Nitrogen Dioxide Inside and Outside 137 Homes ... - ACS Publications

Financial support for this research was provided in part by grants from NASA (NSG 7457 and NAG 1 -16O), NSF (AMT-8109047), the Dow Chemical Co., Biosp...
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Environ. Scl. Technol. 1983, 17, 164-168

Rasmussen, R. A,; Rasmussen, L. E.; Khalil, M. A. K.; Dalluge, R. W. J. Geophys. Res. 1980,85, 7350-7356. Crutzen, P. J.; Heidt, L. E.; Krasnec, J. P.; Pollock, W. H.; Seiler, W. Nature (London) 1979,282, 253-256. Logan, J. A. “Proceedings of the NATO Advanced Study Institute on Atmospheric Ozone: Its Variations and Human Influences”; Aikin, A. C., Ed.; Department of Transportation: Washington, DC, 1980.

(33) Rahn, K. A., University of Rhode Island, personal communication, 1982. Received for review May 24,1982. Accepted November 29,1982. Financial support for this research was provided in part by grants from NASA (NSG 7457 and NAG 1 -16O), NSF (AMT-8109047), the Dow Chemical Co., Biospherics Research Corp., and the Andarz Co.

Nitrogen Dioxide Inside and Outside 137 Homes and Implications for Ambient Air Quality Standards and Health Effects Research John D. Spengler” Department of Environmental Health Sciences, Harvard School of Public Health, Boston, Massachusetts 021 15

Colin P. Duffy Department of Horticulture, University of Wisconsin, Madison, Wisconsin 53706

Richard Letz Department of Physiology, Harvard School of Public Health, Boston, Massachusetts 02115

Theodore W. Tibbitts Department of Horticulture, University of Wisconsin, Madison, Wisconsin 53706

Benjamin 0. Ferris, Jr. Department of Physiology, Harvard School of Public Health, Boston, Massachusetts 02115

Week-long integrated nitrogen dioxide (NO,) measurements were made by using diffusion tube samplers inside and outside 137 homes in Portage, WI, over a 1-year period. The annual mean ambient NO2 concentrations in this rural community were 10-15 pg/m3. NO2levels inside the kitchens of 112 homes with gas stoves averaged about 50 pg/m3 higher, and bedroom levels were about 30 pg/m3 higher, than outdoor levels. Ten percent of the gas-cooking homes had annual average kitchen NO2levels higher than the National Ambient Air Quality Standard of 100 pg/m3. NO2levels inside kitchens of 25 homes with electric stoves were about two-thirds outdoor levels, while corresponding bedroom levels were one-half outdoor levels. Distinct seasonal patterns (higher indoor levels in winter, lower in summer) consistent with changes in normal air-exchange rates were evident in gas-cooking homes. The large variation of NO2 concentrations among homes, likely due to differences in stove use, emission rates, and air-exchange rates, limits the development of prediction models. In addition, this variation would reduce the power of epidemiological studies of respiratory health, which use ambient NO2concentration levels, a simple dichotomous description of stove type and two categories of home cooking fuel to describe exposure.

Introduction Most nitrogen dioxide in the ambient atmosphere is produced by the oxidation of nitrogen oxide formed during high-temperature combustion. Nationally, automobiles and stationary fuel combustion are responsible for equal amounts of NO, emissions and together yield 95% of total estimated emissions. Annual ambient concentration averages range fsom 0.001 ppm in rural areas to approximately 0.08 ppm in some urban areas. While ambient air concentrations in most locations are below the annual 164

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National Ambient Air Quality Standard (NAAQS) of 100 pg/m3 (0.05 ppm), it is now well documented that indoor NO2 concentrations often exceed ambient concentrations when gas-burning appliances are used (1-7). Cooking with gas is identified as the principal source for high indoor concentrations, although gas hot water heaters, gas clothes driers, and gas and kerosene space heaters may contribute to elevated indoor levels. When ambient levels and airexchange rates are low, the indoor long-term concentrations can be 5 times higher than the outdoor concentrations. The short-term 5-min to 1-h concentrations can exceed 1000 pg/m3 (0.5 ppm) (8,9).Concentrations this high are rarely observed in ambient air. In 1977, only ten locations reported days where the short-term peak (1h) exceeded 500 pg/m3. Six of these locations were in southern California (10). Some recent epidemiologic studies indicate the possibility of health effects presumably associated with NO2 exposure. A British study by Melia et al. examined 1810 children, 5-11 years old, living in gas-cooking homes, and 3017 children living in electric-cooking homes (11). Although the relative rates varied between boys and girls and among age groups, a higher prevalence of respiratory symptoms and disease was noted for the children dwelling in gas-cooking homes. Keller et al. (12)reported no difference in respiratory symptoms between household groups living in gas- and electric-cooking homes. This U.S. study has been criticized as being insensitive because of the small number of subjects (ca. 600), which were not age stratified in the analysis. In another U.S.study (8) involving some 8000 children 6-10 years old, residing in six cities, respiratory diseases before age 2 and lower pulmonary functions were associated with the presence of gas stoves. While the data analysis controlled for social status and parental smoking, boys and girls were not treated sepa-

00~13-936X/83/0917-0164$01.50/0 0 1983 American Chemical Society

rately in the analysis. The findings of impaired lung functions for the children living in homes with gas stoves was noted for every city; however, the medical significance is not yet clear. In a recent study of 708 nonsmoking adults Comstock et al. (13) found both increased symptoms of respiratory illness and decreased lung function among individuals from households with gas cooking fuel compared to individuals from electric-cooking households. These effects were present after adjusting for sex, age, and several variables associated with social class. All of these reports of adverse health effects associated with gas stoves are presumed to be related to elevated short-term NO2 concentrations. Effects of both single and repeated exposure to short-term peaks have been reported for a variety of animal and human responses (1). A cooperative research program between the University of Wisconsin at Madison and Harvard School of Public Health has been conducted to improve estimates of exposure to NO2 for children participating in a prospective epidemiologic study (14). As part of this cooperative program nitrogen dioxide concentrations were monitored inside and outside 137 homes in Portage, WI. Homes with gas- and electric-cookingstoves were repeatedly monitored for 1-week intervals over a 1-year period by using passive diffusion samples developed by Palmes et al. (15). The objectives of this study were to characterize spatial and temporal variation of NO2 concentrations between and within homes and to develop a model to predict NO2 exposure at home by using descriptive variables obtained from a health survey as well as ambient NO2 levels.

Table I. Sampling Summary stove type

sample weeksa by period: 07/01/80-08/27/80 08/28/80-09/30/80 10/01/80-11/02/80 11/03/80-12/31/80 01/01/81-02/14/81 02/15/81-03/31/81 04/01/81-05/13/81 05/14/81-06/30/81

(16).

Quality assurance procedures included two blanks and at least one replicate sample for every 18 samples, and analysis of unknowns prepared by an independent laboratory. Our initial determination of laboratory performance showed excellent agreement (within 2%) between the Wisconsin and Harvard laboratories. The Palmes diffusion tubes have been examined by Warren Springs Laboratories (17)and the National Bureau of Standards (18). These laboratories reported similar sensitivity of the present configuration of the diffusion tubes as 1150 (pg/m3) h q d an accuracy of within 10%.

23 21 20 23 22 22 23 20 174 25 4

total homes dropouts a

31 32 31 30 29 28 29 29 240 36 3

72 76 72 76 65 72 66 67 568

76 3

Times three tubes at each location.

ELEC-EEDROOII ELEC-KIlCHEN

.

A

4

GUS-BEDROOI

.ORS-KIlCHEN

0

Procedures Participants were selected from a list of families living in Columbia county around Portage, WI, whose elementary-school-aged children participated in the Harvard Air Pollution/Lung Health Study (24). Letters requesting volunteers were preferentially sent to families with gas cooking. Subsequently, we identified the homes that used natural gas and bottled liquid propane as cooking fuel. At least three diffusion tubes were installed in each home. A kitchen monitor was placed on a wall between 4 and 6 feet above the floor and no closer than 10 feet to the stove. A bedroom sampler was similarly placed on a wall away from corners, windows, and forced air vents, in the bedroom of a child. Outdoor monitors were 4-6 feet above the ground on the north or shady side of the house. In an initial visit volunteers were instructed on how the tubes worked, and suitable sampling locations were selected. In the subsequent sampling intervals tubes were mailed with return envelopes. The participating families were instructed to attach the diffusion tubes to the proper locations, uncap them, note the time and date, recap them after 7 days, record the time and date, and return them by postal service to o w staff at the University of Wisconsin at Madison. Analysis was performed at the Wisconsin State Laboratory of Hygiene following a procedure developed by Palmes et al. (15)and modified by Wolfson

natural liquid gas propane

electric

50

100

NO2 C O N C E N T R A T I O N

150

lug/m31

Figure 1. Cumulative frequency distributions of NO2 concentrations by location and type of cooking fuel.

The sampling protocol required homes to be tested for a week-long (ca. 168 h) period twice per season for a total of 8 sample weeks per home. Because of travel and installation times, groups of approximately 20 homes were sampled each week on a 6-week rotating basis. The six groups of homes remained relatively stable throughout the study.

Results A summary of sample collection is presented in Table I. A small number of observations were lost due to improper capping or recapping in the field or the normal laboratory errors. These losses were not related to stove type or time of sampling. Also, three to four homes of each stove-type dropped out of the study during the sampling year. The number of homes sampled during each time period remained stable over the 1-year period reported. Analysis of replicate pairs indicated a high degree of precision of measurement throughout the study. The difference between replicates was less than 8 pg/m3 for 98% of the replicate pairs. Difference scores did not increase with increasing concentration, indicating that the measurement error was additive, not multiplicative. The calculated estimate of precision (the square root of half the variance of the replicate difference scores) was 1.68 pg/m3. There was no trend toward a change of precision over time nor among outdoor, kitchen, and bedroom locations. Cumulative frequency distributions for outdoor observations and indoor observations by stove type are presented in Figure 1. Summary statistics for these distributions are presented in Table 11. First, note the extremely low outdoor concentrations observed. The mean outdoor concentration was 13 pg/m3. Next, note the large Environ. Sci. Technol., Vol. 17, No. 3, 1983

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Table 11. Summary Statistics of NO, Concentrations

electric kitchen bedroom outdoor natural gas kitchen bedroom outdoor LP gas kitchen bedroom outdoor

mean,

sd

N

m3

m

174 172 173

8.4 6.9 12.8

4.7 4.8 5.6

/a/

mean geoI/Oa metric differmean, ence, Pg/ P€![ m3 m 7.3 5.4 11.7

-4.4b -6.0b

median

l/oa

ratio 0.65 0.51

s

. I

237 238 240

65.5 36.7 15.8

30.7 19.0 6.3

57.4 32.3 15.6

49.ab 20.gb

3.92 2.24

568 568 555

65.6 37.6 11.8

38.4 27.4 5.6

55.9 30.0 10.7

53.7b 25.7b

5.23 2.83

Indoor/outdoor. difference was zero.

p < 0,001 that the true mean 1/0

increase over ambient NO2levels inside gas-cooking homes, with kitchen levels higher than bedroom levels. Finally, note that then NO2 levels inside electric-cooking homes were below the low outdoor levels. Table I1 demonstrates that ambient NO2concentrations outside liquid-propane-cooking (LP) homes were apparently slightly lower than those outside natural gas-cooking (NG) and electric-cooking (E) homes. More of the liquid-propane-cooking homes were located in rural areas where natural gas pipelines were not available. For this reason, difference scores and ratios between each indoor sampler and the outdoor sampler for each site for each sampling period were computed. Mean values for these scores are also presented in Table 11. For example, kitchen NO2concentrations were on the average 49.8 pg/m3 higher than outdoors at that location for NG homes and 53.7 pg/m3 higher for LP homes. Conversely, kitchens with electric stoves had NO2 concentrations consistently (4.4 pg/m3, 35%) lower than the concentration outdoors at that site. In addition, bedroom concentrations were typically 50-80% of kitchen values in gas-cooking homes but still twice as high as outdoor concentrations. Surprisingly, bedrooms concentrations inside electric homes were not only lower than outdoor values (6.0 pg/mS, 49%),but also consistently lower than kitchen values sampled at the same time. This implies greater air infiltration in the kitchen area than in bedrooms. In sum, NO2concentrations inside gas-cooking homes were substantially higher than low outdoor concentrations, while concentrations inside electric cooking homes were lower than the outdoor concentrations. Note that all these differences are evident whether one refers to the means, geometric means, difference scores, or ratios. The greater variance of concentrations inside LP and NG homes compared to those inside E homes or to outdoor values was due to systematic variation over time (seasons) and between homes. Mean concentrations for each sampling period are presented in Figure 2. First, note that NO2 concentrations inside LP and NG homes increased dramatically during the fall and winter seasons. Second, bedroom levels in LP and NG homes showed a similar pattern. Finally, kitchen and bedroom NO2concentrations in E homes decreased slightly during the fall and winter seasons. A three-way analysis of variance model of three stove types (E, NG, LP) by three locations (kitchen, bedroom, outdoor) by the eight sampling periods was fitted both to the concentrations and (separately) to their logs. A sum166

t

Envlron. Sci. Technol., Vol. 17, No. 3, 1983

. 2

. 3

.

.

V

5

PERIOD IJULY. 1980

-

.

.

.

8

7

8

I

JUNE, 19811

Flgure 2. Mean NO2 concentrations for eight sampllng periods by locatlon and type of cooking fuel.

Table 111. Summary of Analyses of Variance concn source

DF

model (R*= 0.532 and 0.685) stove location stove X location period stove X period location X period stove X location. X period

71

F

log concn

P

45.64 0.0001

F

P

87.22 0.0001

2 364.74 0.0001 1150.72 0.0001 2 865.94 0.0001 1181.62 0.0001 4 114.97 0.0001 293.64 0.0001 7 21.55 0.0001 12.65 0.0001 14 4.12 0.0001 9.63 0,0001 14 5.79 0.0001 4.86 0.0001 28 1.08 0.3482 2.21 0.0003

mary of these analyses is presented in Table 111. The model accounted for 53% of the variance of the concentrations and 69% of the variance of their logs. Residuals from fitting the model to the concentrations showed a pattern of increasing spread with increasing concentration. This pattern was not found in the log-concentration residuals, so the following interpretations apply to fitting the model to the logs of the concentrations. The significant main effects of stove, location, and period indicate that NO2 concentrations varied systematically between indoors and outdoors, between gas and electric homes, and over time (Le., over primarily seasons). The significant "stove X location" interaction term can be interpreted (see Figure 2) as demonstrating that NO2 concentrations were higher indoors in gas homes than outdoors, while they were lower indoors than outdoors in electric homes. Finally, the significant "stove X location X period" interaction indicates that the previous pattern differed by sampling period. In an effort to develop a simple prediction model, a regression model with an ambient concentration effect and a period X gas stove factor was fitted separately to the kitchen and bedroom concentrations. Unfortunately, this model only accounted for 39% of the variance of the kitchen values and 28% of the variance of the bedroom values. The ambient concentrations alone accounted for 1%of the variance of the kitchen concentrations for gas homes and 21% for electric homes. The relatively poor fit achieved by the regression model was likely due to the great variation in indoor NO2 concentrations among gas stove homes. Differences among homes in ventilation, gas appliance use, and combustion emission rates likely led to this variation. Mean concentrations for each home (averaged over four to eight sampling periods distributed over the year) ranged between 11and 161 pg/m3. Thirteen homes had overall mean kitchen concentrations exceeding the NAAQS of 100 pg/m3, and two homes had mean bedroom concentrations exceeding that standard. This represents more than 10% of the 112 gas homes sampled. Analysis of residuals from

fitting the analysis of variance model revealed some homes with observed concentrations consistently higher than predicted (and lower for others).

Discussion Indoor NO2 concentrations have been shown to vary among and within homes. Portage, WI, has a clean outdoor environment. Annual average NO2 concentrations range between 7 and 27 pg/m3. In this study, kitchen NO2 levels in homes with gas stoves were about 50 pg/m3 higher than outdoor levels, and NOz levels were about 4 pg/m3 lower than outdoor levels in electric-cooking homes. In addition, there appears to be a strong seasonal effect on indoor NOz levels, presumably caused by differences in air-exchange rates and possibly cooking as well. Also,NO2 concentrations vary spatially within a home, with bedroom levels typically half of kitchen levels in gas-cooking homes. Finally, great variation in long-term average NOz levels exists among homes within the gas-cooking home category. Thirteen gas-cooking homes has average kitchen NO2 concentrations exceeding the NAAQS of 100 pg/m3, an average excess of 103 pg/m3 over their outdoor concentrations. This among-home variation did not allow development of a good regression model for predicting indoor NO2 concentrations from knowledge of ambient NO2levels and type of cooking fuel in the home in our study. Similarly, regression models accounted for little of the variance in the British study (4) and only a modest amount in the Dutch study (20). Knowledge of additional variables such as surrogates for &-exchange rates (air conditioning, storm windows, degree days for heating and cooling), dilution (vent use, room and home volume), removal (surface area, type of floor covering), and additional source terms (fuel use, smoking) might reduce some of this unexplained variance. Excess NOz above ambient levels in kitchens and bedrooms of LP gas homes were about 4 pg/m3 higher than in natural gas homes. These differences are small, and there may have been systematic differences between the LP and NG homes. The LP homes were rural and the NG homes were not. However, an 8% difference in NOz concentrations is consistent with reported differences in NO emission rates from these fuels. Hayhurst and Vince (19) report about 20% more NO formation in flames by “prompt” NO reactions per megajoule of energy for propane combustion than for methane combustion. On average about half of the flame-oxided NO is produced by prompt NO reactions. This slight increase in NO emissions for propane combustion would be expected because “prompt NO” is thought to result from reaction of hydrocarbon fragments (more prevalent in propane vs. methane) with molecular nitrogen. The indoor NO2 concentrations reported here are high but somewhat lower than those reported for homes with NO2 sources in England, The Netherlands, Japan, and in Florida. In a study using diffusion tubes samplers for l-week sampling times in winter in England, Goldstein et al. (4) report a mean across 428 homes with gas stoves of 202 pg/m3 and a mean of 32 pg/m3 in homes without gas stoves, while ambient concentrations were about 35 pg/m3. Lebret et al. (20) report mean levels for 7-day diffusion tube samples across 236 Dutch homes of 118 pg/m3 with unvented instantaneous water heaters, with ambient levels of about 35 pg/m3. In Japan, where unvented space heaters are a major source of indoor NO2, mean dining room levels of 87 pg/m3 and corresponding ambient levels of 36 pg/m3 are reported by Maeda and Nitta (21) for a study of 10 homes in Tokyo using 24-h diffusion badge

samplers. Similarly, Palmes et al. (22) reported week-long integrated averages of 105 pg/m3 in homes with gascooking stoves and levels of over 200 pg/m3 in homes having both gas stoves and unvented gas space heaters. Great variation in NOz concentrations among homes with gas appliances similar to that reported here was evident in these other studies. The results of the present study have implications for both NO2 ambient air quality standards and research on the health effect on NOz. Ten percent of the gas-cooking homes monitored in this relatively pristine area had annual mean kitchen NO2 levels exceeding the U.S. annual NAAQS of 100 pg/m3. This percentage would undoubtedly be higher in most urban and suburban areas in the US.(23). Given that people spend on average almost 70% of their time indoors at home (24,25) and that gas stoves are used for cooking in 49% of American homes (26), this represents an enormous population exposure to NOz. This indoor exposure would render it unlikely that any reliable effects on health could be directly related to ambient NO2 levels. In addition, ambient air quality standards set for public health reasons without reference to indoor levels (where most of the exposure occurs) seem inappropriate. Changes in ambient air quality might result in little change in actual NO2 exposure. The evidence for NOz damage to lung tissue and increased infection rates is substantial at concentrations higher than commonly reported occurring indoors (1,27, 28). In addition, health effects based upon the classification of NO2 exposure by stove type have been reported. These effects were on average small and required large sample sizes to detect them. Epidemiologic studies may not have detected significant and consistent health effects for several reasons: 1. Classifying exposure to NO2 in a population on the basis of cooking fuel has been done in several epidemiological studies. However, in the present study we found large variation in NO2 levels within the gas-cooking home category, with some in the range of the electric-cooking homes. This large variation would be expected to weaken the association between averaged health effects and the indoor pollutant categories. 2. Only a small subset of the population may be affected by gas stove combustion products. This would result in more variation within exposure groups and less power in detecting mean differences between groups. 3. Short-term elevated NOz concentrations may cause the health effects. If the “peak-to-mean” relationship varies greatly from home to home and time to time, then only a subgroup in gas-cooking homes would receive the appropriate stimulus, and a small average effect would result. 4. Pollutants other than NOz may cause the health effects. In one study seeking to relate NOz to respiratory illness, no increase in respiratory illness with increasing NO2 measured in the home was found (29). Gas combustion emission products include water vapor, carbon monoxide, carbon dioxide, nitric oxide, sulfur dioxide, formaldehyde, carbon particles, and sulfate particles (30). Variation in concentrations of the offending agent(s) among gas fuels and over time would weaken any observed effect. 5. Statistical correction for factors correlated with NOz might weaken any observed effect. For example, if socioeconomic status (SES) is controlled for in the analysis, and SES is correlated with NO2 exposure (such as gas stoves being used for supplementary heat in lower SES homes) (31),then removing the variation due to SES would Environ. Scl. Technol., Vol. 17, No. 3, 1983

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reduce the amount of variation attributed to gas cooking fuel alone. Evidence from epidemiologic investigations may be improved by better understanding of individual sensitivity and the nature of the damaging stimulus as well as by better estimates of population exposure. Exposure estimates may be improved dramatically by a better understanding of the factors affecting indoor source strength, air-exchange rates, the peak-to-mean relationship, cooking patterns, and human activity patterns. Acknowledgments

This study would not have been possible without the kind and reliable cooperation of participating families in the Portage, WI, area. We acknowledge the dedicated efforts of Debra Parker of the University of Wisconsin, who assisted in the field sampling and reporting. The coorperation of the Wisconsin State Laboratory of Hygiene and the careful work of Aliene Mason are appreciated. Registry No. Nitrogen dioxide, 10102-44-0. Literature Cited (1) “Air Quality Criteria for Oxides of Nitrogen”;Environmental Criteria and Assessment Office, Office of Research and Development, U S . Environmental Protection Agency, Research Triangle Park, NC, 1980. (2) Belles, F. E.; Himmel, R. L.; DeWerth, D. W. Paper 75.09.1 at the 68th Annual Meeting of the Air Pollution Control Association, Boston, MA, 1975. (3) Thompson, C. R.; Hensel, E. G.; Kats, G. J. Air Pollut. Control Assoc. 1973,23,881-886. (4) Goldstein, B. D.; Melia, R. J. W.; Chinn, S.; Florey, C.; Clark, D.; John, H. H. Int. J. Epidemiol. 1979,8,339-345. (5) Moschandreas, D. J.; Stark, J. W. C.; McFadden, J. C.; Morse, S. S. “Indoor Air Pollution in the Residential Environment”; Geomet, Inc. EPA 600/7-78-229 AB, Washington, D.C., 1978. (6) Spengler, J. D.; Ferris, B. J., Jr.; Dockery, D. W.; Speizer, F. E. Environ. Sei. Technol. 1979,13,1276-1280. (7) Palmes, E. D.; Tomczyk, D.; DiMattio, J. Atmos. Environ. 1977,11,869-872. (8) Speizer, F. E.; Ferris, B. G., Jr.; Bishop, Y. M. M.; Spengler, J. D. A m . Rev. Resp. Dis. 1980,121,3-10. (9) Wade, W. A., 111;Cote, W. A.; Yocom, J. E. J . Air Pollut. Control Assoc. 1975,5,933-939. (10) “The Tenth Annual Report of the Council on Environmental Quality”; Council on Environmental Quality, US. Government Printing Office: Washington, D.C., 1979. (11) Melia, R. J. W.; Florey, C.; Chinn, S. Int. J. Epidemiol. 1979, 8,333-338. (12) Keller, M. D.; Lanese, R. R.; Mitchell, R. I.; Cote, R. W. Enuiron. Res. 1979,19,495-515.

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(13) Comstock, G. W.; Meyer, M. B.; Helsing, K. J.; Tockman,

M. S. Am. Rev. Resp. Dis. 1981,124,143-148. (14) Ferris, B. G., Jr.; Speizer, F. E.; Spengler, J. D.; Dockery, D. W.; Bishop, Y. M. M.; Wolfson, M.; Humble, C. Am. Rev. Resp. Dis. 1979,120,767-779. (15) Palmes, E. D.; Gunnison, A. F.; DiMattio, J.; Tomczyk, C. Am. Ind. Hyg. Assoc. J. 1976,37,570-577. (16) Wolfson, M. ”Modifications to the Palmes Diffusion Tube Preparation and Analysis Methods”; Harvard Six City Study Quality Assurance Document, Boston, MA, 1980: Vol. 11. (17) Apling, A. J.; Stevenson, K. J.; Goldstein, B. D.; Melia, R. J.; Atkins, D. H. F. Warren Spring Laboratory Report LR 311(AP), Herefordshire, England, 1979. (18) Cadof, B. C.; Knox, S. F.; Hodgeson, J. A. “Personal Exposure Samplers for NO,”; Draft report of the National Bureau of Standards, Washington, D.C. 1979. (19) Hayhurst, A. N.; Vince, I. M. Prog. Energy Combust. Sci. 1980,6,35-51. (20) Lebret, E.; Brunekreef, B.; Boleij, J. S. M. Paper presented at the International Symposium on Indoor Air Quality, Health and Energy Conservation, Amherst, MA, Oct 13-16, 1981. (21) Maeda, K.; Nitta, H. Paper presented at the International Symposium on Indoor Air Quality, Health and Energy Conservation, Amherst, MA, Oct 13-16, 1981. (22) Palmes, E. D.; Tomczyk, C.; March, A. W. J . Air Pollut. Control Assoc. 1979,29,392-393. (23) Sexton, K.; Letz, R.; Spengler, J. D. Environ. Res., in press. (24) Szalai, A., Ed. ”The Use of Time: Daily Activities of Urban and Suburban Popultions in Twelve Countries”; Mouton: The Hague, Netherlands, 1972. (25) Chapin, F. S. “Human Activity Patterns in the City”; Wiley-Interscience: New York, 1974. (26) “AnnualHousing Survey, Part F”,Current Housing Reports Series H-150, US. Bureau of the Census, Washington, D.C., 1978. (27) Ferris, B. G., Jr. J. Air Pollut. Control Assoc. 1978,28, 482-497. (28) Lee, S. D. Ed. “Nitrogen Oxides and Their Effects on Health”; Ann Arbor Science: Ann Arbor, MI, 1980. (29) Florey, C.; Melia, R. J. W.; Chinn, S.; Goldstein, B. D.; Brooks, A. G. F.; John, H. H.; Craighead, I. B.; Webster, X. Int. J. Epidemiol. 1979,8,347-353. (30) Traynor, G. W.; Anthon, D. W.; Hollowell, C. D. Atmos. Enuiron., in press. (31) Sterling, T. D.; Kobayashi, D. J. Air Pollut. Control Assoc. 1981,31,162-165.

Received for review May 20,1982.Accepted November 4,1982. This study, a component of HarvardS Air Pollution Respiratory Health Study, was supported by contracts from Wisconsin Power Companies and the Electric Power Research Institute (RP1001-1) and a grant from the National Institute of Environmental Health Sciences (ES01180).