EFFECTS OF INDOOR EXPOSURE TO NITROGEN DIOXIDE ON PULMONARY FUNCTION OF WOMEN LIVING IN URBAN AND RURAL AREAS
Paul Fischer, Bert Brunekreef*, and Klaas Biersteker Department of Environmental Health, University of Wageningen, P. O. Box 238, 6700 AE Wageningen, The Netherlands
Jan S.M. Boleij Department of Air Pollution, University of Wageningen, The Netherlands
Roelof van der Lende and Jan Schouten Department of Epidemiology, University of Groningen, The Netherlands
Philip H. Quanjer Department of Physiology, University of Leiden, The Netherlands
E1 87-372 (Received 9 November 1987; Accepted 15 February 1989)
The heal th effects of indoor NO= pol lu t ion were studied among two popula t ions o f adul t women. One popula t ion was l iv ing in a rural area, one in an urban area. Exposure to NO= was measured in the homes of the complete study populat ion. Over 500 women were studied. Data on pu lmonary funct ion and respi ra tory symptoms were used to assess the respi ra tory heal th of the women. S igni f ican t associa t ions were found be tween exposure to NO2 and pu lmonary funct ion among the non-smoking women l iv ing in the rural area, but not among the smoking women in that area, or among the non-smoking and smoking women l iv ing in the urban area.
INTRODUCTION
In the Netherlands, about 50% of all houses are equipped with gas-fired water heaters. In about 50% of these houses, the water heater is unrented, which can result in high nitrogen dioxide levels indoors. Because of the relatively high proportion of people exposed, a survey was started in 1983 to assess the effects of indoor exposure to NO 2 on pulmonary func- tion and respiratory symptoms. In addition, informa- tion on the smoking habits of the residents was gathered to assess the relationship between passive smoking and respiratory health.
In contrast to studies on the relationship between indoor air pollution and health effects among chil- dren, studies in which adults were involved have
*To whom corrcspondenon should be addressed.
been scarce. Comstock (1981) found an association between the use of gas for cooking and respiratory symptoms as well as pulmonary function in men, but not in women. Helsing (1982), in a re-analysis of the data, however, found the same associations, indepen- dently of gender. Jones (1983) found a marginally significant association between low FEV I and gas cooking in non-smoking women. Keller et al. (1979) were not able to demonstrate any association. Incon- sistency of results seems to be a general feature of studies on health effects of gas cooking/indoor NO 2 exposure (Ogsten et al. 1985). Incomparability of methodology is one potential explanation. Another reason might be that exposure was mostly estimated using the 'gas versus electricity' dichotomy, which may lead to considerable misclassification (Spengler and Socsek 1984, Remijn et al. 1985). By applying the same methodology to different populations and
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376 P. Fischer ct al.
by actually measuring exposure indoors, we have tried to address these issues in our studies.
Studies on health effects due to exposure to to- bacco smoke among non-smoking adults have been relatively scarce as well. Schilling (1977) did not find any association between respiratory desease and smoking of the spouse. White (1980) showed that exposure to tobacco smoke at the workplace can lead to a decrease of lung functions which was equal to the effect of 1 to 10 cigarettes active smoking. Corn- stock (1981) did not find any effect of tobacco smoke among men due to the smoking of their wives. Hels- ing (1982), in a re-analysis, confirmed these results. Kauffmann et al. (1983) presented a paper in which they showed that smoking of the husband of a mini- mum of 10 g of tobacco per day can lead to a decrease of the pulmonary function of the non-smoking wife. The effects were more pronounced among women over 40 years of age, suggesting that the length of exposure determines the magnitude of the effect.
In the first and second part of our survey, in which we assessed the relationship between NO2/tobacco smoke and lung functions among women living in a rural area, we found an association between both exposures and lung functions of the non-smoking women (Fischer et al. 1986a). No associations were seen among the smoking women.
METHODS
Population The survey was performed in three parts. In the
first part, non-smoking women living in a rural area were involved in the survey. In the second part, smok- ing and non-smoking rural women were added, and in the third part, smoking and non-smoking women in an urban area were studied. The results of the first two surveys were previously published (Fischer et al. 1985, 1986a).
In 1965, random samples of the adult populations of Vlaardingen (urban) and Vlagtwedde (rural) were invited to participate in a longitudinal study on the epidemiology of chronic non-specific lung disease (Van der Lende, 1969; Van tier Lende et al. 1981). All non-smoking women in Vlagtwedde who participated in at least the first survey and in the survey of 1979 were invited to participate in the indoor study in 1982. In 1983, all smoking and ex-smoking women in Vlagtwedde who participated in at least the first survey and in the surveys of either 1979, 1982, or both were invited. In 1984, all women in Vlaardingen who had participated in the first survey and who were expected to participate in the 1984 survey were in-
vited to participate in the indoor survey. In this paper, the results of the third part of the survey will be presented, and the complete study results will be discussed.
The study population consisted of a sub-sample of 612 adult women who had already participated in the above-mentioned longitudinal study on the natural history and determinants of chronic, non-specific lung disease (Van der Lende 1969; Van der Lende et al. 1981). In that study, the lung function of the partici- pants has been measured every three years since 1967. In the winters of 1982/1983 to 1984/1985, the houses of a sub-sample of the populat ion were visited. 164 women were visited in the winter of 1982/1983; 149 in the winter 1983/1984; and 299 in the winter of 1984/1985. All the women visited in the first two winters lived in a rural area in the north- eastern part of the Netherlands, where there is little air pollution. In the first winter, only women who had never smoked were invited for the survey. In the second winter, the remaining women were invited to participate, regardless of their smoking habits. In the third winter, all women who had agreed to participate and were living in the urban area were visited in their homes. Of those invited, 84% agreed to participate.
NO2 measurements
During a one week period, NO 2 measurements were performed with Palmes' diffusion tubes in the houses of the participants (Palmes 1976). Measurements were taken in the kitchen, living room, and bedroom. In the second and third winter, the participants were asked to wear a personal sampler during the measure- ment week. The personal sampler was technically identical to the indoor samplers.
In addition to the measurements, information about the time spent in each location was collected during the measurement week using self- administered diary forms.
Passive smoking
Exposure to tobacco smoke was estimated by ask- ing for the amount of tobacco smoked inside the house per day. To refine the passive smoking expo- sure estimates, this information was gathered for the last 18 years in three time periods: the period 1965 to 1970; the period 1970 to 1975; and the period 1975 to 1983. For every period, the respondents were asked for the number of smokers in the house and the amount of tobacco smoked inside the house per day. Active smokers were asked for the current number of cigarettes they smoked per day.
Indoor exposure to nitrogen dioxide 377
Assessment of pulmonary function
The pulmonary funct ion measurements were per formed in the longi tudinal f ield study every three years since 1967 with a Lode D53 water sealed spirometer (Van der Lende 1969). In 1973, additio- nal pulmonary function measurements, performed with a pneumotachograph, were introduced in the survey (Quanjer 1983). The pulmonary function data ob- tained during the regular longitudinal field study were used in the indoor air study to assess the relation between exposure to NO 2 and tobacco smoke, and the pulmonary functions of the residents.
Statistical analyses
The association between the lung functions and the exposures was analyzed using multiple regres- sion analyses, as available in the Statistical Package for the Social Sciences. In the analyses, adjustments were made for age, standing height, and socioeco- nomic status (SES). Educational level of the husband was used as a proxy for SES. No adjustments for allergy score and number of eosinophilic cells in blood had been made. In addition to these adjust- ments, the association between pulmonary function and NO 2 exposure was corrected for passive smok- ing, and the association between pulmonary function and passive smoking was corrected for NO 2 expo- sure. For smoking women, no passive smoking effect was investigated.
Because the first and second parts of the indoor air survey were performed in the same village but in different years, in the analyses of the combined sam- ples a correction was also made for year of NO 2 measurement by incorporating dummy variables for 'year ' in the analysis. This was done because differ- ences in indoor air pollution (NO 2 levels) between the two measurement periods could have originated from differences in outdoor air pollution between the two winter periods.
In addition to the NO z regressions, analyses were also performed using categorical exposure variables (e.g., presence or absence of an unvented, gas fired water heater). This was done to investigate whether actually measuring exposure gave different results from simple, categorical exposure classifications.
The analyses were cross-sectional. Associations between pulmonary change and exposure to NO 2 and tobacco smoke were found to be non-significant in the rural population. This was found to be the case within the urban population as well, and no further results of the longitudinal analysis will be reported.
RESULTS
NO2 measurements
Outdoor NO 2 concentrations were consistently higher in the urban area. Amongst other factors, this is due to a higher traffic density, the presence of a number of oil refineries, and of gas heated greenhouses situa- ted in the area.
In Table 1, the weekly averages for the different locations in the houses and the personal NO 2 expo- sure data are given.
As can be seen from the table, all the NO 2 concen- trations in the urban area were higher than in the rural area. Outdoor NO 2 concentrations during the mea- surement period were measured at stationary points by the National Institute of Public Health and Envi- ronmental Hygiene. Differences in outdoor NO 2 lev- els between the two areas might be responsible for the differences in indoor NO 2 pollution; however, when we break down the indoor concentration levels by the presence of gas fired water heaters (Table 2), it becomes clear that the differences in indoor con- centrations between the two areas are mainly caused by differences in the distribution of indoor sources (Fischer et al., 1986b). It is also clear that the pres- ence of an unvented, gas fired water heater in the kitchen was the main determinant of the indoor NO 2 concentration in the houses in this study.
Table 1. Weekly average NOffi concentrations in the subsequent measurement periods.
Year Area N Kitchen Livingroom Bedroom Personal" Outdoor
'arithmetic concentration in ~g/m s. bin parentheses: standard deviation.
Lung function and NO2 exposure
Before analyzing this relationship, a selection was made in the populat ion. Only cases with full in- formation about age, height, educational level, and amount of passive smoking were selected. The re- suits of the analyses are presented in Tables 3-5. From Table 3, in which the regression coefficients of the pulmonary function parameter on the N O 2 param- eter is given for the non-smokers, it becomes clear that in the group of non-smoking women living in the rural area, a generally negative association was found between the lung function and NO 2 parame- ters, which was in some cases statistically signifi- cant. In the urban area, however, no such association was found.
In Table 4, the associations between several N O 2
exposure measures and several pulmonary function parameters are shown for women who quit smoking. Among the ex-smokers in the rural area, again a generally negative association was found, which was also found in the group of ex-smokers in the urban area. The numbers in this category were small in both areas.
Among the smoking women (Table 5), no gener- ally negative association was found. In some cases, even a statistically significant positive association was found.
Associations between the presence of unvented water heaters and pulmonary function were insignif- icant (Table 6).
Table 3. Adjusted regression coefficients of pulmonary function on NO2 exposure in non-smoking women.
Lung function Livingroom' Personal, estimated' Personal, measured'
• n = 54. 'n = 68. ha = 53. Sn = 6 6 . "for abbreviat ions see Table 3. *p < 0.05. din (mL/s):(IJ.g/m3). **p < 0.01. 'standard error.
Lung function and passive smoking
The association between pulmonary function and exposure to tobacco smoke was only analyzed for non-smoking women (women who had not smoked since 1965). For the analyses, the non-smokers were divided into a group of non-smokers in whose homes 10 or more cigarettes were smoked each day, a group in whose homes 1 to 10 cigarettes were smoked per day, and a group of women in whose homes no one smoked. The results of the analyses are summarized in Table 7. In all the analyses, the lung functions were adjusted for height, age, SES, and estimated NO z exposure. The latter was obtained by multiplying the time spent in the separate locations inside and out- side the home (as estimated from the diaries) with the
concentrations in these locations. This was done be- cause personal exposure data were not obtained for all women. The regression coefficients that are pre- sented in the table represent the mean difference in lung function between the group of women men- tioned in the head of the table and the group of non-smoking women in whose homes no one smoked (the reference group).
The table shows that in the rural group of women exposed to tobacco smoke inside their homes since 1965, pulmonary function was decreased. The de- creases for peak flow (PEF) were statistically signif- icant. In the urban area, no consistent association between exposure to tobacco smoke in the home and pulmonary function was seen.
380 P. Fischer et al.
Table 6. Adjusted regression coefficients of pulmonary function on the presence of an unvented, gas fired water heater as an indicator of exposure to NOz.
Rural Area Urban Area
Lung function non-smokers (118) smokers (56) non-smokers (116) smokers (66)
"number of tobacco units smoked inside the home. bfor abbreviations see Table 3. 'coefficient in mL(/s), difference from reference category of unexposed. dstandard error. *p < 0.05.
The results of the NO 2 measurements showed the importance of the (unvented) water heater as a deter- minant of NO 2 levels in Dutch homes. Although on the average the indoor NO 2 levels in the urban houses were higher, this was mainly due to the different distribution of gas appliances in the urban and rural area.
Consistently negative associations were found between pulmonary function and exposure to NO 2 and tobacco smoke in the home among non-smoking women living in the rural area only. Negative associati- ons between exposure to NO 2 and pulmonary func- tion were found among ex-smokers in both areas. Their numbers were small, however. No consistently negative or positive associations were found among the smoking women.
Apparently, the effects of tobacco smoking com- pletely mask any effects that exposure to NO 2 might have. The difference in results between the urban and the rural area is not an easy one to explain. Method- ology is not involved, as it was exactly the same in both areas. Indoor exposure to NO 2 was higher in the
urban area than in the rural area, so that it cannot be a threshold phenomenon either. One explanation could be that in the urban area, exposure to other air pollu- tants has been of a large enough magnitude to effec- tively mask any NO 2 effect. After adjustment for age and height, the level of pulmonary function was shown to be lower in the urban area than in the rural area (Van der Lende et al., 1981). Contributing causes might be differences in outdoor and indoor air pollution, but also may be other differences in life style and/or genetic factors between the two populations.
Occupational factors do not seem to play an im- portant role, as a large majority of the women was not employed, and indeed had never been employed in occupations potentially threatening to respiratory health. It is not clear to what extent the results might be biased by selection phenomena, as it was not possible to obtain exposure information for women who had left the study.
When we compare the results for the continuous NO 2 exposure variable with those for the categorical one (presence of an unrented water heater), it seems that the results for the continuous variable were some-
Indoor exposure to nitrogen dioxide 381
what clearer than for the categorical one. This is in line with our theoretical expectations. However, in an ongoing study among children, the results in this respect seem to be reversed (Brunekreef et al., 1987). Further analysis of this issue is needed before the question can be resolved as to whether we gained anything by measuring exposure in a smaller popula- tion rather than estimating it in a larger one.
A c k n o w l e d g m e n t - The study was funded, in part, by a grant from the Department of Urban Planning and the Environment.
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