ES&T
Evidence for improved ambient air quality and the need for personal exposure research
John D. Spengler Mary Lou Soczek Department of Environmental Science and Physiology Harvard University School of Public Health Boston, Mass. 02115 The evidence on trends in the quality of the nation's air is encouraging. By conventional measures of total suspended particulates (TSP), sulfur 268A
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dioxide (SO2), carbon monoxide (CO), and lead (Pb), the number of locations exceeding the primary National Ambient Air Quality Standards (NAAQS) has decreased over the past decade. Even ozone (O3) pollution has shown some improvement, with a reduction in peak levels in several urban areas. While widespread violations of the O3 standard still occur in much of the eastern third of the country, auto emission controls and a recent effort to reduce volatile organic compounds
(VOCs) from stationary sources will, it is hoped, make inroads to reducing O3 and other photochemically produced air contaminants. Of the six criteria pollutants—TSP, S 0 2 , CO, O3, Pb, and N0 2 ~-only nitrogen dioxide (NO2) has shown upward trends in several urban areas (EPA, 1983). Overall, steadfastness in our environmental resolve is paying dividends of cleaner air. A closer examination of data on TSP and SO2 offers an explanation for
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© 1984 American Chemical Society
the encouraging trends in ambient air quality. In the early 1970s, there were numerous violations of the N A A Q S near stationary sources. Emissions from paper mills, power plants, iron and steel operations, and smelters were the prime offenders. Regulation of sulfur content in fuel oils greatly reduced urban concentrations of SO2 in many northeastern cities. Relocation of industries and power plants and the building of taller stacks reduced local fumigation conditions resulting from particulate emissions. Figure 1 is a histogram of annual concentrations of SO2 from the valid air quality monitoring sites in 1978. Only 3% of the sites exceeded the 8 0 ^ g / m 3 primary N A A Q S . The cumulative frequency distributions for the 1978 annual mean and ninth-percentile T S P concentrations are displayed in Figure 2. Half of the sites met the N A A Q S secondary standard of 60 Mg/m 3 , and only 25% of the sites violated the primary standard of 75 Mg/m 3 (EPA, 1982). While ground level SO2 concentrations have improved since the late 1960s, the atmospheric emissions of SO2 are estimated to have been reduced only from 30 million metric tons/y in 1968 to about 25 million tons/y by the middle of the following decade. Most of these emissions are concentrated in the industrial states of the eastern and northeastern U.S. A fraction of the sulfur oxide emissions oxidize further to particulate sulfate species, including strong acids (H2SO4 and NH4HSO4). Current concentrations of sulfate particulates are similar to the 1990 predicted annual sulfate concentrations shown proportional to elevation in Figure 3. Concentrations are predicted to still exceed 10 /xg/m 3 over a very large portion of the northeast in 1990. The highest concentrations occur downwind of the intense emission areas. Eastern Ohio, western Pennsylvania, New York, and southern Ontario are reporting high annual and episodic sulfate concentrations. This pattern is expected to change as the area of Texas, Oklahoma, Louisiana, and Arkansas becomes the nation's largest coal-consuming region by the year 2000 (Mitre Corp., 1979). Prevailing wind patterns indicate that Tennessee, Kentucky, and northern portions of Mississippi and Alabama will have increasing sulfate concentrations. While the evidence for improved air quality from reductions in TSP, SO2, and other pollutants in the air is undeniable, similar long-term data for concentrations of fine aerosols, sul-
fates, or nitrates are not available. Where records do exist, some northern urban areas might show marginal improvements, but air quality is degrading in suburban and rural regions of the eastern half of the U.S., parts of the Southwest, urban areas of the West Coast, and mountain state cities with increased domestic wood burning (Cooper and Malek, 1982). This type of pollution is primarily from smallparticle emissions and secondarily formed aerosols from SO2 and N 0 2 conversions. Thus, while the sootiness and dustiness of our industrial or populated urban areas have decreased, there is evidence that concentrations of fine aerosols (primarily the sulfate fractions) in our ambient air have increased over the past decade (Altshuller, 1980). Reports of decreasing visibility over a large area of the eastern U.S. (Husar et al., 1979), particularly in the third quarter of the year, suggest the potential effect of the aerosol (sulfate) burden (Waggoner and Weiss, 1980). No national standards for fine or respirable aerosols, sulfates, or visibility levels have been established. The fact that ambient air quality has improved by the traditional measurements and national standards unfortunately does not necessarily imply
that human exposures to the biologically important contaminants have decreased. Nor does it imply that the population at risk has decreased. Where the primary sources for pollutants are emissions to the ambient air, human exposures are determined by the resulting ambient concentrations of the various pollutants. The pollutant concentrations measured at monitoring stations, however, may not offer an accurate representation of actual human exposures because of the dynamic nature of the source-receptor relationship (Fugas et al., 1982; Tost e s o n e t a l . , 1982). Nevertheless, changes in ambient emissions should somehow be reflected in changes in personal exposures, as is suggested by a recent study relating a drop in blood lead levels to reductions in lead content of gasoline (Annest et al., 1983). On the other hand, where pollutants are emitted into the indoor air (for example, from heating and cooking fuel combustion and evaporation of cleaning fluids), personal exposures to contaminants such as CO, CQ2, formaldehyde, hydrocarbons, and suspended particulate matter are more closely related to the indoor pollutant levels than to levels measured in the ambient air. This is especially important because individuals typi-
FIGURE 1
Annual average sulfur dioxide concentrations (/xg/'m3) for valid continuous sampling sites in the U.S. in 1978
Annual average concentration Source: Environmental Protection Agency, "Air Quality Criteria for Particulate Matter and Sulfur Oxides," Vol. 2, EPA-600/8-82-019D. 1982
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FIGURE 2
Distribution of the mean and 90th percentile TSP concentrations (itg/m3) of valid 1978 sites
Percent of sites reporting annual mean Source: Environmental Protection Agency. "Air Quality Criteria tor Particulate Matter and Sulfur Oxides," Vol. 2, EPA-600/8-82-029b. 1982.
FIGURE 3
Predicted annual sulfate concentrations in 31990 from the Brookhaven long-distance transport model
a m i s s i o n s are from major coal-burning facilities and include area sources. Sulfate concentrations (in Mg/m3) are proportional to altitude and spatially averaged over 32 χ 32-km grids (x, y). Reprinted with permission from Wilson, Colome, Spengler and Wilson, "Health Effects of Fossil Fuel Burning: Assessment and Mitigation"; © 1980, Ballinger Publishing Company.
cally spend more than two-thirds of their time indoors (Szalai, 1972; Chapin, 1974). Exposure monitoring techniques Human exposures to air pollutants can be studied with direct measure 270A
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ments (personal monitoring) or with estimates of exposures from data from fixed monitoring sites. In both ap proaches, knowledge of time -activity patterns is important. Direct personal monitoring has become possible with the development of sampling "badges"
and lightweight portable sampling systems (Wallace and Ott, 1982). For the most part, the type of sample ob tained (continuous or integrated) de pends on available instrumentation for the pollutant under investigation. Most studies of personal exposures to respirable particles (RSP), Pb, N 0 2 , and VOCs, for example, have taken inte grated samples over several hours to several days, whereas continuous monitors with strip chart recorders or data loggers have routinely been used to measure human exposures to CO. Continuous sampling records of personal exposures, along with timeactivity data, provide the most detailed sampling information, enabling re searchers to determine the relative contributions of various sources to peak as well as to integrated (timeaveraged) pollutant concentrations. If continuous sampling records are not available or are impossible to obtain, information on the relative contribu tions of various locations or sources to total exposures measured with inte grated sampling methods can be esti mated with regression techniques, using time-activity data and the total mass of the pollutant collected during the sampling period. In many studies using integrated samples, several lo cations (home, office, and outdoors) are sampled simultaneously to provide specific information on pollutant concentrations in the various microenvironments in which individuals spend significant portions of time. It is possible to determine the chemical and elemental compositions of integrated particle samples to identify various source contributions. The success of this technique of source apportionment depends, however, on obtaining uniquely identifiable trac ers for the sources of interest (Gor don, 1980; Cooper and Watson, 1980). Tracer metals and compounds have been used as markers of the penetra tion rates of aerosols of outdoor origin (Alzona et al., 1979; Moschandreas and Stark, 1982; McCarthy et al., 1982). Analysis of the elemental composition of particulate emissions from a variety of household products and tobacco smoke has indicated that when the assumptions of source uniqueness are valid, such techniques offer considerable promise (Colome et al., 1982). Indirect approaches to estimating personal exposures have been at tempted in several studies and offer distinct advantages over direct per sonal measurements (Duan, 1982). Fixed or semiportable continuous re cording instruments are more available
Indoor air: A view from developing countries Increased attention to personal exposure monitoring is one result of the realization that most people do not spend much time on the roofs of post offices, or in other outdoor locations where air pollution monitoring has traditionally been done. When human ill-health is the end point of interest, it is nec essary to monitor in the places where people spend their time. In most cultures of the world, this means monitoring indoors is more important than monitoring outdoors. Because of a long preoccupation with ambient monitoring, this attention to indoor monitoring is in itself a revolution in thinking for the air pollution community. There are, however, other articles of conventional wisdom about air pollution that are threatened by the growing focus on personal monitoring. For example, in the past most lay and expert observers would probably have agreed that the important exposures to air pollution occur as a result of the combustion of fossil fuels in the industrial or automobile-dense cities of developed countries. If indoor exposures were mentioned at all they would be associated with occupational exposures in jobs held mainly by men. Recently, however, it has become clear that the conven tional wisdom needs revamping. The most common indoor environment and the most common fuel in the world today are the same as those which have dominated most of human history—the village house burning traditional biomass fuels such as wood, crop residues, and dried animal dung. In small-scale combustion such as in simple cook stoves, biomass fuels have relatively high emission factors for RSP, CO, and hydrocarbons. In addition, the hydrocarbon portion contains significant quantities of polyaromatic compounds that are known to be mutagenic and, in some cases, carci nogenic.
Cooking with biomass. Β [a] P exposure could be heavy It seems that about half of the world's households cook daily with biomass fuels, often indoors with little ventilation. Hundreds of millions of people also rely on these fuels for space heating in stoves that basically amount to open com bustion. In these circumstances, a simple calculation can easily show that indoor concentrations must be high. For example, a typical cookstove might have a burn rate (F) of about 1 kg/h of wood in a room of about 40 m 3 mixing volume ( V). In such conditions the emission factor (£) might be 10 g/kg for particulates and the ventilation rate (S) about 10 air changes per hour. Assuming perfect mixing and equilib rium conditions, the indoor particulate concentration (X) could be estimated by:
Outdoor exposures. Testing with personal monitor or, in this example, 25 mg/m 3 . This is 125 times the Japanese 1-h standard and more than 400 times the present annual U.S. standard for particulate matter. While there is much anecdotal material in the literature about smoky village kitchens, to date there are only a handful of monitoring studies. These have shown, however, that in door concentrations and personal exposures of women cooks can indeed be orders of magnitude higher than those con sidered of concern in urban situations. Exposures to RSP, for example, have been measured to be 2-56 mg/m 3 during the 3 h of daily cooking. Benzo[a]pyrene exposures averaging nearly 4000 ng/m 3 have been found as well (Smith et al., 1983). This concentration results in inhalation of the same amount of B[a]P during 3 h of cooking as would be inhaled by smoking about 20 packs of cigarettes—13,000 ng. Although the list of rural epidemiological studies of bio mass smoke exposures is also small, it is consistent with the existence of the kind of effects of the sort expected from exposures to cigarette smoke, with the possible exception of lung cancer (Smith et al., 1984). Fortunately, there seem to be economically and technically feasible ways to reduce exposures of this type through cleaner fuels, better stoves, and enhanced ventilation. Institutional and social barriers, however, may be difficult to overcome in some areas. Personal monitoring requires going to where the people are. Doing so presents another challenge to the conventional wisdom. It may be that the most important exposures to many of the principal categories of pollutants are among women in rural areas of developing countries where agriculture is the principal way of life and biomass is the principal fuel. Kirk R. Smith East-West Center Honolulu, Hawaii
X(g/m 3 ) = FEZ VS
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than are personal samplers. A sub stantial number of high-quality con tinuous instruments and size-selective particle samplers are available. Con centrations obtained in various microenvironments are coupled with in dividual time-activity data to produce time-weighted exposure estimates. Indirect methods are usually less expensive than direct methods; how ever, the best of these measurements still provide only estimates of actual personal exposures. Statistical prob lems with the estimates exist and the prediction errors can be quite large (Letz et al., in press). Comparisons of continuous and integrated sampling records and more research on predic tion errors are needed. Research findings Results from continuous and inte grated personal sampling studies have provided information on relationships between concentrations in the ambient air and levels to which individuals are exposed. Several important exposure studies reported over the past 15 years have demonstrated that personal ex posures to many pollutants show little or no relationship to outdoor mea surements and that personal exposures are often higher than, though poorly correlated with, outdoor measure ments. The purposes of many of the studies summarized in Tables 1 -6 have been to document the relationship be tween personal and fixed-location sampling, both indoors and outdoors; to establish exposure categories; to develop predictive equations; and to quantify contributions of specific sources or activities. Examination of results from studies of personal exposures to particulates illustrates the problem with assuming that ambient measurements reflect actual personal exposures. Particles of all sizes are classified together in the TSP standard, but only a fraction of the particles are inhalable (
