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Personal NO2 exposures of high school students
Environmentlnternational, Vol. 12, pp. 413-417, 1986 Printed in the USA. All rights reserved.
0160-4120/86 $3.00 + .00 Copyright © 1986 Pergamon Journals Ltd.
PERSONAL NO, EXPOSURES OF HIGH SCHOOL STUDENTS Pieter Clausing and Jan Karel Mak Agricultural University,Department of Air Pollution, Wageningen, The Netherlands
John D. Spengler a and Richard Letz Department of EnvironmentalScience and Physiology, Harvard School of Public Health, Boston, Massachusetts 02115, USA (Received 1 June 1985; Accepted 17 September 1985) Personal NO2 exposures and time activity patterns were measured during two sampling periods, for 500 junior high school students (aged 12-14) in Watertown, MA. NO2 concentrations in the bedroom, living room, and kitchen were measured for 100 students in each period with Palmes' NO2 diffusion tubes. Indoor and personal NO2 levels were closely related to the type of cooking fuel used in the home. The correlation between outdoor and personal NO2 levels was not significant. Step-wise multiple regression techniques were used to fit models to estimate personal exposures. Models including indoor NO2 concentrations explained 60% to 90% of the variance in personal NO2 exposures. When indoor concentrations were excluded, only about 40% of the variance was explained by cooking fuel, presence of pilot lights, and NO2 concentration at school. Exposures of students measured in both periods were used to determine within-person and between-person variability. Within-person variability was found to be large, indicating that a single person exposure measurement is not sufficient to characterize a person's exposure.
Introduction T h e s u s p e c t e d h e a l t h e f f e c t s o f NO2 h a v e b e e n i n v e s t i g a t e d in m a n y e p i d e m i o l o g i c a l s t u d i e s , w i t h i n c o n s i s t e n t r e s u l t s ( F l o r e y et a l . , 1979; K e l l e r et al., 1979; S p e i z e r et a l . , 1980). In s o m e o f t h e s e s t u d i e s the t y p e o f c o o k i n g fuel u s e d at h o m e w a s u s e d as a m e a s u r e o f e x p o s u r e to NO2 ( F l o r e y et al., 1979; K e l l e r et al., 1979; S p e i z e r et a l . , 1980), a n d in o t h e r s t u d i e s the i n d o o r NO2 c o n c e n t r a t i o n s w e r e u s e d ( G o l d s t e i n et a l . , 1979; F l o r e y et a l . , 1979). T h e r e l a t i o n b e t w e e n indoor NO 2 concentrations and personal NO 2 exposures has only been investigated for small study popu l a t i o n s ( N i t t a a n d M a e d a , 1981; Q u a c k e n b o s s et al., 1981). It w a s f o u n d that i n d o o r NO2 l e v e l s a r e i m p o r tant in d e t e r m i n i n g p e r s o n a l N O 2 e x p o s u r e s , p a r t i c u l a r l y w h e n u n v e n t e d N O 2 s o u r c e s s u c h as g a s s t o v e s o r h o t w a t e r h e a t e r s a r e p r e s e n t ( M o s c h a n d r e a s et al., 1981; Q u a c k e n b o s s et a l . , 1981; S p e n g l e r et al., 1981). T h e r e l a t i v e l y l a r g e f r a c t i o n o f t i m e that p e o p l e s p e n d i n s i d e t h e i r h o m e s ( S p e n g l e r et al., 1981) c o n aTo whom all correspondence should be addressed.
t r i b u t e s to the i m p o r t a n c e o f i n d o o r N O 2 l e v e l s . W i t h i n the f r a m e w o r k o f the H a r v a r d A i r P o l l u t i o n / H e a l t h S t u d y ( F e r r i s et al., 1979), a s t u d y w a s c o n d u c t e d to i n v e s t i g a t e the l e v e l a n d v a r i a b i l i t y o f NO2 e x p o s u r e s o f j u n i o r h i g h s c h o o l s t u d e n t s in W a t e r t o w n , M A , a n d the r e l a t i o n s h i p b e t w e e n p e r s o n a l e x p o s u r e s a n d i n d o o r NO2 l e v e l s . V a r i a b i l i t y o f the e x p o s u r e v a r i a b l e s o v e r t i m e for the s a m e p e r s o n o r h o m e (unit) is a l s o o f interest. W h e n u s e d in a h e a l t h - e f f e c t s s t u d y , a r a n d o m e r r o r in the e x p o s u r e v a r i a b l e w i l l d e c r e a s e the c o r r e l a t i o n b e t w e e n the m e a s u r e o f h e a l t h e f f e c t a n d the e x p o s u r e v a r i a b l e s . It w i l l a l s o t e n d to b i a s the e s t i m a t e o f h e a l t h e f f e c t t o w a r d s z e r o . A n e s t i m a t e o f this t y p e o f e r r o r c a n b e o b t a i n e d b y r e p e a t i n g the e x p o s u r e m e a s u r e m e n t s . T h e v a r i a n c e in the e x p o s u r e v a r i a b l e s c a n t h e n b e d e c o m p o s e d in a w i t h i n - u n i t v a r i a n c e a n d a b e t w e e n - u n i t v a r i a n c e . W h e n the w i t h i n - u n i t v a r i a n c e is s m a l l c o m p a r e d to the b e t w e e n - u n i t v a r i a n c e , this i n d i c a t e s that m o s t o f the v a r i a t i o n in the e x p o s u r e v a r i a b l e is d u e to d i f f e r e n c e s b e t w e e n units. A r e l a t i v e l y l a r g e w i t h i n - u n i t v a r i a n c e i n d i c a t e s that it w i l l b e difficult to e s t a b l i s h b e t w e e n - u n i t d i f f e r e n c e s on 413
414 the basis o f a single measurement per unit. The relationship between these components of variance o f the exposure variables and the estimated effect is more extensively discussed by B r u n e k r e e f et al. (1985) and Ozkaynak et al. (1985). Although initially this study was not set up for this purpose, the data were used to get some notion o f the variability in time of the exposure variables.
Methods and Materials The study was carried out during two periods: four days in N o v e m b e r and three days in December 1982. Students in the seventh and eighth grade (aged 12-14) from the two public high schools in Watertown, MA, were asked to volunteer for the study. Letters for permission to participate were sent to the parents. Objectives and set-up o f the study were explained in the letter. Class visits were made to explain the study to the students, to give instructions and to distribute the materials. The study started with 250 students in each period. Personal NO2 exposures o f all students were measured. In a subpopulation o f 100 students in each period, NOz concentrations were also measured within the home (kitchen, living room, and bedroom) and outside the home. Palmes' NO2 diffusion tubes were used for all NOz measurements. The students received the NOz monitors at school and were instructed where to place the monitors in their homes. NO2 was also monitored in a few locations in both participating schools. Directly after the NOz measurements, the NO2 monitors were analyzed using the method described by Palmes et al. (1976) with modifications by Wolfson (1980). During the sampling days, all participants kept track of their activities in a small diary. Time spent at home, outside, in travel, in school, and in other indoor locations was recorded to the nearest half hour. Time that the personal monitor was not worn by the person, other irregularities with the monitor, and time spent in the kitchen during stove or oven use were also recorded. Questionnaires concerning home characteristics with a potential influence on indoor NO2 levels (among others: cooking fuel; presence o f a pilot light on the stove; presence and use o f a ventilation system in the kitchen; number o f cigarettes smoked in the house; heating fuel) were filled out by all students. For the second period the questionnaires were simplified; only the questions about cooking fuel, pilot light, ventilation system, and the use of oven and burners were maintained.
Data Analysis Multiple linear regression analysis with the SPSS statistical package (Nie et al., 1975; Nie and Hull,
P. Clausing et al. 1981) was used to test various models to estimate personal N O z exposures. The selection of the variable sets used in this analysis was based on expected contributions of variables to personal exposures and on preliminary statistical analysis. The following sets of independent variables were introduced in the models: 1. measured indoor NO2 concentrations; 2. NO2 concentration at school and d u m m y variables for cooking fuel, presence o f pilot light, presence and use o f ventilation in the kitchen, and sex; 3. set 2 plus time fractions spent in each of the five locations; 4. set 3 plus various combinations o f other home characteristics. The time spent in the kitchen during stove or oven use was not included in the analysis since the answering of this question in the diary was often incomplete. The data from each sampling period were handled separately. For each model the multiple correlation coefficient (R 2) and the standard error o f estimate (SEE) are presented. The stepwise inclusion method was used, examining variables at each step for entry in or removal from the regression equation at a significance level of 0.10. The exposure variables from students who participated in both periods were analyzed using the analysis of variance technique " R e l i a b i l i t y , " available in the SPSS statistical package (Nie and Hull, 1981). Analysis was p e r f o r m e d for the whole group and for the gas and the electric group separately. Analysis was also performed after exclusion o f four o f the personal exposures " o u t l i e r s " and one o f the indoor concentrations. The within-unit variance and the between-unit variance are presented.
Results Data Base
About 150 (60%) complete and useful data sets were retrieved in each period, o f which 67 and 48 sets contained h o m e measurements. A number of the students never gave the materials back, some did not cooperate well enough, or did not understand the proceedings. In some cases the NO2 data were useless because exposure times were unknown since diaries were filled out improperly or not returned. Personal exposures were r e m o v e d from the data file when the monitor was worn for less than 80% o f the time. About 140 students participated in both periods, resuiting in 83 useful data sets for the analysis o f variance. Twenty-four of these contained indoor NO2 measurements.
Personal NO2 exposures
415 Table 1. Indoor, outdoor and personal NO2 concentrations (in ttg/m ~) classified for cooking fuel and sampling period. Gas
Period
Electric
Location
Mean
S.D.*
Range
n
Mean
S.D.*
Range
personal bedroom living room kitchen outdoors**
46 45 52 74 37
16 16 18 27 6
23-134 15-86 21-97 29-161 27-65
128 60 60 60 72
26 19 19 24 37
9 4 4 13 6
12-50 12-25 14-27 13-74 27-65
35 17 18 18 72
personal bedroom living room kitchen outdoors**
49 46 60 86 46
16 24 27 41 6
17-109 10-131 12-125 11-195 36-61
127 50 51
22 14 16
6 5 5
51
18
8
28
46
6
12-40 6-21 8-22 6-29 36-61
36 10 10 10 28
*Standard deviation **The outdoor concentrations are averaged over the gas and the electric homes.
Table 2. Summary statistics of time activity data. Percentages of time spent in each location
Period 1
Period 2
Location
Mean
S.D.
Range
n
Mean
S.D.
Range
n
Home School Outside Travel Other indoors
66.4 19.0 7.1 1.4 6.2
8.8 4.2 5.2 2.3 6.0
39.1-100 0-30.7 0-26.0 0-18.8 0.29.7
177 177 177 177 177
63.8 26.0 5.9 0.7 3.7
6.9 4.4 4.5 1.4 4.5
39.9-86.6 8.3-34.7 0-22.2 0-11.1 0-26.4
192 192 192 192 192
NO2 M o n i t o r i n g
Multiple Linear Regression Analysis
A v e r a g e i n d o o r , o u t d o o r a n d p e r s o n a l NO2 l e v e l s are given for each cooking fuel and sampling period in T a b l e 1. I n b o t h p e r i o d s i n d o o r c o n c e n t r a t i o n s a n d p e r s o n a l e x p o s u r e s w e r e c l o s e l y r e l a t e d to the t y p e o f c o o k i n g f u e l u s e d at h o m e . P e a r s o n p r o d u c t - m o m e n t correlations between personal exposure and indoor c o n c e n t r a t i o n s w e r e h i g h a n d s i g n i f i c a n t in b o t h e l e c tric ( e x c e p t f o r t h e k i t c h e n ) a n d g a s h o m e s in b o t h p e r i o d s . C o r r e l a t i o n s b e t w e e n i n d o o r NO2 c o n c e n t r a t i o n s in the d i f f e r e n t l o c a t i o n s w e r e a l s o h i g h in b o t h e l e c t r i c a n d g a s h o m e s in the s e c o n d p e r i o d a n d in g a s h o m e s in t h e first p e r i o d . C o r r e l a t i o n s b e t w e e n outdoor concentrations and personal exposures were not s i g n i f i c a n t . T h e m e a n NO2 c o n c e n t r a t i o n s m e a s u r e d i n s i d e t h e t w o s c h o o l s w e r e 28 a n d 34 ~ g / m 3 in the first p e r i o d a n d 22 a n d 29 fxg/m 3 in t h e s e c o n d p e r i o d .
T a b l e 3 s h o w s the r e s u l t s o f the r e g r e s s i o n a n a l y s i s o n t h e first t h r e e v a r i a b l e sets. M o d e l 1 ( v a r i a b l e set 1) e x p l a i n e d 6 0 % (first p e r i o d ) a n d 8 3 % ( s e c o n d p e r i o d ) o f the v a r i a t i o n in p e r s o n a l e x p o s u r e . T h e r e g r e s s i o n c o e f f i c i e n t s f o r b e d r o o m NO2 c o n c e n t r a t i o n a n d l i v i n g r o o m NO2 c o n c e n t r a t i o n w e r e s i g n i f i c a n t . T h e r e g r e s s i o n o n v a r i a b l e set 2 r e s u l t e d in an e x p l a i n e d v a r i a n c e o f 3 9 % in b o t h p e r i o d s . C o o k i n g fuel, p i l o t l i g h t , s c h o o l c o n c e n t r a t i o n , a n d s e x w e r e s i g n i f i c a n t in the first p e r i o d , a n d c o o k i n g fuel w a s s i g n i f i c a n t in the s e c o n d p e r i o d . A d d i t i o n o f t i m e frac-
Activity Data T h e a v e r a g e p e r c e n t a g e s o f t i m e s p e n t in e a c h o f the five l o c a t i o n s a r e p r e s e n t e d in T a b l e 2. V a r i a t i o n in a c t i v i t y p a t t e r n s is s m a l l . I n b o t h p e r i o d s , o v e r 9 0 % o f the t i m e w a s s p e n t i n d o o r s a n d 6 5 % o f t h e t i m e w a s s p e n t i n s i d e the h o m e .
Table 3. Results of multiple regression analysis of three sets of independent variables on personal NO2 exposures. Independent variables
Period
Set 1 Set 2
1 1
Set 3 Set 1 Set 2 Set 3
1 2 2 2
n
R a (070)
S.E.E. 0tg/m ~)
67
60
8.0
153 153 48 146 146
39 39 83 39 42
11.1 11.1 6.5 14.0 13.8
P. Clausing et al.
416 Table 4. Components of variance of personal NO2 exposures and indoor NO~ concentrations.
Exposure variable
Within-unit variance
Between-unit variance
n
personal exposure personal exposure* bedroom concentration bedroom concentration** living room concentration living room concentration** kitchen concentration kitchen concentration**
107 45 159 55 221 142 440 271
124 127 316 380 424 486 903 1034
77 73 24 23 24 23 25 24
*after exclusion of four outliers **after exclusion of one outlier
Table 5. Components of variance of personal NOa exposures for the gas and the electric group separately. Between-Unit Variance
Within-Unit Variance
n
80 9
117 53
62 15
Gas Electric
tions to variable set 2 did not result in a significantly better fit of the regression equation, neither did addition of other home characteristics to variable set 3. Analysis o f Variance The results o f the analysis o f variance are given in Tables 4 and 5. Table 4 shows the results for the whole group, with and without outliers. The within-person variance is almost equal to the between-person variance when outliers are included. Without outliers the remaining cases exhibit more stability over the two periods. The indoor concentrations have a larger total variance, and the within-home variance is about half of the between-home variance. Exclusion o f one outlier gave a further reduction o f the ratio between within- and between-home variance. Table 5 shows the results for the gas and the electric group separately.
Discussion Cooperation o f the students was not exceptional. The diary appeared to be a large burden to the students and, in particular, the question about the time that the monitor was not worn was often not answered. This may have introduced a bias in the personal exposure measurements. Overall, the procedure used seems a feasible method o f cheaply and accurately acquiring NO2 exposure data and related variables o f large study groups. The results o f the NO2 monitoring clearly show a relationship between the type o f cooking fuel used at
home and the indoor and personal N O 2 concentration. Within the gas group, however, there is still substantial variation. The variation of the indoor concentrations in electric homes, in particular in bedroom and living room, is much smaller. For all locations an overlap exists between the range for the gas group and the range for the electric group. The results o f the regression analysis emphasize the importance o f indoor NO2 concentrations for personal exposures. In particular, in a population such as this, with little variation in time activity patterns, most of the variation in personal exposures will be due to variation in exposure at home. Although the questionnaire about the home characteristics was kept simple and thus did not cover all potential predictors for personal NO2 exposures, the conclusion that a model containing only h o m e characteristics does not give a satisfactory estimate of personal exposure seems to be justified. Variations in use of gas appliances and in air exchange in the home, for instance, are not reflected by this model. Neither is variation in the decay rate of NO2 inside the home. The model containing indoor NO2 concentrations does, at least partially, reflect the influence o f these variables and, indeed, gives better results. Thus, because there are not efficient methods available to measure or estimate ventilation rates and NO2 emission and decay rates in a great number o f homes, it seems that indoor NO2 measurements are necessary to provide health effect studies with adequate NO2 exposure data. Additional information about short-term variation of indoor NO2 concentrations in relation to the presence o f the occupants in the indoor location, may further improve the estimates of personal exposures at home and, thus, the estimates of the total personal exposures. Since the study was not set up to measure variation in time of the exposure variables, the results o f the analysis o f variance should be interpreted with care. The results show that a considerable variation in time may exist in measurements of exposure at the same person or home. This is consistent with the results from a study on personal NO2 exposures o f 35 housewives by Noy et al. (1985), where it was found that the within-person variance was almost as large as the between-person variance. B r u n e k r e e f et al. (1985) found that indoor NO2 concentrations were fairly stab l e - r e l a t i v e l y small within-home variance compared to between-home v a r i a n c e - - i n fall, winter, and spring. Acknowledgements--We wish to thank Watertown Public Schools, their Junior High Science teachers, and Mr. George Buckley for their assistance in administrating this study. Tony Majahad and Helen Miklas' assistance with the NO2 analysis was also appreciated. Partial funding for this research was provided by a cooperative contract with U.S. EPA (CR80853601), NIEHS grant (ES-01108), and EPRI contract (RP-1001).
Personal NO2 exposures
References Brunekreef, B. and Hock, F. (1985) Variability of indoor NO2 measurements, Environ. Int. 12, 000-000. Ferris, B. G., Speizer, F. E., Spengler, J. D., Dockery, D. W., Bishop, Y. M. M., Wolfson, M., and Humble, C. (1979) Effects of sulfur oxides and respirable particles on human health. Methodology and demography of populations in study, Am. Rev. Resp. Dis. 120, 767-779. Florey, C. du V., Melia, R. J. W., Chinn, S., Goldstein, B. D., Brooks, A. G. F., John, H. H., Cralghead, I. B., and Webster, X. (1979) The relation between respiratory illness in primary school children and the use of gas for cooking. III. Nitrogen dioxide, respiratory illness, and lung function, Int. J. Epidemiol. 8, 347-353. Goldstein, B. D., Melia, R. J. W., Chinn, S., Florey, C., Clark, D., and John, H. H. (1979) The relationship between respiratory illness in primary school children and the use of gas for cooking. II. Factors affecting nitrogen dioxide levels in the home, Int. J. Epidemiol. 8, 339-345. Keller, M. D., Lanese, R. R., Mitchell, R. J., and Cote, R. W. (1979) Respiratory illness in households using gas and electricity for cooking. I. Survey of incidence, Environ. Res. 11,495-503. Moschandreas, D. J., Zabransky, J., and Pelton, D. J. (1981) Comparison of indoor and outdoor air quality. EA-1733, Research Project 1309, Electric Power Research Institute, Palo Alto, CA. Nie, N. H., Hull, C. H., Jenkins, J. G., Steinbrermer, K., and Bent, D. H. (1975) SPSS Statistical Package for the Social Sciences, 2nd ed. McGraw-Hill, New York, NY. Nie, N. H. and Hull, C. H. (1981) SPSS Update 7-9. McGraw-Hill,
417 New York, NY. Nitta, H. and Maeda, K. (1981) Personal exposure monitoring to nitrogen dioxide, Environ. Int. 8, 1-6, 243-248. Noy, D., Willers, H., Winkes, H., Brunekreef, B., Lebret, E., and Boleij, L. (1985) Estimating human exposure to nitrogen dioxide: Results from a personal monitoring study among housewives, Environ. Int. 12, 000-000. Ozkaynak, H., Ryan, P. B., Spengler, J. D., and Laird, N. M. (1985) Bias due to misclassification of personal exposures in epidemiologic studies of indoor and outdoor air pollution, Environ. Int. 12, (current issue). Palmes, E. D., Gunnison, A. F., Dimattio, J., and Tomczyk, C. (1976) Personal sampler for nitrogen dioxide, Am. Ind. Hyg. Assoc. 37, 570-577. Quackenboss, J. J., Kanarek, M. S., Spengler, J. D., and Letz, R. (1981) Personal monitoring for nitrogen dioxide exposure: Methodological considerations for a community study, Environ. Int. 8, 249-258. Speizer, F. E., Ferris, B. G., Bishop, Y. M. M., and Spengler, J. D. (1980) Respiratory disease rates and pulmonary function in children associated with NO2 exposure, Am. Rev. Resp. Dis. 121, 3-10. Spengler, J. D., Duffy, C. P., Letz, R., Tibbitts, T. W., and Ferris, B. G., Jr. (1983) Nitrogen dioxide inside and outside 137 homes and implications for ambient air quality standards and health effects research, Environ. Sci. Technol. 17, 164-168. Wolfson, J. M. (1980) Modifications to the Palmes diffusion tube preparation and analysis methods. Quality assurance document, Vol. 2, Harvard Six City Study, Harvard School of Public Health, Boston, MA.
0160-4120/86 $3.00 + .00 Copyright © 1986 Pergamon Journals Ltd.
PERSONAL NO, EXPOSURES OF HIGH SCHOOL STUDENTS Pieter Clausing and Jan Karel Mak Agricultural University,Department of Air Pollution, Wageningen, The Netherlands
John D. Spengler a and Richard Letz Department of EnvironmentalScience and Physiology, Harvard School of Public Health, Boston, Massachusetts 02115, USA (Received 1 June 1985; Accepted 17 September 1985) Personal NO2 exposures and time activity patterns were measured during two sampling periods, for 500 junior high school students (aged 12-14) in Watertown, MA. NO2 concentrations in the bedroom, living room, and kitchen were measured for 100 students in each period with Palmes' NO2 diffusion tubes. Indoor and personal NO2 levels were closely related to the type of cooking fuel used in the home. The correlation between outdoor and personal NO2 levels was not significant. Step-wise multiple regression techniques were used to fit models to estimate personal exposures. Models including indoor NO2 concentrations explained 60% to 90% of the variance in personal NO2 exposures. When indoor concentrations were excluded, only about 40% of the variance was explained by cooking fuel, presence of pilot lights, and NO2 concentration at school. Exposures of students measured in both periods were used to determine within-person and between-person variability. Within-person variability was found to be large, indicating that a single person exposure measurement is not sufficient to characterize a person's exposure.
Introduction T h e s u s p e c t e d h e a l t h e f f e c t s o f NO2 h a v e b e e n i n v e s t i g a t e d in m a n y e p i d e m i o l o g i c a l s t u d i e s , w i t h i n c o n s i s t e n t r e s u l t s ( F l o r e y et a l . , 1979; K e l l e r et al., 1979; S p e i z e r et a l . , 1980). In s o m e o f t h e s e s t u d i e s the t y p e o f c o o k i n g fuel u s e d at h o m e w a s u s e d as a m e a s u r e o f e x p o s u r e to NO2 ( F l o r e y et al., 1979; K e l l e r et al., 1979; S p e i z e r et a l . , 1980), a n d in o t h e r s t u d i e s the i n d o o r NO2 c o n c e n t r a t i o n s w e r e u s e d ( G o l d s t e i n et a l . , 1979; F l o r e y et a l . , 1979). T h e r e l a t i o n b e t w e e n indoor NO 2 concentrations and personal NO 2 exposures has only been investigated for small study popu l a t i o n s ( N i t t a a n d M a e d a , 1981; Q u a c k e n b o s s et al., 1981). It w a s f o u n d that i n d o o r NO2 l e v e l s a r e i m p o r tant in d e t e r m i n i n g p e r s o n a l N O 2 e x p o s u r e s , p a r t i c u l a r l y w h e n u n v e n t e d N O 2 s o u r c e s s u c h as g a s s t o v e s o r h o t w a t e r h e a t e r s a r e p r e s e n t ( M o s c h a n d r e a s et al., 1981; Q u a c k e n b o s s et a l . , 1981; S p e n g l e r et al., 1981). T h e r e l a t i v e l y l a r g e f r a c t i o n o f t i m e that p e o p l e s p e n d i n s i d e t h e i r h o m e s ( S p e n g l e r et al., 1981) c o n aTo whom all correspondence should be addressed.
t r i b u t e s to the i m p o r t a n c e o f i n d o o r N O 2 l e v e l s . W i t h i n the f r a m e w o r k o f the H a r v a r d A i r P o l l u t i o n / H e a l t h S t u d y ( F e r r i s et al., 1979), a s t u d y w a s c o n d u c t e d to i n v e s t i g a t e the l e v e l a n d v a r i a b i l i t y o f NO2 e x p o s u r e s o f j u n i o r h i g h s c h o o l s t u d e n t s in W a t e r t o w n , M A , a n d the r e l a t i o n s h i p b e t w e e n p e r s o n a l e x p o s u r e s a n d i n d o o r NO2 l e v e l s . V a r i a b i l i t y o f the e x p o s u r e v a r i a b l e s o v e r t i m e for the s a m e p e r s o n o r h o m e (unit) is a l s o o f interest. W h e n u s e d in a h e a l t h - e f f e c t s s t u d y , a r a n d o m e r r o r in the e x p o s u r e v a r i a b l e w i l l d e c r e a s e the c o r r e l a t i o n b e t w e e n the m e a s u r e o f h e a l t h e f f e c t a n d the e x p o s u r e v a r i a b l e s . It w i l l a l s o t e n d to b i a s the e s t i m a t e o f h e a l t h e f f e c t t o w a r d s z e r o . A n e s t i m a t e o f this t y p e o f e r r o r c a n b e o b t a i n e d b y r e p e a t i n g the e x p o s u r e m e a s u r e m e n t s . T h e v a r i a n c e in the e x p o s u r e v a r i a b l e s c a n t h e n b e d e c o m p o s e d in a w i t h i n - u n i t v a r i a n c e a n d a b e t w e e n - u n i t v a r i a n c e . W h e n the w i t h i n - u n i t v a r i a n c e is s m a l l c o m p a r e d to the b e t w e e n - u n i t v a r i a n c e , this i n d i c a t e s that m o s t o f the v a r i a t i o n in the e x p o s u r e v a r i a b l e is d u e to d i f f e r e n c e s b e t w e e n units. A r e l a t i v e l y l a r g e w i t h i n - u n i t v a r i a n c e i n d i c a t e s that it w i l l b e difficult to e s t a b l i s h b e t w e e n - u n i t d i f f e r e n c e s on 413
414 the basis o f a single measurement per unit. The relationship between these components of variance o f the exposure variables and the estimated effect is more extensively discussed by B r u n e k r e e f et al. (1985) and Ozkaynak et al. (1985). Although initially this study was not set up for this purpose, the data were used to get some notion o f the variability in time of the exposure variables.
Methods and Materials The study was carried out during two periods: four days in N o v e m b e r and three days in December 1982. Students in the seventh and eighth grade (aged 12-14) from the two public high schools in Watertown, MA, were asked to volunteer for the study. Letters for permission to participate were sent to the parents. Objectives and set-up o f the study were explained in the letter. Class visits were made to explain the study to the students, to give instructions and to distribute the materials. The study started with 250 students in each period. Personal NO2 exposures o f all students were measured. In a subpopulation o f 100 students in each period, NOz concentrations were also measured within the home (kitchen, living room, and bedroom) and outside the home. Palmes' NO2 diffusion tubes were used for all NOz measurements. The students received the NOz monitors at school and were instructed where to place the monitors in their homes. NO2 was also monitored in a few locations in both participating schools. Directly after the NOz measurements, the NO2 monitors were analyzed using the method described by Palmes et al. (1976) with modifications by Wolfson (1980). During the sampling days, all participants kept track of their activities in a small diary. Time spent at home, outside, in travel, in school, and in other indoor locations was recorded to the nearest half hour. Time that the personal monitor was not worn by the person, other irregularities with the monitor, and time spent in the kitchen during stove or oven use were also recorded. Questionnaires concerning home characteristics with a potential influence on indoor NO2 levels (among others: cooking fuel; presence o f a pilot light on the stove; presence and use o f a ventilation system in the kitchen; number o f cigarettes smoked in the house; heating fuel) were filled out by all students. For the second period the questionnaires were simplified; only the questions about cooking fuel, pilot light, ventilation system, and the use of oven and burners were maintained.
Data Analysis Multiple linear regression analysis with the SPSS statistical package (Nie et al., 1975; Nie and Hull,
P. Clausing et al. 1981) was used to test various models to estimate personal N O z exposures. The selection of the variable sets used in this analysis was based on expected contributions of variables to personal exposures and on preliminary statistical analysis. The following sets of independent variables were introduced in the models: 1. measured indoor NO2 concentrations; 2. NO2 concentration at school and d u m m y variables for cooking fuel, presence o f pilot light, presence and use o f ventilation in the kitchen, and sex; 3. set 2 plus time fractions spent in each of the five locations; 4. set 3 plus various combinations o f other home characteristics. The time spent in the kitchen during stove or oven use was not included in the analysis since the answering of this question in the diary was often incomplete. The data from each sampling period were handled separately. For each model the multiple correlation coefficient (R 2) and the standard error o f estimate (SEE) are presented. The stepwise inclusion method was used, examining variables at each step for entry in or removal from the regression equation at a significance level of 0.10. The exposure variables from students who participated in both periods were analyzed using the analysis of variance technique " R e l i a b i l i t y , " available in the SPSS statistical package (Nie and Hull, 1981). Analysis was p e r f o r m e d for the whole group and for the gas and the electric group separately. Analysis was also performed after exclusion o f four o f the personal exposures " o u t l i e r s " and one o f the indoor concentrations. The within-unit variance and the between-unit variance are presented.
Results Data Base
About 150 (60%) complete and useful data sets were retrieved in each period, o f which 67 and 48 sets contained h o m e measurements. A number of the students never gave the materials back, some did not cooperate well enough, or did not understand the proceedings. In some cases the NO2 data were useless because exposure times were unknown since diaries were filled out improperly or not returned. Personal exposures were r e m o v e d from the data file when the monitor was worn for less than 80% o f the time. About 140 students participated in both periods, resuiting in 83 useful data sets for the analysis o f variance. Twenty-four of these contained indoor NO2 measurements.
Personal NO2 exposures
415 Table 1. Indoor, outdoor and personal NO2 concentrations (in ttg/m ~) classified for cooking fuel and sampling period. Gas
Period
Electric
Location
Mean
S.D.*
Range
n
Mean
S.D.*
Range
personal bedroom living room kitchen outdoors**
46 45 52 74 37
16 16 18 27 6
23-134 15-86 21-97 29-161 27-65
128 60 60 60 72
26 19 19 24 37
9 4 4 13 6
12-50 12-25 14-27 13-74 27-65
35 17 18 18 72
personal bedroom living room kitchen outdoors**
49 46 60 86 46
16 24 27 41 6
17-109 10-131 12-125 11-195 36-61
127 50 51
22 14 16
6 5 5
51
18
8
28
46
6
12-40 6-21 8-22 6-29 36-61
36 10 10 10 28
*Standard deviation **The outdoor concentrations are averaged over the gas and the electric homes.
Table 2. Summary statistics of time activity data. Percentages of time spent in each location
Period 1
Period 2
Location
Mean
S.D.
Range
n
Mean
S.D.
Range
n
Home School Outside Travel Other indoors
66.4 19.0 7.1 1.4 6.2
8.8 4.2 5.2 2.3 6.0
39.1-100 0-30.7 0-26.0 0-18.8 0.29.7
177 177 177 177 177
63.8 26.0 5.9 0.7 3.7
6.9 4.4 4.5 1.4 4.5
39.9-86.6 8.3-34.7 0-22.2 0-11.1 0-26.4
192 192 192 192 192
NO2 M o n i t o r i n g
Multiple Linear Regression Analysis
A v e r a g e i n d o o r , o u t d o o r a n d p e r s o n a l NO2 l e v e l s are given for each cooking fuel and sampling period in T a b l e 1. I n b o t h p e r i o d s i n d o o r c o n c e n t r a t i o n s a n d p e r s o n a l e x p o s u r e s w e r e c l o s e l y r e l a t e d to the t y p e o f c o o k i n g f u e l u s e d at h o m e . P e a r s o n p r o d u c t - m o m e n t correlations between personal exposure and indoor c o n c e n t r a t i o n s w e r e h i g h a n d s i g n i f i c a n t in b o t h e l e c tric ( e x c e p t f o r t h e k i t c h e n ) a n d g a s h o m e s in b o t h p e r i o d s . C o r r e l a t i o n s b e t w e e n i n d o o r NO2 c o n c e n t r a t i o n s in the d i f f e r e n t l o c a t i o n s w e r e a l s o h i g h in b o t h e l e c t r i c a n d g a s h o m e s in the s e c o n d p e r i o d a n d in g a s h o m e s in t h e first p e r i o d . C o r r e l a t i o n s b e t w e e n outdoor concentrations and personal exposures were not s i g n i f i c a n t . T h e m e a n NO2 c o n c e n t r a t i o n s m e a s u r e d i n s i d e t h e t w o s c h o o l s w e r e 28 a n d 34 ~ g / m 3 in the first p e r i o d a n d 22 a n d 29 fxg/m 3 in t h e s e c o n d p e r i o d .
T a b l e 3 s h o w s the r e s u l t s o f the r e g r e s s i o n a n a l y s i s o n t h e first t h r e e v a r i a b l e sets. M o d e l 1 ( v a r i a b l e set 1) e x p l a i n e d 6 0 % (first p e r i o d ) a n d 8 3 % ( s e c o n d p e r i o d ) o f the v a r i a t i o n in p e r s o n a l e x p o s u r e . T h e r e g r e s s i o n c o e f f i c i e n t s f o r b e d r o o m NO2 c o n c e n t r a t i o n a n d l i v i n g r o o m NO2 c o n c e n t r a t i o n w e r e s i g n i f i c a n t . T h e r e g r e s s i o n o n v a r i a b l e set 2 r e s u l t e d in an e x p l a i n e d v a r i a n c e o f 3 9 % in b o t h p e r i o d s . C o o k i n g fuel, p i l o t l i g h t , s c h o o l c o n c e n t r a t i o n , a n d s e x w e r e s i g n i f i c a n t in the first p e r i o d , a n d c o o k i n g fuel w a s s i g n i f i c a n t in the s e c o n d p e r i o d . A d d i t i o n o f t i m e frac-
Activity Data T h e a v e r a g e p e r c e n t a g e s o f t i m e s p e n t in e a c h o f the five l o c a t i o n s a r e p r e s e n t e d in T a b l e 2. V a r i a t i o n in a c t i v i t y p a t t e r n s is s m a l l . I n b o t h p e r i o d s , o v e r 9 0 % o f the t i m e w a s s p e n t i n d o o r s a n d 6 5 % o f t h e t i m e w a s s p e n t i n s i d e the h o m e .
Table 3. Results of multiple regression analysis of three sets of independent variables on personal NO2 exposures. Independent variables
Period
Set 1 Set 2
1 1
Set 3 Set 1 Set 2 Set 3
1 2 2 2
n
R a (070)
S.E.E. 0tg/m ~)
67
60
8.0
153 153 48 146 146
39 39 83 39 42
11.1 11.1 6.5 14.0 13.8
P. Clausing et al.
416 Table 4. Components of variance of personal NO2 exposures and indoor NO~ concentrations.
Exposure variable
Within-unit variance
Between-unit variance
n
personal exposure personal exposure* bedroom concentration bedroom concentration** living room concentration living room concentration** kitchen concentration kitchen concentration**
107 45 159 55 221 142 440 271
124 127 316 380 424 486 903 1034
77 73 24 23 24 23 25 24
*after exclusion of four outliers **after exclusion of one outlier
Table 5. Components of variance of personal NOa exposures for the gas and the electric group separately. Between-Unit Variance
Within-Unit Variance
n
80 9
117 53
62 15
Gas Electric
tions to variable set 2 did not result in a significantly better fit of the regression equation, neither did addition of other home characteristics to variable set 3. Analysis o f Variance The results o f the analysis o f variance are given in Tables 4 and 5. Table 4 shows the results for the whole group, with and without outliers. The within-person variance is almost equal to the between-person variance when outliers are included. Without outliers the remaining cases exhibit more stability over the two periods. The indoor concentrations have a larger total variance, and the within-home variance is about half of the between-home variance. Exclusion o f one outlier gave a further reduction o f the ratio between within- and between-home variance. Table 5 shows the results for the gas and the electric group separately.
Discussion Cooperation o f the students was not exceptional. The diary appeared to be a large burden to the students and, in particular, the question about the time that the monitor was not worn was often not answered. This may have introduced a bias in the personal exposure measurements. Overall, the procedure used seems a feasible method o f cheaply and accurately acquiring NO2 exposure data and related variables o f large study groups. The results o f the NO2 monitoring clearly show a relationship between the type o f cooking fuel used at
home and the indoor and personal N O 2 concentration. Within the gas group, however, there is still substantial variation. The variation of the indoor concentrations in electric homes, in particular in bedroom and living room, is much smaller. For all locations an overlap exists between the range for the gas group and the range for the electric group. The results o f the regression analysis emphasize the importance o f indoor NO2 concentrations for personal exposures. In particular, in a population such as this, with little variation in time activity patterns, most of the variation in personal exposures will be due to variation in exposure at home. Although the questionnaire about the home characteristics was kept simple and thus did not cover all potential predictors for personal NO2 exposures, the conclusion that a model containing only h o m e characteristics does not give a satisfactory estimate of personal exposure seems to be justified. Variations in use of gas appliances and in air exchange in the home, for instance, are not reflected by this model. Neither is variation in the decay rate of NO2 inside the home. The model containing indoor NO2 concentrations does, at least partially, reflect the influence o f these variables and, indeed, gives better results. Thus, because there are not efficient methods available to measure or estimate ventilation rates and NO2 emission and decay rates in a great number o f homes, it seems that indoor NO2 measurements are necessary to provide health effect studies with adequate NO2 exposure data. Additional information about short-term variation of indoor NO2 concentrations in relation to the presence o f the occupants in the indoor location, may further improve the estimates of personal exposures at home and, thus, the estimates of the total personal exposures. Since the study was not set up to measure variation in time of the exposure variables, the results o f the analysis o f variance should be interpreted with care. The results show that a considerable variation in time may exist in measurements of exposure at the same person or home. This is consistent with the results from a study on personal NO2 exposures o f 35 housewives by Noy et al. (1985), where it was found that the within-person variance was almost as large as the between-person variance. B r u n e k r e e f et al. (1985) found that indoor NO2 concentrations were fairly stab l e - r e l a t i v e l y small within-home variance compared to between-home v a r i a n c e - - i n fall, winter, and spring. Acknowledgements--We wish to thank Watertown Public Schools, their Junior High Science teachers, and Mr. George Buckley for their assistance in administrating this study. Tony Majahad and Helen Miklas' assistance with the NO2 analysis was also appreciated. Partial funding for this research was provided by a cooperative contract with U.S. EPA (CR80853601), NIEHS grant (ES-01108), and EPRI contract (RP-1001).
Personal NO2 exposures
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