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Personal exposure to nitrogen dioxide in Switzerland
The Science of the Total Environment 215 Ž1998. 243]251
Personal exposure to nitrogen dioxide in Switzerland C. Monna,U , O. Brandli ¨ b , C. Schindler c , U. Ackermann-Liebrich c , P. Leuenberger d , SAPALDIA Team1 a
Department of Hygiene and Applied Physiology, En¨ironmental Hygiene Group, Federal Institute of Technology, ETH-Z, Clausiusstrasse 25, CH-8092 Zurich, Switzerland ¨ b Zurcher Hohenklinik Wald, CH-8636 Wald, Zurich, Switzerland ¨ ¨ c Institute for Social and Pre¨enti¨e Medicine, Uni¨ersity of Basle, Steinengraben 49, CH-4051 Basle, Switzerland d Di¨ision of Pneumology, Centre Hospitalier Uni¨ersitaire Vaudois, Rue du Bugnon, CH-1011 Lausanne, Switzerland Received 3 December 1997; accepted 16 January 1998
Abstract Home indoor and outdoor levels, and personal exposures to NO 2 were determined for more than 500 subjects in a subpopulation of SAPALDIA by using passive samplers. The overall personal NO 2 average was found to be 27 m g my3, the overall indoor average 21 m g my3 and the overall outdoor average 31 m g my3. Personal NO 2 levels ranged between the outdoor and indoor levels, with the exception of study areas with low NO 2 concentrations. In the winter, the indoorroutdoor ratios were lower than in the summer. Outdoor NO 2 levels were higher in winter. In some study areas, indoor NO 2 levels were lower in the winter than in the summer due to reduced ventilation but this was not consistent. Personal NO 2 concentrations were very similar during all seasons. Gas-cooking and smoking were important factors for elevated indoor and personal NO 2 levels Žcontribution: 5 m g my3, 2 m g my3 , respectively.. Personal exposure to NO 2 correlated best with the indoor NO 2 concentrations. Q 1998 Elsevier Science B.V. Keywords: Nitrogen dioxide; Personal exposure; Indoor; Outdoor; Passive sampling; Gas-cooking; Exposure assessment
U
Corresponding author. Tel.: q41 1 6324629; fax: q41 1 6321318; e-mail: [email protected] SAPALDIA ŽSwiss Study on Air Pollution and Lung Diseases in Adults. Team: Study Director: P. Leuenberger ŽP., Programme ŽP., Director: U. Ackermann-Liebrich ŽE.. In¨estigators: P. Alean ŽAM., K. Blaser ŽA., G. Bolognini ŽP., J.P. Bongard ŽP., O. Brandli ¨ P. Braun ŽP., C. Bron ŽL., M. Brutsche ŽL., C. Defila ŽM., G. Domenighetti ŽP., S. Elsasser ŽL., L. Grize ´ ŽS., P. Guldimann ŽL., P. ŽE., J.C. Luthy ŽL., B.W. Martin ŽE., T. Medici ŽP., Hufschmied ŽL., W. Karrer ŽP., H. Keller-Wossidlo ŽO., R. Keller ŽP., N. Kunzli ¨ C. Monn ŽAM., A.G. Peeters ŽPA., A. Perruchoud ŽP., A. Radaelli ŽL., C. Schindler ŽS., J. Schwartz ŽS., M. Schoni ¨ ŽP., G. Solari ŽP., J.M. Tschopp ŽP., B. Villiger ŽP., B. Wuthrich ŽA., J.P. Zellweger ŽP., E. Zemp ŽE.. Specialities: A, Allergology; AM, Air ¨ Pollution Monitoring; E, Epidemiology; L, Local Assistant MD; PA, Palynology; P, Pneumology; M, Meteorology; O, Occupational Medicine; S, Statistics. 1
0048-9697r98r$19.00 Q 1998 Elsevier Science B.V. All rights reserved. PII S0048-9697Ž98.00124-7
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1. Introduction Exposure is a term associated with some uncertainty. Exposure to an atmospheric pollutant may be defined as contact with the pollutant of a known concentration for a defined time period; this leads to information on an exposure-dose ŽCommittee on Advances in Assessing Human Exposure to Airborne Pollutants, 1991.. A simpler definition defines exposure as any contact of a human with a pollutant. In air pollution epidemiology, the use of Žpollutant. concentrations is more widespread than the use of a dose Žwhich is mainly used in biomarker studies.. Exposure levels can be determined directly or indirectly ŽCommittee on Advances in Assessing Human Exposure to Airborne Pollutants, 1991.. Direct approaches use personal measurements at the individuals or biomarkers from body fluids. Indirect approaches use ‘surrogates’, such as outdoor or indoor concentration and also models. Microenvironmental models are based on known concentrations in these so-called ‘microenvironments’ Ža location with a homogeneous pollution distribution. and known time activity Žor budget. pattern. In order to establish and validate such models it is crucial to have a large-scale database of microenvironmental levels, time-activity patterns, and personal measurements. For NO 2 , a simple technology Žpassive samplers. is available for the determination of personal exposures. Passive samplers have the advantage that they can be used to determine individual exposures but also for establishing large-scale databases with indoor and outdoor data, which can be used to establish and validate models. Within the framework of SAPALDIA ŽSwiss Study on Air Pollution and Lung Diseases in Adults., personal exposure to nitrogen dioxide was measured in a subsample of more than 500 subjects from the eight study regions from December 1992 to December 1993. In addition, parallel measurements were conducted outdoors of the home and indoors in the bedroom. To date, little information on personal exposure to NO 2 of adults in Switzerland has been available, but in 1985r1986 in a study with children, personal levels of NO 2 were determined ŽBraun et al., 1992..
In this study, a strong relationship between personal exposure levels of NO 2 and indoor levels was found. Exposure in adults, however, shows higher variation and covers more microenvironments. A preliminary study was conducted in 1992, in which personal exposure of adults to NO 2 and O 3 was investigated ŽMonn et al., 1993.. Moreover, in recent years, many studies have been conducted comparing indoor with outdoor levels, and they showed that indoor levels of NO 2 are between 50 and 90% of outdoor levels, in the absence of indoor sources ŽMonn et al., 1993; Spengler et al., 1994; Tomingas and Grover, 1990; Wanner et al., 1990.. In the absence of indoor sources, NO 2 levels are a function of the outdoor concentrations and the ventilation rate, and the ventilation rate shows a seasonal pattern. In Switzerland, air conditioning is rarely used so that ventilation depends on the outdoor air temperature. Indoor sources, such as gas stoves and gas heaters, may lead to indoor NO 2 levels exceeding levels measured outdoors. Such homes have to be treated separately when using NO 2 as an indicator of ambient pollution, as the mixture of gas combustion emissions is different from the mixture of outdoor pollutants. The purpose of this analysis was to evaluate the relationship between personal NO 2 levels with indoor and outdoor levels at home. As from all participants of the passive sampler study, information on indoor sources Žgas-stoves, smoking status ., ventilation habits, situation of home with respect to streets as well as on occupational exposure was available, the effects of these parameters on personal exposure to NO 2 were additionally estimated. In a multiple regression model, the effects of these parameters on personal NO 2 exposures were quantified. 2. Methods 2.1. Participants SAPALDIA is an eight-centre study with four urbanrsuburban regions ŽGeneva, Basle, Lugano, Aarau., two rural regions ŽWald, Payerne. and two alpine regions ŽMontana, Davos.. In each region, more than 500 subjects were
C. Monn et al. r The Science of the Total En¨ironment 215 (1998) 243]251
invited to participate in the follow-up study. From this sample, a stratified random subsample was selected in each area. Stratification was to ensure that any time measurements provided a faithful picture of differences in NO 2 levels across subregions. The measurement program covered a full year. In larger study regions Žurban, suburban areas., where a higher range NO 2 concentration was expected, the subsample was greater than in regions, where a more homogeneous distribution was expected. Table 1 shows the number of selected subjects for each region. Subjects participated between December 1993 and December 1994, most during three periods of 1 month. In the first week of each period, personal, indoor and outdoor levels were measured, and in the next 3 consecutive weeks only outdoor levels were measured. As personal sampling requires much discipline, it was carried out only in the first week. From all subjects, information on cooking methods, home ventilation, smoking, occupational exposure and the characteristics of home with respect to traffic density was available. 2.2. Measurements NO 2 was measured with Palmes tubes ŽPalmes et al., 1976.. An adaptation of the method was used for measuring ambient NO 2 concentrations ŽHangartner, 1990.. Metal grids were coated with triethanol-amine, dissolved in a mixture of acetonerwater. Samplers were posted to local SAPALDIA offices from where they were sent to subjects on a weekly basis. Subjects were given verbal instruction before their first measurement Table 1 Measurements in each region and the total number of personal measurements Area
Number of persons Total number of weekly measuring simultaneously personal observations
Geneva Basle Lugano Aarau Payerne Wald Davos Montana
16 28 16 20 16 16 16 12
131 337 204 204 169 203 194 144
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period and written instructions were included with each weekly mailing. The shipping time of the tubes was between one and 2 days. After exposure, the time protocols and the exposed tubes were sent back to the local teams where they were stored in a dark and preferably cold place. Twice a month, the exposed tubes were sent to the central analytical lab at the ETH where analysis took place within 1 day. The maximum storing time of an exposed tube was 2 weeks Žminimum 1 week.. After returning, the samplers were analysed according to Saltzmann ŽNEDA. and NO 2 concentration in volume of air were calculated according to Fick’s law of diffusion. Samplers were exposed at home outdoors Žoutside the window of the bedroom. and indoors Žin the bedroom, 1.5]2 m above ground.. Moreover, subjects carried the samplers for personal sampling on outer clothing in the breast]head region for 1 week. For each passive sampler, the exposure period was 1 week. Samplers which were exposed more than 2 weeks or less than 3 days were excluded from statistical analyses. For weekly exposures, the precision of the passive sampler was found to be "5% ŽHangartner, 1990.. Each subject participating in the ‘cross-sectional’ study was asked the following questions Žamong others.: v
v v
What type of cooking facility do you use: gas cooker or electric cooker? Are you a current smoker: yes or no? Do you open the windows of the bedroom during winter regularly or seldom? Žthese two answers represents the two extremes of three possibilities: regularly, sometimes, seldom.
All statistical analyses Žsummary statistics, multiple regressions. were carried out in SASW , version 6.08 for PC. 3. Results 3.1. Comparison between indoor, outdoor and personal NO2 concentrations (all subjects) Fig. 1 shows the average values of outdoor,
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Fig. 1. Comparison between indoor, outdoor and personal levels Žfull year. across study centres. ŽGe: Geneva, Lu: Lugano, Ba: Basle, Aa: Aarau, Pa: Payerne, Wa: Wald, Da: Davos, Mo: Montana.. Mean values " standard deviation.
indoor and personal measurements. Comparing the outdoor concentrations, a gradient between the study sites was found. Looking at the indoor levels, a gradient was found between the urban
sites, but from Aarau to Montana the variation of indoor levels was much smaller. Indoor levels were about 70]80% of the outdoor levels. For the personal levels, a gradient was found between the five most polluted regions and to a lesser extent in the least polluted regions. In the three least polluted regions ŽDavos, Wald, Montana., personal levels were even greater than the outdoor levels Žsignificant for Montana P- 0.001.. This is probably explained by the fact that subjects, living in these regions, go to work at sites exhibiting higher pollution concentrations. The overall averages of indoor NO 2 levels were 21 m g my3 , of personal NO 2 levels 27 m g my3 and of outdoor levels 31 m g my3 . In the suburban, rural and alpine sites, the personal NO 2 values were closer to the outdoor levels than to the indoor levels. In the urban regions, where indoor sources of NO 2 were used, personal NO 2 levels were closer to the indoor levels. Boxplots of personal exposure to NO 2 among study areas are shown in Fig. 2. Highest medians were found at Geneva and Lugano and lowest at
Fig. 2. Boxplots of personal exposures to NO 2 for each study region. ŽWhite line inside box: median. Box limits: quartiles: 25th percentile Žlower. and 75th percentile Župper.. Whiskers Žwithin dotted line.: maxima observation Žup or down. within interquartile range Ž25th]75th percentile. multiplied with 1.5 Žstandard span.. Lines outside whiskers: single observations, outliers.
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Fig. 3. Comparison between indoor, outdoor and personal levels Žwinter period: October]March. across study centres. ŽGe: Geneva, Lu: Lugano, Ba: Basle, Aa: Aarau, Pa: Payerne, Wa: Wald, Da: Davos, Mo: Montana.. Mean values " standard deviation.
Montana, Wald and Davos. The variation of personal NO 2 levels is very great at all sites. At Aarau and Basle the concentration range was found to be very wide. The influence of season on the pollution concentrations are presented in Figs. 3 and 4. The full year was devided into winter ŽOctober] March. and summer ŽApril]September.. NO 2 outdoor levels during winter were about 25]40% higher than during summer. Winter indoor levels were in the range of the summer levels, in some regions higher, in others slightly lower. The most interesting finding, however, concerns the personal levels for which no significant differences between summer and winter were found. 3.2. Indoor sources 3.2.1. Gas-cooking Gas-cookers and gas-stoves are known to be important sources of indoor NO 2 . In SAPALDIA, gas cookers are used in the urban areas of Geneva, Basle and Lugano. Table 2 shows the averages of indoor and personal exposure levels for homes with gas-cookers and homes with electric cookers. The indoor levels were significantly higher Ž P - 0.005. in homes with gas-cookers. NO 2 indoor
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Fig. 4. Comparison between indoor, outdoor and personal levels Žsummer period: April]September. across study centres. ŽGe: Geneva, Lu: Lugano, Ba: Basle, Aa: Aarau, Pa: Payerne, Wa: Wald, Da: Davos, Mo: Montana.. Mean values " standard deviation.
levels were 5]8 m g my3 higher in homes with gas-cooker than in homes with electric cookers. The same finding was made forthe personal levels Žhighly significant P- 0.005 for all three regions.. Personal NO 2 levels of subjects living in homes with gas-cooker were on average 5 ] 6 m g my3 higher than personal NO 2 levels in homes with electric cookers. In a further statistical analysis it was controlled for outdoor levels since homes using gas tend to be more frequent in parts of town with higher pollution levels. When controlling for outdoor levels, the effect of gas cooking was still highly significant Ž P- 0.001.. 3.2.2. Smoking An investigation was carried out to determine if smokers’ indoor and personal levels were higher than those of non-smokers. In the questionnaire, subjects were asked if they were current smokers or not. Fig. 5 shows a comparison between personal levels of smokers and non-smokers across areas. Personal NO 2 levels among study sites were constantly higher in smokers than in non-smokers. ŽThe proportions of smokers were: Aa: 24%, Ba: 22%, Dav: 25%, Lu: 30%, Mo: 16%, Pa: 30%, Wa: 21%, Ge: 26%.. Because of the potential
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Table 2 Indoor and personal NO 2 levels in homes using gas-cooking and electric cooking Area
Percentage of Indoor Indoor Personal Personal homes using gas electric gas electric gas-cooking cooking cooking cooking cooking
Geneva 31 Basle 43 Lugano 49
40 Ž13. 29 Ž12. 36 Ž12.
32 Ž10. 24 Ž10. 30 Ž11.
45 Ž12. 35 Ž10. 40 Ž13.
38 Ž12. 31 Ž12. 35 Ž11.
Mean values and standard deviation Žin parentheses . all in m g my3 .
confounding with outdoor levels and gas-cooking, a model was set up which accounted for these factors. Highly significant difference Ž P- 0.01. was found at Wald, and significant differences Ž P- 0.05. were found at Lugano, Basle and Aarau. When controlling for outdoor NO 2 and gascooking in a model which included all regions, the positive effect of smoking on indoor NO 2 levels remained significant Ž P- 0.001..
Fig. 5. Personal NO 2 levels for smokers and non-smokers. ŽGe: Geneva, Lu: Lugano, Ba: Basle, Aa: Aarau, Pa: Payerne, Wa: Wald, Da: Davos, Mo: Montana.. Mean values " standard deviation.
3.3. Multiple models for personal exposure
In a second analysis, the relationships were calculated for subjects without indoor sources Žsmokers and gas-cooking.. The coefficients of determination did not improve in this models.
3.3.1. Correlation between indoor, outdoor and personal le¨els Univariate regression models were calculated to estimate indoor]outdoor, personal]outdoor and personal]indoor relationships ŽTable 3.. All three correlations were highly significant Ž P0.001.. The highest coefficient of determination was found between personal NO 2 levels and indoor levels Ž R 2 s 0.5149..
3.3.2. Model for predicting NO2 indoor le¨els: As the indoor NO 2 level is the best predictor for personal NO 2 levels, a multiple regression model which included outdoor NO 2 levels, gascooking, smoking status and ventilation habits, was calculated. Within this multiple regression model the parameter estimate of all factors can be made when controlling for all other parame-
Table 3 Linear regression analyses for indoorroutdoor, outdoorrpersonal and personalrindoor relationships
All subjects Indoor]outdoor Personal]outdoor Personal]indoor Excluding smokers and gas-cooking Indoor]outdoor Personal]outdoor Personal]indoor
Number of observations
Intercept
Slope
R2
1544 1494 1501
6.2 13.7 12.4
0.470 0.449 0.731
0.3734 0.3307 0.5149
968 943 943
6.1 13.7 11.6
0.402 0.384 0.738
0.3368 0.2739 0.4689
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ters. It gives, the net-effect of all parameters on the dependent variable Žindoor NO 2 .: w NO 2 x indoorss a q b 1 w NO 2 x outdoors q b 2 w gas - cookingx q b 3 w smokingx q b4 w ventilation x Table 4 shows parameter estimates when controlling for outdoor levels, gas-cooking, smoking and ventilation. Binary variables are: cooking status Ž0 s electric cooking, 1 s gas-cooking., smoking Ž0 s no, 1 s yes. and ventilation in the bedroom in winter Ž0 s seldom, 1 s regularly.. The slope between indoor and outdoor levels was 0.545; this is an estimate of average NO 2 indoorroutdoor ratios. For the binary-variable ‘gas-cooking’ Žparameter estimate: 5.2" 0.5 m g my3 . the parameter estimate was highly significant; for ‘smoking status’ Žparameter estimate: 2.2" 0.7 m g my3 . and ‘ventilation’ Žparameter estimate: 2.2" 0.8 m g my3 . the parameter estimates were significant. 3.4. Estimate of the ‘measurement error’ In order to obtain an ‘un-attenuated’ regression coefficient in the relationship between exposure and health, the variance within subjects should be small compared with the variation between the subjects. Brunekreef et al. Ž1987. and Armstrong Ž1990. conducted data analyses in order to quantify effects of measurement errors Žthe definition does not apply for the analytical error only but includes the error of misclassification.. As a measure for the attenuation, the variation
249
term l was defined as the ratio between the ‘within subject’ and the ‘between subject’ variance. Generally spoken, this is the ratio between the ‘measurement error’ and the ‘true error’. An estimate of the attenuation would then be defined as 1rŽ1 q lrk . for repeated Ž k . measurements. Table 5 shows the calculated l for the indoor home, the personal and the outdoor home NO 2 levels Žaverages over all study subjects.. Personal sampling and indoor sampling were repeated three times over a period of 1 year in these subjects Ž k s 3.. Outdoor measurements were repeated twelve times Žthree series of 4 consecutive weeks.. It has to be pointed out that all these values represent an estimates over all regions Žarithmetic means.. Table 5 shows that for the indoor and personal measurements, the l is smaller than for the outdoor measurements, which indicates a stronger temporal variation of the outdoor levels. On the other hand, the estimate of the attenuation factor is much smaller for the outdoor data, because the repetition of the measurement was four times greater than for the indoor and personal data. The precision of the passive samplers was within "5%, hence this contribution to the ‘measurement error’ is quite small. 4. Discussion Average personal exposure to NO 2 was measured for more than five hundred adult subjects in eight regions of Switzerland. This is one of the largest data base of measured personal NO 2 levels in adults. These values were compared with the average indoor and outdoor concentration at home. The average NO 2 personal concentration
Table 4 Multiple regression model for predicting NO 2 indoor levels Ž N s 502, R 2 s 0.584. Parameter
Parameter estimate
95% confidence interval lower limit
95% confidence interval upper limit
P-value
Intercept Outdoor conc. Gas-cooking Smoking Ventilation
0.841 0.545 5.28 2.22 2.12
y1.09 0.49 3.56 0.73 0.80
2.77 0.60 7.00 3.71 3.53
0.3936 0.0001 0.0001 0.0036 0.0020
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Table 5 Estimates of attenuation Measuring site
Estimate for l
Attenuation estimate 1rŽ1 q lrk .
Personal Indoor home Žbedroom. Outdoor home
0.67 0.59 0.80
0.82 0.84 0.94
Ž27 m g my3 . was between the average outdoor Ž31 m g my3 . and indoor levels Ž21 m g my3 .. In three regions with lowest outdoor NO 2 pollution ŽWald, Davos and Montana., average personal levels were higher than average outdoor levels. This may be explained by the fact that a considerable fraction of the population commutes to places of higher pollution Že.g. from Montana down to the valley, or from Wald to larger cities.. Indoor concentrations were between 50% and 80% Žoverall average. of the outdoor levels, except for homes with NO 2 indoor sources Žgas-cooker, smoking.. A seasonal stratification of the indoor]outdoor]personal relationships showed that during wintertime, ŽOctober]March. the indoor]outdoor gradient was higher than in summer ŽApril]September.. Outdoor NO 2 levels were higher in winter but the ventilation was lower. Personal exposure, however, was almost identical during the winter and the summer period. The higher indoor NO 2 levels in the summer and the greater amount of time spent outdoors lead to personal levels similar to winter, despite NO 2 pollution levels outdoors are higher in winter. Correlations between indoor levels and personal levels were higher than between outdoor levels and personal levels. The coefficient of determination for the personal]outdoor relationship was 0.33, and for the indoor]personal relationship 0.51. This has to expected, as a substantial fraction of time outside work is spent at home indoors. A model for predicting indoor NO 2 levels Žas a surrogate for personal NO 2 levels. showed that cooking status and outdoor air were good predictors. The contribution of gas-cooking to indoor NO 2 was estimated at 5 m g my3 . Smoking and ventilation habits were significant predictors as well. There are hence different sources for
NO 2 in indoor air and careful distinctions have to be made when indoor NO 2 is used as indicator of pollution: NO 2 from gas-cooking stands for another pollution mixture than NO 2 form outdoor pollution. From gas-cooking, acids such as nitrous acid ŽHNO2 . can be more abundant in the indoor environment of gas cooking homes ŽBrauer et al., 1990.. A large study conducted in Los Angeles ŽSpengler et al., 1994. showed median indoor NO 2 levels lower than and personal and outdoor levels which were very similar overall. The distribution of personal exposure reflected the outdoor spatial variations. In addition, Schwab et al. Ž1994. showed that the variability of indoorroutdoor relationship can vary from year to year and estimations for long-term exposure have to made carefully. Outdoor and indoor NO 2 levels do not reflect individuals exposure fully. Spatial outdoor NO 2 differences and differences in time-activity pattern might lead to great variability of personal NO 2 exposure among study sites. It was shown that personal sampling of NO 2 is very important if information of individual exposure is needed. When planning a study, it has to be evaluated if the expense of measuring individual personal pollution levels or if the approach of modelling should be chosen. Prediction of individual levels without measuring personal pollution levels can be done by assessing time-activity patterns, and conducting selective additional measurements in other microenvironments Že.g. some selected workplaces, commuting sites: train, tram, cars., and collecting information on indoor sources Žgas-stoves., and smoking status. For NO 2 , personal sampling can be favoured because a simple technique is available. For other pollutants however, i.e. suspended particulate matter there is no method available for sampling individual samples widespread and the approach of modelling might be easier. Acknowledgements SAPALDIA is part of the Swiss National Research Programme 26A supported by the Swiss
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National Foundation Žgrant No. 4026-28099. and by the Federal Office of Education and Science. SAPALDIA Basle is part of the European Respiratory Survey. References Armstrong BG. The effect of measurement errors on relative risk regressions. Am J Epidemiol 1990;132:1176]1184. Brauer M, Koutrakis P, Keeler GJ, Spengler JD. Indoor and outdoor concentrations of inorganic acidic aerosols and gases. J Air Waste Manage Assoc 1990;41:171]181. Braun C, Ackermann U, Schwartz J, Gnehm HP, Rutishauser M, Wanner H-U. Air pollution and respiratory symptoms in preschool children. Am Rev Respir Dis 1992;145:42]47. Brunekreef B, Noy D, Clausing P. Variability of exposure measurements in environmental epidemiology. Am J Epidemiol 1987;125:892]898. Committee on Advances in Assessing Human Exposure to Airborne Pollutants. Human exposure assessment for airborne pollutants. Washington, DC, National Academy Press, 1991:19.
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Hangartner M. Aktuelle Aufgaben der Messtechnik in der Luftreinhaltung. Kolloquium Heidelberg 29]31, VDIBerichte No. 838, 1990. Monn C, Frauchiger P, Wanner HU. Assessment of personal exposure to nitrogen-dioxide and ozone. In: Proceedings of Indoor Air ’93, vol. 3. Helsinki, 1993;319]323. Palmes ED, Gunnison AF. Personal sampler for nitrogen-dioxide. J Am Ind Hyg Assoc 1976;10:570]577. Schwab M, Dermott M, Spengler JD, Samet JM, Lambert WE. Seasonal and yearly patterns of indoor nitrogen dioxide levels: data from Albuquerque, New Mexico. Indoor Air 1994;4:8]22. Spengler JD, Schwab M, Ryan PB, Colome S, Wilson AL, Billick I, Becker E. Personal exposure to nitrogen dioxide in the Los Angeles basin. J Air Waste Manage Assoc 1994;44:39]47. Tomingas R, Grover YP. Schadstoffe in Wohnungen mit und ohne G asanlagen. Staub-Reinhaltung der Luft 1990;50:391]394. Wanner HU, Braun C, Monn C. Measurementof nitrogen dioxide indoor and outdoor concentrations with passive sampling devices. Proceedings of Indoor Air ’90, vol. 2. Toronto, 1990:503]507.
Personal exposure to nitrogen dioxide in Switzerland C. Monna,U , O. Brandli ¨ b , C. Schindler c , U. Ackermann-Liebrich c , P. Leuenberger d , SAPALDIA Team1 a
Department of Hygiene and Applied Physiology, En¨ironmental Hygiene Group, Federal Institute of Technology, ETH-Z, Clausiusstrasse 25, CH-8092 Zurich, Switzerland ¨ b Zurcher Hohenklinik Wald, CH-8636 Wald, Zurich, Switzerland ¨ ¨ c Institute for Social and Pre¨enti¨e Medicine, Uni¨ersity of Basle, Steinengraben 49, CH-4051 Basle, Switzerland d Di¨ision of Pneumology, Centre Hospitalier Uni¨ersitaire Vaudois, Rue du Bugnon, CH-1011 Lausanne, Switzerland Received 3 December 1997; accepted 16 January 1998
Abstract Home indoor and outdoor levels, and personal exposures to NO 2 were determined for more than 500 subjects in a subpopulation of SAPALDIA by using passive samplers. The overall personal NO 2 average was found to be 27 m g my3, the overall indoor average 21 m g my3 and the overall outdoor average 31 m g my3. Personal NO 2 levels ranged between the outdoor and indoor levels, with the exception of study areas with low NO 2 concentrations. In the winter, the indoorroutdoor ratios were lower than in the summer. Outdoor NO 2 levels were higher in winter. In some study areas, indoor NO 2 levels were lower in the winter than in the summer due to reduced ventilation but this was not consistent. Personal NO 2 concentrations were very similar during all seasons. Gas-cooking and smoking were important factors for elevated indoor and personal NO 2 levels Žcontribution: 5 m g my3, 2 m g my3 , respectively.. Personal exposure to NO 2 correlated best with the indoor NO 2 concentrations. Q 1998 Elsevier Science B.V. Keywords: Nitrogen dioxide; Personal exposure; Indoor; Outdoor; Passive sampling; Gas-cooking; Exposure assessment
U
Corresponding author. Tel.: q41 1 6324629; fax: q41 1 6321318; e-mail: [email protected] SAPALDIA ŽSwiss Study on Air Pollution and Lung Diseases in Adults. Team: Study Director: P. Leuenberger ŽP., Programme ŽP., Director: U. Ackermann-Liebrich ŽE.. In¨estigators: P. Alean ŽAM., K. Blaser ŽA., G. Bolognini ŽP., J.P. Bongard ŽP., O. Brandli ¨ P. Braun ŽP., C. Bron ŽL., M. Brutsche ŽL., C. Defila ŽM., G. Domenighetti ŽP., S. Elsasser ŽL., L. Grize ´ ŽS., P. Guldimann ŽL., P. ŽE., J.C. Luthy ŽL., B.W. Martin ŽE., T. Medici ŽP., Hufschmied ŽL., W. Karrer ŽP., H. Keller-Wossidlo ŽO., R. Keller ŽP., N. Kunzli ¨ C. Monn ŽAM., A.G. Peeters ŽPA., A. Perruchoud ŽP., A. Radaelli ŽL., C. Schindler ŽS., J. Schwartz ŽS., M. Schoni ¨ ŽP., G. Solari ŽP., J.M. Tschopp ŽP., B. Villiger ŽP., B. Wuthrich ŽA., J.P. Zellweger ŽP., E. Zemp ŽE.. Specialities: A, Allergology; AM, Air ¨ Pollution Monitoring; E, Epidemiology; L, Local Assistant MD; PA, Palynology; P, Pneumology; M, Meteorology; O, Occupational Medicine; S, Statistics. 1
0048-9697r98r$19.00 Q 1998 Elsevier Science B.V. All rights reserved. PII S0048-9697Ž98.00124-7
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1. Introduction Exposure is a term associated with some uncertainty. Exposure to an atmospheric pollutant may be defined as contact with the pollutant of a known concentration for a defined time period; this leads to information on an exposure-dose ŽCommittee on Advances in Assessing Human Exposure to Airborne Pollutants, 1991.. A simpler definition defines exposure as any contact of a human with a pollutant. In air pollution epidemiology, the use of Žpollutant. concentrations is more widespread than the use of a dose Žwhich is mainly used in biomarker studies.. Exposure levels can be determined directly or indirectly ŽCommittee on Advances in Assessing Human Exposure to Airborne Pollutants, 1991.. Direct approaches use personal measurements at the individuals or biomarkers from body fluids. Indirect approaches use ‘surrogates’, such as outdoor or indoor concentration and also models. Microenvironmental models are based on known concentrations in these so-called ‘microenvironments’ Ža location with a homogeneous pollution distribution. and known time activity Žor budget. pattern. In order to establish and validate such models it is crucial to have a large-scale database of microenvironmental levels, time-activity patterns, and personal measurements. For NO 2 , a simple technology Žpassive samplers. is available for the determination of personal exposures. Passive samplers have the advantage that they can be used to determine individual exposures but also for establishing large-scale databases with indoor and outdoor data, which can be used to establish and validate models. Within the framework of SAPALDIA ŽSwiss Study on Air Pollution and Lung Diseases in Adults., personal exposure to nitrogen dioxide was measured in a subsample of more than 500 subjects from the eight study regions from December 1992 to December 1993. In addition, parallel measurements were conducted outdoors of the home and indoors in the bedroom. To date, little information on personal exposure to NO 2 of adults in Switzerland has been available, but in 1985r1986 in a study with children, personal levels of NO 2 were determined ŽBraun et al., 1992..
In this study, a strong relationship between personal exposure levels of NO 2 and indoor levels was found. Exposure in adults, however, shows higher variation and covers more microenvironments. A preliminary study was conducted in 1992, in which personal exposure of adults to NO 2 and O 3 was investigated ŽMonn et al., 1993.. Moreover, in recent years, many studies have been conducted comparing indoor with outdoor levels, and they showed that indoor levels of NO 2 are between 50 and 90% of outdoor levels, in the absence of indoor sources ŽMonn et al., 1993; Spengler et al., 1994; Tomingas and Grover, 1990; Wanner et al., 1990.. In the absence of indoor sources, NO 2 levels are a function of the outdoor concentrations and the ventilation rate, and the ventilation rate shows a seasonal pattern. In Switzerland, air conditioning is rarely used so that ventilation depends on the outdoor air temperature. Indoor sources, such as gas stoves and gas heaters, may lead to indoor NO 2 levels exceeding levels measured outdoors. Such homes have to be treated separately when using NO 2 as an indicator of ambient pollution, as the mixture of gas combustion emissions is different from the mixture of outdoor pollutants. The purpose of this analysis was to evaluate the relationship between personal NO 2 levels with indoor and outdoor levels at home. As from all participants of the passive sampler study, information on indoor sources Žgas-stoves, smoking status ., ventilation habits, situation of home with respect to streets as well as on occupational exposure was available, the effects of these parameters on personal exposure to NO 2 were additionally estimated. In a multiple regression model, the effects of these parameters on personal NO 2 exposures were quantified. 2. Methods 2.1. Participants SAPALDIA is an eight-centre study with four urbanrsuburban regions ŽGeneva, Basle, Lugano, Aarau., two rural regions ŽWald, Payerne. and two alpine regions ŽMontana, Davos.. In each region, more than 500 subjects were
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invited to participate in the follow-up study. From this sample, a stratified random subsample was selected in each area. Stratification was to ensure that any time measurements provided a faithful picture of differences in NO 2 levels across subregions. The measurement program covered a full year. In larger study regions Žurban, suburban areas., where a higher range NO 2 concentration was expected, the subsample was greater than in regions, where a more homogeneous distribution was expected. Table 1 shows the number of selected subjects for each region. Subjects participated between December 1993 and December 1994, most during three periods of 1 month. In the first week of each period, personal, indoor and outdoor levels were measured, and in the next 3 consecutive weeks only outdoor levels were measured. As personal sampling requires much discipline, it was carried out only in the first week. From all subjects, information on cooking methods, home ventilation, smoking, occupational exposure and the characteristics of home with respect to traffic density was available. 2.2. Measurements NO 2 was measured with Palmes tubes ŽPalmes et al., 1976.. An adaptation of the method was used for measuring ambient NO 2 concentrations ŽHangartner, 1990.. Metal grids were coated with triethanol-amine, dissolved in a mixture of acetonerwater. Samplers were posted to local SAPALDIA offices from where they were sent to subjects on a weekly basis. Subjects were given verbal instruction before their first measurement Table 1 Measurements in each region and the total number of personal measurements Area
Number of persons Total number of weekly measuring simultaneously personal observations
Geneva Basle Lugano Aarau Payerne Wald Davos Montana
16 28 16 20 16 16 16 12
131 337 204 204 169 203 194 144
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period and written instructions were included with each weekly mailing. The shipping time of the tubes was between one and 2 days. After exposure, the time protocols and the exposed tubes were sent back to the local teams where they were stored in a dark and preferably cold place. Twice a month, the exposed tubes were sent to the central analytical lab at the ETH where analysis took place within 1 day. The maximum storing time of an exposed tube was 2 weeks Žminimum 1 week.. After returning, the samplers were analysed according to Saltzmann ŽNEDA. and NO 2 concentration in volume of air were calculated according to Fick’s law of diffusion. Samplers were exposed at home outdoors Žoutside the window of the bedroom. and indoors Žin the bedroom, 1.5]2 m above ground.. Moreover, subjects carried the samplers for personal sampling on outer clothing in the breast]head region for 1 week. For each passive sampler, the exposure period was 1 week. Samplers which were exposed more than 2 weeks or less than 3 days were excluded from statistical analyses. For weekly exposures, the precision of the passive sampler was found to be "5% ŽHangartner, 1990.. Each subject participating in the ‘cross-sectional’ study was asked the following questions Žamong others.: v
v v
What type of cooking facility do you use: gas cooker or electric cooker? Are you a current smoker: yes or no? Do you open the windows of the bedroom during winter regularly or seldom? Žthese two answers represents the two extremes of three possibilities: regularly, sometimes, seldom.
All statistical analyses Žsummary statistics, multiple regressions. were carried out in SASW , version 6.08 for PC. 3. Results 3.1. Comparison between indoor, outdoor and personal NO2 concentrations (all subjects) Fig. 1 shows the average values of outdoor,
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Fig. 1. Comparison between indoor, outdoor and personal levels Žfull year. across study centres. ŽGe: Geneva, Lu: Lugano, Ba: Basle, Aa: Aarau, Pa: Payerne, Wa: Wald, Da: Davos, Mo: Montana.. Mean values " standard deviation.
indoor and personal measurements. Comparing the outdoor concentrations, a gradient between the study sites was found. Looking at the indoor levels, a gradient was found between the urban
sites, but from Aarau to Montana the variation of indoor levels was much smaller. Indoor levels were about 70]80% of the outdoor levels. For the personal levels, a gradient was found between the five most polluted regions and to a lesser extent in the least polluted regions. In the three least polluted regions ŽDavos, Wald, Montana., personal levels were even greater than the outdoor levels Žsignificant for Montana P- 0.001.. This is probably explained by the fact that subjects, living in these regions, go to work at sites exhibiting higher pollution concentrations. The overall averages of indoor NO 2 levels were 21 m g my3 , of personal NO 2 levels 27 m g my3 and of outdoor levels 31 m g my3 . In the suburban, rural and alpine sites, the personal NO 2 values were closer to the outdoor levels than to the indoor levels. In the urban regions, where indoor sources of NO 2 were used, personal NO 2 levels were closer to the indoor levels. Boxplots of personal exposure to NO 2 among study areas are shown in Fig. 2. Highest medians were found at Geneva and Lugano and lowest at
Fig. 2. Boxplots of personal exposures to NO 2 for each study region. ŽWhite line inside box: median. Box limits: quartiles: 25th percentile Žlower. and 75th percentile Župper.. Whiskers Žwithin dotted line.: maxima observation Žup or down. within interquartile range Ž25th]75th percentile. multiplied with 1.5 Žstandard span.. Lines outside whiskers: single observations, outliers.
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Fig. 3. Comparison between indoor, outdoor and personal levels Žwinter period: October]March. across study centres. ŽGe: Geneva, Lu: Lugano, Ba: Basle, Aa: Aarau, Pa: Payerne, Wa: Wald, Da: Davos, Mo: Montana.. Mean values " standard deviation.
Montana, Wald and Davos. The variation of personal NO 2 levels is very great at all sites. At Aarau and Basle the concentration range was found to be very wide. The influence of season on the pollution concentrations are presented in Figs. 3 and 4. The full year was devided into winter ŽOctober] March. and summer ŽApril]September.. NO 2 outdoor levels during winter were about 25]40% higher than during summer. Winter indoor levels were in the range of the summer levels, in some regions higher, in others slightly lower. The most interesting finding, however, concerns the personal levels for which no significant differences between summer and winter were found. 3.2. Indoor sources 3.2.1. Gas-cooking Gas-cookers and gas-stoves are known to be important sources of indoor NO 2 . In SAPALDIA, gas cookers are used in the urban areas of Geneva, Basle and Lugano. Table 2 shows the averages of indoor and personal exposure levels for homes with gas-cookers and homes with electric cookers. The indoor levels were significantly higher Ž P - 0.005. in homes with gas-cookers. NO 2 indoor
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Fig. 4. Comparison between indoor, outdoor and personal levels Žsummer period: April]September. across study centres. ŽGe: Geneva, Lu: Lugano, Ba: Basle, Aa: Aarau, Pa: Payerne, Wa: Wald, Da: Davos, Mo: Montana.. Mean values " standard deviation.
levels were 5]8 m g my3 higher in homes with gas-cooker than in homes with electric cookers. The same finding was made forthe personal levels Žhighly significant P- 0.005 for all three regions.. Personal NO 2 levels of subjects living in homes with gas-cooker were on average 5 ] 6 m g my3 higher than personal NO 2 levels in homes with electric cookers. In a further statistical analysis it was controlled for outdoor levels since homes using gas tend to be more frequent in parts of town with higher pollution levels. When controlling for outdoor levels, the effect of gas cooking was still highly significant Ž P- 0.001.. 3.2.2. Smoking An investigation was carried out to determine if smokers’ indoor and personal levels were higher than those of non-smokers. In the questionnaire, subjects were asked if they were current smokers or not. Fig. 5 shows a comparison between personal levels of smokers and non-smokers across areas. Personal NO 2 levels among study sites were constantly higher in smokers than in non-smokers. ŽThe proportions of smokers were: Aa: 24%, Ba: 22%, Dav: 25%, Lu: 30%, Mo: 16%, Pa: 30%, Wa: 21%, Ge: 26%.. Because of the potential
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Table 2 Indoor and personal NO 2 levels in homes using gas-cooking and electric cooking Area
Percentage of Indoor Indoor Personal Personal homes using gas electric gas electric gas-cooking cooking cooking cooking cooking
Geneva 31 Basle 43 Lugano 49
40 Ž13. 29 Ž12. 36 Ž12.
32 Ž10. 24 Ž10. 30 Ž11.
45 Ž12. 35 Ž10. 40 Ž13.
38 Ž12. 31 Ž12. 35 Ž11.
Mean values and standard deviation Žin parentheses . all in m g my3 .
confounding with outdoor levels and gas-cooking, a model was set up which accounted for these factors. Highly significant difference Ž P- 0.01. was found at Wald, and significant differences Ž P- 0.05. were found at Lugano, Basle and Aarau. When controlling for outdoor NO 2 and gascooking in a model which included all regions, the positive effect of smoking on indoor NO 2 levels remained significant Ž P- 0.001..
Fig. 5. Personal NO 2 levels for smokers and non-smokers. ŽGe: Geneva, Lu: Lugano, Ba: Basle, Aa: Aarau, Pa: Payerne, Wa: Wald, Da: Davos, Mo: Montana.. Mean values " standard deviation.
3.3. Multiple models for personal exposure
In a second analysis, the relationships were calculated for subjects without indoor sources Žsmokers and gas-cooking.. The coefficients of determination did not improve in this models.
3.3.1. Correlation between indoor, outdoor and personal le¨els Univariate regression models were calculated to estimate indoor]outdoor, personal]outdoor and personal]indoor relationships ŽTable 3.. All three correlations were highly significant Ž P0.001.. The highest coefficient of determination was found between personal NO 2 levels and indoor levels Ž R 2 s 0.5149..
3.3.2. Model for predicting NO2 indoor le¨els: As the indoor NO 2 level is the best predictor for personal NO 2 levels, a multiple regression model which included outdoor NO 2 levels, gascooking, smoking status and ventilation habits, was calculated. Within this multiple regression model the parameter estimate of all factors can be made when controlling for all other parame-
Table 3 Linear regression analyses for indoorroutdoor, outdoorrpersonal and personalrindoor relationships
All subjects Indoor]outdoor Personal]outdoor Personal]indoor Excluding smokers and gas-cooking Indoor]outdoor Personal]outdoor Personal]indoor
Number of observations
Intercept
Slope
R2
1544 1494 1501
6.2 13.7 12.4
0.470 0.449 0.731
0.3734 0.3307 0.5149
968 943 943
6.1 13.7 11.6
0.402 0.384 0.738
0.3368 0.2739 0.4689
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ters. It gives, the net-effect of all parameters on the dependent variable Žindoor NO 2 .: w NO 2 x indoorss a q b 1 w NO 2 x outdoors q b 2 w gas - cookingx q b 3 w smokingx q b4 w ventilation x Table 4 shows parameter estimates when controlling for outdoor levels, gas-cooking, smoking and ventilation. Binary variables are: cooking status Ž0 s electric cooking, 1 s gas-cooking., smoking Ž0 s no, 1 s yes. and ventilation in the bedroom in winter Ž0 s seldom, 1 s regularly.. The slope between indoor and outdoor levels was 0.545; this is an estimate of average NO 2 indoorroutdoor ratios. For the binary-variable ‘gas-cooking’ Žparameter estimate: 5.2" 0.5 m g my3 . the parameter estimate was highly significant; for ‘smoking status’ Žparameter estimate: 2.2" 0.7 m g my3 . and ‘ventilation’ Žparameter estimate: 2.2" 0.8 m g my3 . the parameter estimates were significant. 3.4. Estimate of the ‘measurement error’ In order to obtain an ‘un-attenuated’ regression coefficient in the relationship between exposure and health, the variance within subjects should be small compared with the variation between the subjects. Brunekreef et al. Ž1987. and Armstrong Ž1990. conducted data analyses in order to quantify effects of measurement errors Žthe definition does not apply for the analytical error only but includes the error of misclassification.. As a measure for the attenuation, the variation
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term l was defined as the ratio between the ‘within subject’ and the ‘between subject’ variance. Generally spoken, this is the ratio between the ‘measurement error’ and the ‘true error’. An estimate of the attenuation would then be defined as 1rŽ1 q lrk . for repeated Ž k . measurements. Table 5 shows the calculated l for the indoor home, the personal and the outdoor home NO 2 levels Žaverages over all study subjects.. Personal sampling and indoor sampling were repeated three times over a period of 1 year in these subjects Ž k s 3.. Outdoor measurements were repeated twelve times Žthree series of 4 consecutive weeks.. It has to be pointed out that all these values represent an estimates over all regions Žarithmetic means.. Table 5 shows that for the indoor and personal measurements, the l is smaller than for the outdoor measurements, which indicates a stronger temporal variation of the outdoor levels. On the other hand, the estimate of the attenuation factor is much smaller for the outdoor data, because the repetition of the measurement was four times greater than for the indoor and personal data. The precision of the passive samplers was within "5%, hence this contribution to the ‘measurement error’ is quite small. 4. Discussion Average personal exposure to NO 2 was measured for more than five hundred adult subjects in eight regions of Switzerland. This is one of the largest data base of measured personal NO 2 levels in adults. These values were compared with the average indoor and outdoor concentration at home. The average NO 2 personal concentration
Table 4 Multiple regression model for predicting NO 2 indoor levels Ž N s 502, R 2 s 0.584. Parameter
Parameter estimate
95% confidence interval lower limit
95% confidence interval upper limit
P-value
Intercept Outdoor conc. Gas-cooking Smoking Ventilation
0.841 0.545 5.28 2.22 2.12
y1.09 0.49 3.56 0.73 0.80
2.77 0.60 7.00 3.71 3.53
0.3936 0.0001 0.0001 0.0036 0.0020
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Table 5 Estimates of attenuation Measuring site
Estimate for l
Attenuation estimate 1rŽ1 q lrk .
Personal Indoor home Žbedroom. Outdoor home
0.67 0.59 0.80
0.82 0.84 0.94
Ž27 m g my3 . was between the average outdoor Ž31 m g my3 . and indoor levels Ž21 m g my3 .. In three regions with lowest outdoor NO 2 pollution ŽWald, Davos and Montana., average personal levels were higher than average outdoor levels. This may be explained by the fact that a considerable fraction of the population commutes to places of higher pollution Že.g. from Montana down to the valley, or from Wald to larger cities.. Indoor concentrations were between 50% and 80% Žoverall average. of the outdoor levels, except for homes with NO 2 indoor sources Žgas-cooker, smoking.. A seasonal stratification of the indoor]outdoor]personal relationships showed that during wintertime, ŽOctober]March. the indoor]outdoor gradient was higher than in summer ŽApril]September.. Outdoor NO 2 levels were higher in winter but the ventilation was lower. Personal exposure, however, was almost identical during the winter and the summer period. The higher indoor NO 2 levels in the summer and the greater amount of time spent outdoors lead to personal levels similar to winter, despite NO 2 pollution levels outdoors are higher in winter. Correlations between indoor levels and personal levels were higher than between outdoor levels and personal levels. The coefficient of determination for the personal]outdoor relationship was 0.33, and for the indoor]personal relationship 0.51. This has to expected, as a substantial fraction of time outside work is spent at home indoors. A model for predicting indoor NO 2 levels Žas a surrogate for personal NO 2 levels. showed that cooking status and outdoor air were good predictors. The contribution of gas-cooking to indoor NO 2 was estimated at 5 m g my3 . Smoking and ventilation habits were significant predictors as well. There are hence different sources for
NO 2 in indoor air and careful distinctions have to be made when indoor NO 2 is used as indicator of pollution: NO 2 from gas-cooking stands for another pollution mixture than NO 2 form outdoor pollution. From gas-cooking, acids such as nitrous acid ŽHNO2 . can be more abundant in the indoor environment of gas cooking homes ŽBrauer et al., 1990.. A large study conducted in Los Angeles ŽSpengler et al., 1994. showed median indoor NO 2 levels lower than and personal and outdoor levels which were very similar overall. The distribution of personal exposure reflected the outdoor spatial variations. In addition, Schwab et al. Ž1994. showed that the variability of indoorroutdoor relationship can vary from year to year and estimations for long-term exposure have to made carefully. Outdoor and indoor NO 2 levels do not reflect individuals exposure fully. Spatial outdoor NO 2 differences and differences in time-activity pattern might lead to great variability of personal NO 2 exposure among study sites. It was shown that personal sampling of NO 2 is very important if information of individual exposure is needed. When planning a study, it has to be evaluated if the expense of measuring individual personal pollution levels or if the approach of modelling should be chosen. Prediction of individual levels without measuring personal pollution levels can be done by assessing time-activity patterns, and conducting selective additional measurements in other microenvironments Že.g. some selected workplaces, commuting sites: train, tram, cars., and collecting information on indoor sources Žgas-stoves., and smoking status. For NO 2 , personal sampling can be favoured because a simple technique is available. For other pollutants however, i.e. suspended particulate matter there is no method available for sampling individual samples widespread and the approach of modelling might be easier. Acknowledgements SAPALDIA is part of the Swiss National Research Programme 26A supported by the Swiss
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National Foundation Žgrant No. 4026-28099. and by the Federal Office of Education and Science. SAPALDIA Basle is part of the European Respiratory Survey. References Armstrong BG. The effect of measurement errors on relative risk regressions. Am J Epidemiol 1990;132:1176]1184. Brauer M, Koutrakis P, Keeler GJ, Spengler JD. Indoor and outdoor concentrations of inorganic acidic aerosols and gases. J Air Waste Manage Assoc 1990;41:171]181. Braun C, Ackermann U, Schwartz J, Gnehm HP, Rutishauser M, Wanner H-U. Air pollution and respiratory symptoms in preschool children. Am Rev Respir Dis 1992;145:42]47. Brunekreef B, Noy D, Clausing P. Variability of exposure measurements in environmental epidemiology. Am J Epidemiol 1987;125:892]898. Committee on Advances in Assessing Human Exposure to Airborne Pollutants. Human exposure assessment for airborne pollutants. Washington, DC, National Academy Press, 1991:19.
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Hangartner M. Aktuelle Aufgaben der Messtechnik in der Luftreinhaltung. Kolloquium Heidelberg 29]31, VDIBerichte No. 838, 1990. Monn C, Frauchiger P, Wanner HU. Assessment of personal exposure to nitrogen-dioxide and ozone. In: Proceedings of Indoor Air ’93, vol. 3. Helsinki, 1993;319]323. Palmes ED, Gunnison AF. Personal sampler for nitrogen-dioxide. J Am Ind Hyg Assoc 1976;10:570]577. Schwab M, Dermott M, Spengler JD, Samet JM, Lambert WE. Seasonal and yearly patterns of indoor nitrogen dioxide levels: data from Albuquerque, New Mexico. Indoor Air 1994;4:8]22. Spengler JD, Schwab M, Ryan PB, Colome S, Wilson AL, Billick I, Becker E. Personal exposure to nitrogen dioxide in the Los Angeles basin. J Air Waste Manage Assoc 1994;44:39]47. Tomingas R, Grover YP. Schadstoffe in Wohnungen mit und ohne G asanlagen. Staub-Reinhaltung der Luft 1990;50:391]394. Wanner HU, Braun C, Monn C. Measurementof nitrogen dioxide indoor and outdoor concentrations with passive sampling devices. Proceedings of Indoor Air ’90, vol. 2. Toronto, 1990:503]507.