that has penetrated indoors and remains suspended). This ambient PM that infiltrates indoors contributes to the total concentration of indoor PM. Personal exposure to total PM (E t ) is defined as that PM collected by a monitor worn by a person and sampled from a point near the breathing zone (but not impacted by exhaled breath). Personal exposure PM includes ambient PM while the person is outside, indoor PM while the person is indoors, and PM generated by personal activities whether indoors or outdoors. As explained above, indoor PM includes indoor-generated PM and ambientindoor PM. E t may be divided into exposure to ambient-generated PM (E ag ), indoor-generated PM (E ig ), and personal activity PM (E pact ). E ag is the sum of exposure to ambient particles (C a ) while outdoors and to C ai while indoors. E ig is exposure to indoor-generated PM while indoors. Personal activity sources can exist either indoors or outdoors. These are microscale particle-generating activities that primarily influence the exposure of the person performing the activity, either a particle-generating Volume 50 July 2000 Journal of the Air & Waste Management Association 1171

7 activity (hobby or occupation) or a physical activity that brings the subject into the undiluted particle plume from a local source (e.g., jogging on a busy street, holding a lit cigarette between puffs, or operating a vacuum cleaner). Thus, exposure to personal activity PM can be measured only by a personal monitor carried by the subject, because a stationary monitor located away from the subject will not measure the high PM concentration resulting from that activity. The personal cloud 30,31 is given by the difference between a personal monitor measurement and an area-representative measurement several meters away and is a narrower definition than personal activity PM. (A person wearing a personal monitor who stops by a smokefilled tavern on the way home is exposed to smoke in the tavern. If the tavern is not included in the measured microenvironments, the PM collected by the personal monitor will be captured as E pact but would not be considered a personal cloud exposure.) The sum of exposure to indoor-generated PM and personal activity PM gives exposure to nonambient PM (E na [E na = E ig + E pact ]). There are some semantic problems with the choice of terminology. Particulate matter is used instead of particles because an individual particle found indoors could consist of an ambient particle that had infiltrated indoors and coagulated with a particle generated indoors and, in addition, could have material condensed on it that resulted from chemical reactions indoors. For the sum of exposure to ambient PM while outdoors and ambient-indoor PM (ambient, infiltrated indoor PM) while indoors, ambient-generated PM is used. PM of ambient origin has been used for this combination. However, PM of ambient origin could be interpreted to include PM resuspended from previously deposited ambient-indoor PM or PM, of the same composition as some component of ambient PM, suspended from bulk material, such as soil tracked indoors. In this paper, PM suspended or resuspended by indoor activities is considered indoorgenerated PM because its temporal variation will be determined by indoor activities rather than by the ambient PM concentration. Ambient-indoor PM is defined as ambient PM that has infiltrated indoors (i.e., PM that has penetrated from outdoors to indoors, and that has not been removed by deposition or filtration but remains suspended in the air). This definition excludes PM resuspended from previously deposited ambient-indoor PM or PM suspended from tracked-in soil. Several additional types of PM may need to be considered. Individual PM covers PM that only the individual is exposed to and which is not sampled by a personal monitor (e.g., PM in cigarette smoke inhaled during active smoking). Individual PM is not included in personal exposure. Studies of personal exposure sometimes monitor outdoor PM (i.e., in the backyard of the home), as well as personal, indoor, and ambient PM concentrations. Outdoor PM may include, in addition to ambient PM, PM emitted in the backyard (i.e., on private property) or by other local sources that impact people nearby but do not make a significant contribution to a community particle monitor (i.e., smoke from a neighbor s barbecue grill). Unless an outdoor measurement is made near an indoor environment where a local (microscale) source of PM is operating, it is generally assumed that the outdoor concentration can be represented adequately by the concentration measured by a community monitor. Sourceoriented PM is a special type of outdoor PM. Sometimes particle samplers are sited to monitor PM from a specific local source (e.g., a busy street corner, across the street from a coke oven). Such monitors are useful for compliance monitoring but are not useful for estimating a community exposure. It is clear that only C a, C i, and E t can be measured directly. However, the following also need to be known: personal exposure to ambient-generated PM (E ag ), personal exposure to indoor-generated PM (E ig ), and personal exposure to personal activity PM (E pact ). A major challenge for exposure analysts is to split C i into C ig and C ai. EXPOSURE MODELS: EQUATIONS AND PARAMETERS Personal exposure to PM by inhalation is defined as the concentration of PM measured by sampling at a point near the breathing zone but not impacted by exhaled air. (This paper addresses exposure issues for those health effects caused by inhalation of particles. It is recognized that exposure by ingestion or absorption through the skin following deposition on food or skin may be important, especially for toxic metals or organic compounds. Such issues, however, are outside the scope of this paper.) It is customary to express personal exposure as a concentration, usually µg/m 3, by dividing the integral of the concentration over time by the measurement time period (T). Following the formalism of the NAS report, 12 personal exposure may be expressed as where E is personal exposure, C(t) is the time-variant concentration, and t is the time that the person experiences a specific concentration. For PM, a more precise expression is where E t is the personal exposure to all classes of particles in all microenvironments. In the first term, C j is the concentration of PM (of all sizes and classes) in each microenvironment j, t j is the time spent in microenvironment j, (1) (2) 1172 Journal of the Air & Waste Management Association Volume 50 July 2000

8 and the summation is over all microenvironments j. T is the sum of t j over all j. The integral is used to emphasize that, although most studies measure the average concentration over 24 hr (or sometimes 12 or 48 hr), the actual concentration will vary with time. Thus, an error is introduced if an average concentration is used to create a timeweighted average exposure but the subject is in that environment for only a fraction of the averaging time. In the second term, C pact, is the concentration of PM resulting from a specific personal activity, t is the time spent in that personal activity, and the summation is over all instances of personal activity that lead to an exposure not recorded by either the ambient or the indoor PM measurement. Equation 2 also may be written as where E j is the time-weighted personal exposure to PM in each microenvironment j and E pact is the time-weighted personal exposure to PM generated by personal activity. 32 E pact may be determined as the difference between the measured E t and the time-weighted average of the E j, calculated from C j and t j. Thus, in the concept of exposure by microenvironments, personal exposure is the exposure to total PM (of all sizes and classes) summed over the different microenvironments in which a person spends time, plus exposure to PM generated by personal activity. Important microenvironments are outdoors, indoors, in transit, and occupational. This exposure model has been applied to concentration data in a number of studies. 28,33-38 A different approach is recommended for PM. Many problems in PM epidemiology and exposure assessment can be more readily resolved if personal exposure is conceived of as exposure to several classes of PM. Thus, personal exposure to PM can be expressed as the sum of exposure to several different and independent classes of particles, summed over all microenvironments in which exposure occurs. In the above formalism, personal exposure, in terms of the three classes of particles of special interest, may be expressed as where E ag is personal exposure to ambient-generated PM (the sum of ambient PM while outdoors and ambient PM, which has infiltrated indoors, while indoors); E ig is personal exposure to indoor-generated PM; and E pact is personal exposure to PM from personal activity. These three classes of PM cannot be measured directly but must be calculated or estimated from other measurable quantities. E pact is the same as in eq 3 and can be calculated as described above. For E ag and E ig, a key parameter is the (3) (4) concentration of ambient PM that has infiltrated indoors (C ai ). Once C ai has been determined, C ig can be calculated Once C ig has been determined, E ag and E ig may be calculated by where t a stands for time in the ambient microenvironments, t j for time in all other j, and T. These three classes of particles may vary independently on a per-minute, hourly, and daily basis. 39,40 They are also likely to differ in terms of sources, composition, and properties (chemical, physical, and toxicologic), as well as in exposure relationships. The mass balance model has been used by many exposure analysts 27, 32-36,41-46 to describe the concentration of particles within an indoor volume where V = volume of the well-mixed indoor air (m 3 ); C i = concentration of indoor PM = C ai + C ig ; v = volumetric air exchange rate between indoors and outdoors (m 3 /hr); P = penetration ratio, the fraction of ambient (outdoor) PM that is not removed from ambient air during its entry into the indoor volume; C a = concentration of PM in the ambient air (µg/m 3 ); C ai = concentration of ambient-indoor PM in the indoor volume V (µg/m 3 ); C ig = concentration of indoor-generated PM (µg/m 3 ); k = removal rate (per hr); and Q i = indoor sources of particles (µg/hr). Q i contains a variety of indoor, particle-generation sources, including generation by combustion or mechanical processes, condensation of vapors formed by combustion or chemical reaction, suspension from bulk material, and resuspension of previously deposited PM. The removal rate, k, includes dry deposition to interior surfaces by diffusion, impaction, electrostatic forces, and gravitational fallout. It may include other removal processes such as filtration by forced air heating, ventilation, or air-conditioning (HVAC) or by independent air cleaners. All parameters except V are functions of time. P and k also are functions of particle aerodynamic diameter and v. The mass balance model (eq 8) may be solved for C ai and C ig at equilibrium by assuming that all variables remain constant. 268 Replace dc i with dc ai + dc ig and let dc ai and dc ig = 0. Then, PM from ambient-indoor PM (C ai ) and indoor-generated PM (C ig ), at equilibrium, are given by (5) (6) (7) (8) Volume 50 July 2000 Journal of the Air & Waste Management Association 1173

9 (9) (13) (10) (14) where a = v/v, the number of air exchanges per hr. Equations 9 and 10 are valid only when the parameters k, a, C a, and Q i are not changing rapidly and when the C s are averaged over several hours. The equations probably will not be valid, for example, for air-conditioned homes, for homes with HVAC or air cleaners that cycle on and off, or for ambient pollutants with rapidly varying concentrations. For these conditions, non-equilibrium versions of the mass balance model 26,29,47 should be used. However, the equilibrium model provides a useful, if simplified, example of the basic relationships. Note that in this formulation, Q i for resuspension of previously deposited PM and suspension of soil tracked into the home is included as indoor sources, although some of this material may have the same composition as some components of ambient PM. One goal of this formulation is to divide E t into exposure to classes of PM that vary independently and can be used as independent variables in epidemiologic analyses and risk assessment. Resuspension will vary with cleaning and activity within the home, not with ambient concentrations. Therefore, it is appropriate to consider resuspended or suspended PM as indoor-generated PM even if its composition is similar to that of some components of ambient PM. Deposited PM, whether fine-mode or coarse-mode, is resuspended as coarse-mode PM (bulk material also is suspended as coarse-mode PM). These considerations, therefore, are irrelevant for fine-mode particles but are important for coarse-mode particles. Equation 9 may be rearranged to give C ai /C a, the equilibrium fraction of ambient PM that is found indoors, defined as the infiltration factor (F INF ). 27 F INF will be a function of particle aerodynamic diameter because P and k are both a function of particle aerodynamic diameter. (11) If, for the time T, over which the exposure parameter is calculated, y = the fraction of time that an individual spent outdoors, and (1 y) = the fraction of time spent indoors, then E ag may be expressed as (12) It will be convenient to express E ag as the product of C a and a personal exposure factor (F PEX ); therefore, Thus, PM personal exposures of interest, E ag, E ig, and E pact, may be calculated from the parameter F INF ; measurements of the observable quantities, C a, C i, and E t ; and diary records of time in various microenvironments. (In the analysis that follows, we assume, for simplicity, that the subject is either indoors at home, or outdoors in the community. If the subject were to spend an appreciable amount of time in other indoor microenvironments [office, school, etc.], then it would be necessary to follow the same procedure and separately measure the subject s personal exposures and the indoor concentrations while in those non-residence indoor microenvironments and determine the correct infiltration factors.) F INF can be measured directly or calculated from measured or estimated values of the parameters a, k, and P. The use of a mass balance model to separate personal exposure into two components because of exposure to ambient and nonambient concentrations is not novel. This approach, based on eq 3, as given in Duan 32 and called superposition of component concentrations, has been applied using multiple microenvironments to carbon monoxide, volatile organic compounds, 36 and particles. 28,37 However, in these studies, and in most of the exposure literature, the ambient and nonambient components are added to yield a personal exposure from all sources of the pollutant. The use of the mass balance model, ambient concentrations, and exposure parameters to estimate exposure to ambient-generated PM and exposure to indoor-generated PM separately as different classes of exposure, as suggested by Wilson and Suh, 14,48 and demonstrated in this paper, apparently has not been of interest to exposure analysts. Summary of Exposure Symbols Meaning of Subscripts. a = ambient i = indoor g = generated n = non pact = personal activity t = total Concentration Variables. Ambient PM (C a ) Indoor PM (C i ) Components of Indoor PM. Ambient-Indoor PM (C ai ) Indoor-Generated PM (C ig ) 1174 Journal of the Air & Waste Management Association Volume 50 July 2000

10 Exposure Variables. Exposure to indoor-generated PM (E ig ) Exposure to ambient-generated PM (E ag ) Exposure to personal activity PM (E pact ) Exposure to nonambient-generated PM (E na = E ig + E pact ) Exposure to total PM (E t = E ig + E ag + E pact ) Infiltration Factor (F INF = C ai /C a = Pa/[a + k]) where P is penetration factor, a is air exchange rate, and k is deposition rate Exposure Factor, F PEX = E ag /C a = y + (1 - y) F INF where y is the fraction of time spent outdoors DETERMINATION OF THE CONCENTRATION OF AMBIENT-INDOOR PM AND PERSONAL EXPOSURE TO AMBIENT-GENERATED PM Statistical Fitting Statistical techniques for fitting data to a theoretical model can be used to determine which average values of penetration factor (P), deposition rates (k), and indoor generation rates (Q i ) give the best fit to a large database of indoor and outdoor data. These techniques, applied to the PTEAM database by Özkaynak et al., 49,50 gave values of k and P for both PM 2.5 and PM 10 and for day, night, and 24-hr time periods. 49,50 Average values of the reciprocal of the air exchange rates were measured so the average F INF could be calculated by eq 11. Average values are given in Table 3. The nonlinear optimization procedure used by Özkaynak et al. 49,50 initially found a solution for the daytime data with P = Because P must be equal to or less than 1.0, the solution was constrained by setting P = 1 and the values of the other parameters were recalculated. 49,50 The calculated values, however, have some uncertainty beyond that caused by violations of the equilibrium assumption. For example, as shown in Table 3, the 24-hr average air exchange rate (a = 0.97 hr -1 ) is less than the day or nighttime value. Abt et al. 52 and Long et al. 53 also have used statistical techniques to estimate exposure parameters from data. and PM 10, using k s estimated from PTEAM data 51 and assuming P = 1 for all particle sizes, is shown in Figure 1. If, in addition, the fractions of time each individual spends indoors and outdoors are known, the individual E ag can be calculated (eq 13). Sufficient information is available in the PTEAM database to allow this calculation. Times in various microenvironments for each person for each time period were available from PTEAM diary entries. The value of y used here includes time outdoors plus time commuting or otherwise in automobiles. Ambient concentrations and parameters from the PTEAM study 49 were used to calculate E ag. Figures 2a and 2b show the relationship of E ag with ambient and outdoor PM concentrations for the PTEAM subjects. E ag is well correlated to both outdoor and central site C a. The difference between the intercepts, 2 µg/m 3 for the regression of E ag with backyard and 14 µg/m 3 with central site, indicates a lower concentration at the central site than at the backyard sites. This difference was also observed in the regression of the central site with the backyard concentrations, R 2 = 0.57 and backyard = 1.03 central site (EPA; 13 calculated using PTEAM data from Pellizzari et al. 54 ). There is some uncertainty in the E ag values because of uncertainties in the penetration ratio, the deposition rate, and higher values of the air exchange rates, taken from the PTEAM analysis. Calculation If average values of the reciprocal of the air exchange rate (1/a) have been measured, and values of the penetration ratio (P) and deposition rates (k) are known or can be assumed, it is possible to calculate C ai from C a and calculate F INF (eq 11). The variation of F INF as a function of a for SO 4 Figure 1. Variation in F INF as a function of air exchange rate (a) for SO 4 and PM 10. Table 3. Summary of the mean values of PM variables from the PTEAM study. 49,50 PM 2.5 PM 2.5 PM 2.5 PM 10 PM 10 PM 10 Day Night 24-hr Day Night 24-hr Deposition Rate k (hr -1 ) Air Exchange Rate a (hr -1 ) Penetration Ratio P Infiltration Factor = Pa/(a + k) F INF Volume 50 July 2000 Journal of the Air & Waste Management Association 1175

11 Figure 2. Relationship between exposure to ambient-generated PM 10 and (a) backyard concentrations (daytime) or (b) central site monitor concentrations (daytime). Using similar techniques, Mage et al. 3 calculated E na = E ig + E pact and showed that it was not correlated with the outdoor concentration measured at the home (R 2 = 0.005). The individual C ig also has been estimated by subtracting the individual C ai from the individual C i. As might be expected, the correlation between C ai and C ig is low, R 2 = for all points, and R 2 = if two points with extremely high C ig are eliminated. The distributions of daytime F INF and F EXP for PM 10, calculated similarly using data from PTEAM, are shown in Figures 3a and 3b. The type of exposure assessment information shown in Figure 3b can be used with ambient concentration data to develop a population-based risk assessment for pollutants for which there is an accepted exposureeffect relationship. However, for PM, there is only an ambient concentration-effect relationship. The question, then, is to what extent can epidemiologists use such exposure distribution information to improve the reliability of existing ambient concentration-effect relationships or to move toward an exposure-effect relationship? It would seem that such exposure assessment information could be used in epidemiologic studies to obtain exposure-dose relationships, to account for F EXP being less than one, and to correct for city-to-city (or even season-toseason) variations in F EXP. Figure 3. Distributions of (a) F INF and (b) F EXP for daytime PM 10 for the PTEAM study. The 0.55 bar represents the number of observations 0.50 < F Use of a Tracer Species Which Has No Indoor Source The indoor/ambient concentration ratio (C i /C a ) for a PM component with no indoor source gives F INF directly for particles with the same aerodynamic diameter. Suh et al. 55,56 measured SO 4 in PM 2.5 on a 24-hr basis and found high correlations and only small differences in concentration between personal, indoor, and outdoor samples for non-air-conditioned homes. Homes with air-conditioning had lower infiltration ratios because of their lower air exchange rates. The PTEAM measurements of SO 4 also showed little difference between personal, indoor, and outdoor SO Although neither study used SO 4 as a tracer, the F INF and F PEX values from SO 4 could be used to estimate F INF and F PEX for other PM components with similar size distributions. SO 4 has been used as a tracer by Leaderer et al. 57 The contributions of ambient source-categories to personal or indoor concentrations, as determined by source-apportionment techniques, 58 also can be used as tracers. Values of F PEX and F INF for SO 4 obtained from various techniques are listed in Table 4. The ideal tracer has no indoor sources, can be measured continuously (or at 1-hr intervals or less), is present in high 1176 Journal of the Air & Waste Management Association Volume 50 July 2000

12 enough concentrations to permit precise measurements, does not evaporate or react, and preferably has occasional sharp changes in ambient concentration. SO 4 is a candidate because it has minimal indoor sources, can be measured continuously (1-min time interval), occasionally has large day-to-day variations, and has smaller variations on a daily basis. SO 4 is the major component of PM in the eastern United States. Although it is not the major component in the western states, the concentrations in most communities are adequate for use as a tracer. However, SO 4 in PM 1 would be a better tracer than SO 4 in PM 2.5 because PM 2.5 can contain SO 4 from coarse sea salt, soil, or construction dust particles, whose size distribution extends down into the 1- to 2.5-µm region. Such coarse-mode SO 4 does not infiltrate as efficiently as finemode, secondary SO 4. Coarse-mode SO 4 also may have indoor sources because of the resuspension of SO 4 -containing material tracked in or deposited previously. SO 4 - containing particles do increase in size with increasing relative humidity. At a relative humidity above about 90%, hygroscopic particles will grow enough to separate from nonhygroscopic particles. Therefore, if P changes significantly with particle size, SO 4 would not be as good a tracer at a very high relative humidity. Dockery and Spengler 27 found indoor-generated SO 4 in homes with cigarette smoking and gas cooking stoves (perhaps because of the use of matches). However, the regression of ambient vs indoor SO 4 for nonsmoking homes gave an intercept of 0.4 µg/m 3. This indicates that indoor-generated SO 4 is very low in nonsmoking homes. Lead might be a useful tracer in communities where leaded gasoline is still used. Another potential tracer is fine-mode black carbon, which can be measured continuously by an aethelometer if no combustion is allowed indoors. Other stable fine-mode compounds, not generated indoors, are also candidates. Coarse-mode tracers are not so readily available. Material of coarse-mode chemical composition may exist indoors because of the previous deposition of particles or bulk material brought in on shoes or clothes. This material then may be suspended or resuspended by air currents generated by human activity or Table 4. F PEX and F INF for fine-mode PM based on use of SO 4 as a tracer. F PEX = Personal/Ambient = E t /C a SO 4 as Tracer PTEAM 49, regression 0.70 Suh 55, non-ac regression 0.88 F INF = Ambient-Indoors/Ambient = C ai /C a SO 4 as Tracer PTEAM 49, regression 0.85 Suh 55, non-ac 0.86 ± 0.16 with AC 0.69 ± 0.32 central air-conditioning or heating systems. Tracers are good only for particles of comparable size distribution. It may not be possible to find tracers for coarse-mode PM, except by source apportionment techniques that separate ambient sources from indoor sources. 58 Measurement of F INF A measurement of C a and C i will give the infiltration ratio (F INF ) if there are no indoor sources (i.e., C i = C ai ). This technique, with impactor measurements, was used by Thatcher and Layton 59 to study indoor and outdoor concentrations for an uninhabited house in Berkeley, CA. They measured C a, C i, and a, and they estimated k, as a function of particle size, by stirring up dust and measuring its decay. They assumed C i = C ai and used these parameters in eq 11 to calculate P. Thatcher and Layton 59 often are quoted as proving that P = 1. However, 10 of their 13 measurements of P gave physically impossible values of P > 1. This suggests that, even in uninhabited homes, some coarse-mode particles are generated, either by suspension or resuspension. Thatcher and Layton s studies 59 of particle resuspension did show that the resuspension rate was very low for particles with <1-µm aerodynamic diameter. Using SO 4 in PM 2.5 as a tracer, Suh et al.55 found an average F INF of 0.86 ± 0.16 for non-air- conditioned homes and a value of 0.69 ± 0.32 for air-conditioned homes. Abt et al. 39 and Long et al. 53 report measurements of F INF as a function of particle size (from 0.1 to 0.5 µm and from 0.7 to 10 µm) from nighttime measurements of C a and C i, assuming C i = C ai or C ig = 0 at night. F INF was highest for accumulation-mode particles (diameter of µm) and decreased for larger and smaller particles. Therefore, F INF can be expected to be a function of particle aerodynamic size. Any resuspension of coarse-mode particles indoors because of air currents from infiltration, forced air ventilation, or thermal gradients, or indoor particle emissions from any other source, would cause the inferred value of C ai to be higher and lead to a larger F INF. On the other hand, the presence in PM 2.5 of coarse-mode particles that are more rapidly removed because of a higher k would lead to a lower value of F INF than would be the case for a measurement of accumulation-mode particles. The high values of F INF observed by Thatcher and Layton 59 and the resulting values of P near 1 could result from unrecognized indoor sources. It might be expected that P would be near 1 for penetration through open windows or doors. However, based on studies of flow through tubes, 9 aerosol physicists would expect penetration through cracks in the building envelope to be less than 1, highest for accumulation-mode particles and lower for larger and smaller particles as observed by Abt et al. 39 and Long et Volume 50 July 2000 Journal of the Air & Waste Management Association 1177