Evidence for improved ambient air aualitv and the need for'pers'onal exposure research
John D. Spengler Mary Lou Soczek Department of Environmental Science and Physiology Haruard University School of Public Health Boston, Mass. 021 I5 The evidence on trends in the quality of the nation's air is encouraging. By conventional measures of total suspended particulates (TSP), sulfur 298A
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dioxide (SOz), carbon monoxide (CO), and lead (Pb), the number of locations exceeding the primary National Ambient Air Quality Standards (NAAQS) has decreased over the past ) has decade. Even ozone (03pollution shown some improvement, with a reduction in peak levels in several urban areas. While widespread violations of the 0 3 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 prcduced air contaminants. Of the six criteria pollutants-TSP, SO2, CO, 0 3 , Pb, and NOZ-only nitrogen dioxide (N02) 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 1.5010
@ 1984 American Chemical Society
the encouraging trends in ambient air quality. In the early 197Os, there were numerous violations of the NAAQS 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 80-pg/m3 primary NAAQS. The cumulative frequency distributions for the 1918 annual mean and ninth-percentile TSP concentrations are displayed in Figure 2. Half of the sites met the NAAQS secondary standard of 60 pg/m3, and only 25% of the sites violated the primary standard of 75 pg/m3 (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 (H2S04 and NHdHSO4). 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 pg/m3 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, SOz, 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 NO1 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 fme 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; Tosteson et al., 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, CO2, 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 (figlm') for valid continuous sampling sites in the U.S. in 1978 120
\J\
-110 120 130 140 150 1f Annual average concentration ~
Source: Envirmmental ProtectionAQency, "ANT Qualify Crlferla lor ParflCulale Matter and Sulfur OxldeJ." Vol. 2,EP&WD"J-82019b. 1982
Environ.Sci. Technol.. Vol. 18. No. 9, 1984
2691
FIGURE 2
Distribution of the mean and 90th percentile TSP concentrations bg/m3) of valid 1978 sites S 90th percentile
mmean
11 0.01 0.1 0.51 2
5 10 2030 506070 80 90 95 98 99 99.8 99.99 Percent of sites reponing annual mean
Sourn: EnvironmenlalProtectionAgency "A#, OvaliN Criteria lor Parti~ulateMatlei and Sulfur O~ides: MI. 2, EP&W1118-824Wb. 1982.
FIGURE 3
Predicted annual sulfate concentrations in 1990 from the Brookhaven long-distance transport model'
4
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 measure2101
Enviim. Scl. Tsdnol., Vol. 18. NO.9, 1984
ments (personal monitoring) or with estimates of exposures from data from fixed monitoring sites. In both aDproaches, knowlage of time--activiiy oatterns is imoortant. Direct oersonal konitoring 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 obtained (continuous or integrated) depends on available instrumentation for the pollutant under investigation. Most studies of personal exposures to respirable particles (RSP), Pb, N02,and VOCs, for example, have taken integrated 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 researchers 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 contributions of various locations or sources to total exposures measured with integrated sampling methods can be estimated 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 locations (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 tracers for the sources of interest (Gordon, 1980; Cooper and Watson, 1980). Tracer metals and compounds have been used as markers of the penetration 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 attempted in several studies and offer distinct advantages over direct personal measurements (Duan, 1982). Fixed or semiportable continuous recording instruments are more available
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than are personal samplers. A substantial number of high-quality continuous instruments and size-selective particle samplers are available. Concentrations obtained in various microenvironments are coupled with individual time-activity data to produce time-weighted exposure estimates. Indirect methods are usually less expensive than direct methods; however, the best of these measurements still provide only estimates of actual personal exposures. Statistical problems 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 prediction errors are needed. Research findings Results from continuous and integrated 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 exposures to many pollutants show little or no relationship to outdoor measurements and that personal exposures are often higher than, though poorly correlated with, outdoor measurements. The purposes of many of the studies summarized in Tables 1-6 have been to document the relationship between 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 15 pg the particles are inhalable (I aerodynamic diameter), and a still smaller fraction are respirable. The ratio of inhalable particulates (IP) to TSP (1P:TSP) has been shown to vary widely (0.49-0.88) in different cities, at different sites, and on different days within cities, owing to different sources of emissions and varying meteorological conditions (Feldman et al., 1981). Studies of human exposures to particles have usually measured the respirable fraction of the TSP, simultaneously taking integrated measures of personal exposures and indoor and outdoor concentrations. Measurements of personal exposures to RSP have been found to be strongly 272A
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correlated with indoor RSP levels, but only weakly with ambient levels. In addition, personal exposures have shown greater variability than outdoor samples (Dockery and Spengler, 1981 a). Personal and indoor levels have been found to be consistently higher than outdoor levels, and particulate matter from cigarette smoke has accounted for much of the higher indoor and personal concentrations. In fact, a study of 12-h exposures of nonsmoking adults demonstrated that reported passive smoke exposure increased the mean exposure by 20 pg/m3 (Spengler and Tosteson, 1981). These patterns are well illustrated with results from a 1981 Harvard School of Public Health study conducted in Kingston-Harriman, Tenn. (Spengler et al., 1984). The ambient and personal RSP measures were uncorrelated, and ambient concentrations explained less than 2% of the variability in personal exposures. The indoor RSP measure, however, explained nearly 50%. Smoke exposure alone accounted for 15% of the variability. A time-weighted model approxima ting the separate contributions of indoor and outdoor microenvironments to personal exposures only modestly improved the predictive capability of the indoor measure, explaining 65% of the variance (Spengler et al., 1981; Spengler et al., 1984). If the pollutant, however, is primarily of outdoor origin, outdoor concentrations are important determinants of personal exposures. Studies on exposures to SOz, for example, have shown that personal exposures are determined more by outdoor than by indoor concentrations. A study in Toronto, Canada, found that personal and outdoor SO2 levels were higher than indoor concentrations (Silverman et al., 1982). Analysis of particulate samples from Watertown, Mass., and Steubenville, Ohio, showed that the outdoor sulfate levels gave fairly good predictions of personal sulfate exposures (Dockery and Spengler, 1981 b). However, matches and mercaptans in gas cooking fuels have been identified as sources of indoor respirable sulfates (Dockery and Spengler, 1977). Studies of human exposures to CO in Los Angeles, Calif., have shown that high exposures occur during commuting periods and other times when individuals are near heavy traffic (Ziskind et al., 1982). Measurements from fixed-site monitors in a Boston, Mass., study generally underestimated 1-h commuter exposures, but adequately reflected 8-h averages (Cor-
tese and Spengler, 1976). In a Denver, Colo., study, cigarette smoke was associated with increased C O concentrations both in indoor environments and in personal exposures (Jabara and Keefe, 1980). The early results of an extensive EPA study on commuter CO exposure underscore the effects of in-transit exposures. Using continuous analyzers mated to activity data loggers, 814 commuting-hour samples from 638 Washington, D.C., subjects and 900 samples from 445 Denver, Colo., residents were obtained. The mean hourly concentrations were well below the 1-h C O NAAQS-8,3 ppm for Washington and 11.3 ppm for Denver. However, 1% of the Washington samples and 3.5% of the Denver samples exceeded 35 ppm (Ackland, 1984). Ambient NO2 levels have been shown to represent indoor levels and actual personal exposures only where there are no significant indoor sources. In a Topeka, Kan., study, individuals living in homes with gas stoves had NO2 exposures that were significantly higher than concentrations measured outdoors (Dockery et al., 1981). Researchers in Japan have been studying urinary concentrations of hydroxyproline and creatinine ( H 0 P : C ratios) and environmental factors such as direct and passive smoking and personal NO2 exposures. Earlier studies had shown elevated NO2 exposures for individuals from homes with gas and kerosene heaters (Nitta and Maeda, 1982). In Yanagisawa’s study of children and adult women, H 0 P : C ratios had a significant, direct correspondence to personal NO2 concentrations and passive smoke exposure. Personal NO2 and passive smoke exposures, however, were uncorrelated. This study suggests that hydroxyproline, a metabolite of body collagen degradation, is associated with two ubiquitous environmental contaminants (Yanagisawa et al., 1983 a; Yanagisawa et al., 1983 b). Summer and winter measurements of personal, indoor, and outdoor NO2 levels were taken in an 82-home study in Portage, Wis., to develop and test predictive models of personal exposures. A wide range in the variability in personal exposures could not be accounted for by stratification by stove types, as previous research had suggested. The best predictive model for personal exposures, explaining approximately 70%of the variance, included a direct measurement of home concentrations for each subject. The relationship between personal and
,nBLE 1
1
Studies of personal exposures to carbon monoxide ion am Cambridge, Mass.
Researchw to& CO meeSuements with
CO ex
lskind el al., 1982
638 subjects. 814 CO samples from Winter. 1982-83
concentrations ma lactors in lung disea
I ,T
Studies of personal exposures to nitrogen dioxide
Topeka, Kan.
NO. levels of homes with are thee times h i g h tha
aml OuMw samples also taken.
.
1 .
.. .. . ..
, 274A
Summer. 1982
Envirm. Sci. Technol.. VoI. 18, Nc. 9,1984
.'.. i
..
. .. . . . ..,. ,.
20 mle Mho01 children,24-h RSP, SO2, and No, samples, one day. Indoor and o u M m measurements a160 laken.
clowt assochtion of (wlnier). No coRe!ations in
em and spengler. 1 d School of Public
m e t a l . , igao 3steson, 1981; T W 381:Tostmm et al.
samples also taken.
PerSMal exposures to
Researcher took TSP and B[a] samples from village women during 4% min cwklng periods.
Personal exwares to a the combustionof "tradi
Sexton et SI., 1983; Sexton, 1983
I Envirm. Sa.Technol.. Vol. 18, No. 9. 19e4
2754
Studies of personal exposures to lead
Fuga% et ai., 1972;
Spring and winter, 1972
Wallace et ai.. 1982 Total Exposure Assessment Methodology mAM) Study
lllzzari etal., 1982: Sparaclno etal., 82: Wallace et al., 1983
Summar. fall. and winter, 1981
Personal and outdoor VOC concentrations are not well m e l a t HIOUS underestimation of personal exposures is possible due to low concanhations at wtdwr monitors.
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TABLE 6
Studies of personal exposures to ozone Houston. Tex.
Stock et al.. 1982 University of Texas school of Public Health Study
27 aslhmatlc subjects. 0 3 samples. weaklong NO? and formaldehyde samples. 12-h RSP samples, plus activity logs. Indoor.
outdoor. and central monitoring Site measurements for allstudy also taken. Summer and fall. 1981
Clear Lake City. Tex. Selwyn et al.. 1983 University of Texas School of Public Health Study
outdoor concentrations was strongest in the summer, when subjects spent more time outdoors and natural ventilation was highest (Quackenboss et al., 1984). Exposures to 0, occur predominantly outdoors. A study in Houston, Tex., found that indoor environments typically had negligible (
