INDOOR AIR QUALITY IN ENGLISH HOMES - NITROGEN DIOXIDE

INDOOR AIR QUALITY IN ENGLISH HOMES - NITROGEN DIOXIDE SKD Coward, GJ Raw, JW Llewellyn and DI Ross BRE, Watford WD25 9XX, UK ABSTRACT BRE has conducted a national survey of air pollutants in 876 homes in England, to increase knowledge of pollutant levels and the factors associated with high concentrations. Homes were monitored, using passive samplers, for nitrogen dioxide (NO 2 ), carbon monoxide, formaldehyde and volatile organic compounds. This paper reports the findings on 14-day time-weighted average NO 2 levels. Outdoor NO 2 concentrations were higher in autumn/winter and in urban areas. Kitchen levels were higher than outdoor levels in homes with gas cooking. Bedroom levels were lower than outdoors due to sink effects. Indoor NO 2 levels were highest in urban homes, older homes, terraced houses, homes with gas cooking and smokers homes. Seasonal variation in indoor levels resulted mainly from indoor sources, rather than outdoor variations. The WHO air quality guideline for annual exposure to NO 2 was exceeded in more than 50% of kitchens that had a gas oven. INDEX TERMS Nitrogen dioxide, Homes, Sources, National survey, Diffusive sampling INTRODUCTION BRE has conducted a national survey of air pollutants in 876 homes in England (Coward et al, 2001). The survey was designed to increase knowledge of pollutant levels and the factors associated with high concentrations. This paper reports the findings on nitrogen dioxide (NO 2 ) levels. Results for the other pollutants that were monitored (carbon monoxide, formaldehyde and volatile organic compounds) are reported in companion papers (Raw et al, 2002; Brown et al, 2002a, 2002b). NO 2 levels were measured using Palmes diffusion tubes (Atkins et al, 1978). At least eighteen earlier studies of indoor NO 2 levels using this method, and their findings, have been reviewed and compared by the UK Institute for Environment and Health, 1996. METHOD The survey structure and methodology are described in a companion paper (Raw et al, 2002). Householders completed questionnaires on the homes, occupants and activities in the home. NO 2 was measured once in each home, using Palmes diffusion tubes, as two-week time-weighted averages in kitchens, main bedrooms and outside the homes. The statistical analysis was carried out in three stages, each using logarithms of concentrations to correct for skewed distributions. First, descriptive statistics were produced. Bivariate analysis (analysis of variance (ANOVA) or t-tests) was then used to identify associations between concentrations and possible determinants (e.g. the presence of a gas cooker). ANOVA was then used to show which associations were secondary, in the sense that Contact author email: cowards@bre.co.uk 467

they could be explained by a factor that more directly determines pollutant levels. The factors reported here are those that emerged as determinants in their own right. The independent variables used in the analysis are listed by Raw et al (2002) but only those that had a significant effect are mentioned in this paper. NO 2 CONCENTRATIONS NO 2 levels were measured successfully in 845 homes, with 812 homes providing results for all three locations (kitchen, bedroom and outdoors). Minimum, maximum, geometric mean and percentile values are shown in Table 1. Seasonal NO 2 levels are shown in Table 2, grouped according to the month when the samplers were opened: spring (March-May); summer (June-August); autumn (September-November) and winter (December-February). NO 2 levels were higher in kitchens than in bedrooms, because many homes had cookingrelated sources in the kitchen. In each season, bedroom levels were significantly lower than the levels outdoors, most likely due to the removal of infiltrating NO 2 by indoor sinks. Indoor sources would generally have less impact in bedrooms than in kitchens because they are more likely to be in the kitchen. NO 2 levels in bedrooms were closest to outdoor levels in summer, when it would be expected that windows were more likely to be opened. This would result in a higher ventilation rate and thus a smaller impact of NO 2 removal by sinks. Table 1. Statistics for NO 2 concentrations (µg m -3 ) Geometric Percentiles Location Minimum Maximum mean 10% 50% 75% 95% Kitchen 0.8 620.0 21.8 7.2 21.8 40.1 90.0 Bedroom 0.4 752.6 11.9 4.4 12.1 19.8 38.1 Outdoors 1.0 151.6 20.9 9.9 22.5 32.4 48.9 Table 2. NO 2 levels (µg m -3 ) by season Season Spring Summer Autumn Winter Kitchen 17.2 23.3 23.7 22.3 Bedroom 10.1 14.6 12.7 11.0 Outdoors 15.1 17.0 26.7 22.4 Bedroom and kitchen levels were significantly correlated (p<0.001) with each other (r=0.78) and with outdoor levels (r=0.43 for both bedrooms and kitchens). DETERMINANTS OF OUTDOOR NO 2 CONCENTRATION The most important effects on outdoor NO 2 concentration were of season (F=35.8, p<0.001, Table 2) and area type (F=26.6, p<0.001, Table 3). Some other significant effects can probably be explained by the degree of urbanisation, i.e.: differences between regions: lowest in the South West (14.6 µg m -3 ), highest in London (37.5 µg m -3 ) differences between dwelling types: lowest in bungalows (16.7 µg m -3 ), highest in terraced homes (25.6 µg m -3 ) age of home: lowest in post-1985 homes (16.8 µg m -3 ), highest in pre-1941 (22.7 µg m -3 ). 468

Table 3. Geometric mean outdoor NO 2 (µg m -3 ) by area type Rural Suburban Urban Central urban 16.1 21.9 25.0 33.1 DETERMINANTS OF INDOOR NO 2 CONCENTRATION Cooking fuel The analysis of cooking fuel compared three groups: homes with a natural gas oven, homes with some natural gas cooking (hob and/or grill) but no gas oven, and homes with no fossil fuel cooking. The mean kitchen and bedroom levels are given in Table 4. The difference between the groups was highly significant (kitchens F=343, p<0.001; bedrooms F=128, p<0.001) with each group significantly different from the other two (all p<0.001). Table 4. Geometric mean indoor NO 2 (µg m -3 ) by cooking fuel Cooking fuel Kitchen N Bedroom N Natural gas oven 42.8 338 18.2 338 Natural gas cooking but no gas oven 22.4 128 12.8 128 No fossil fuel cooking 11.5 356 7.9 354 Season Mean concentrations are given in Table 2. The effect of season was significant (kitchens F=4.9, p<0.01; bedrooms F=7.4, p<0.001). In kitchens, levels were significantly lower in spring than in other seasons. In bedrooms, levels were highest in summer and the seasonal pattern was similar to that seen in kitchens with no fossil fuel cooking (Table 5). For kitchen NO 2, there was a significant interaction between season and cooking fuel: season had a greater effect in homes with a gas oven, where NO 2 levels were lowest in spring and highest in winter (Table 5). In homes with some gas cooking but no gas oven, the effect was much smaller, but in the same direction. In electric-cooking homes, NO 2 levels were highest in summer and autumn and, again, the variation small. It seems likely that there was a higher ventilation rate in summer and that this allowed more NO 2 from gas cooking to escape, but allowed more to enter from outdoors, thus increasing NO 2 levels in electric cooking homes. It is also plausible that there was more cooking in the cooler months. Of greater importance is the implication that indoor seasonal variation results mainly from indoor sources. Table 5. Geometric mean kitchen NO 2 (µg m -3 ) by cooking fuel and season Cooking fuel Spring Summer Autumn Winter Gas oven 32.4 41.0 45.4 50.4 Gas cooking but no oven 19.1 21.9 23.1 23.5 No fossil fuel cooking 9.0 13.1 14.1 10.0 Heating Heating systems fell into four main groups in relation to their effects on NO 2, as shown in Table 6. There was a significant effect on kitchen NO 2 (F=41.8, p<0.001). The two central heating groups were combined for ANOVAs, which showed significant interactions between heating system and cooking fuel (Table 7). However, one group, consisting of a single home (electric heating, gas cooking but no oven) distorted the kitchen results (without this home, the interaction was not significant). Among homes with no fossil fuel cooking, bedroom NO 2 was lowest in homes with electric heating and highest in homes with individual gas heaters. Having a gas oven appeared to mask any effect of heating system on bedroom NO 2. 469

Not surprisingly, the effect of heating system was greater in the winter but this effect was not significant because of confounding with cooking fuel. Table 6. Geometric mean indoor NO 2 (µg m -3 ) by grouped heating system Main heating system Kitchen Bedroom Electric 10.1 6.8 Natural gas or oil central heating with radiators 22.9 12.3 Natural gas warm air central heating 31.4 18.9 Individual gas heaters 49.7 19.5 Table 7. Geometric mean indoor NO 2 (µg m -3 ) by heating system and cooking fuel Main heating system Cooking Kitchen N Bedroom N Electric Gas oven 38.3 10 21.8 10 Gas cooking but no gas oven 550.4 1 36.6 1 No fossil fuel cooking 7.7 69 5.6 68 Natural gas or oil Gas oven 40.2 274 17.4 274 central heating Gas cooking but no gas 21.9 125 12.6 125 oven No fossil fuel cooking 13.1 245 8.7 244 Individual natural Gas oven 63.5 42 21.9 42 gas heaters Gas cooking but no gas 22.9 1 17.5 1 oven No fossil fuel cooking 18.4 8 12.5 8 Unflued heating The use of an unflued fixed or portable gas heater, including the use of a gas oven for heating, was significantly related to NO 2 levels in kitchens (F=54.5, p<0.001) and bedrooms (F=19.0, p<0.001, Table 8). However, fewer than 10% of homes used unflued heating. Table 8. Geometric mean kitchen and bedroom NO 2 (µg m -3 ) by unflued heater use Any use of unflued heating Yes No Kitchen NO 2 44.7 20.3 Bedroom NO 2 17.2 11.5 Smoking Householders were asked whether anyone smoked regularly in the home. Bedroom NO 2 was significantly higher (t=4.0, p<0.001) in smokers homes (14.1 µg m -3 ) than in other homes (11.1 µg m -3 ) but the difference was small. Extract fans There was a significant interaction between heating system and the presence of an extract fan, for both kitchen and bedroom NO 2 (Table 9). While the presence of an extract fan did not have a significant effect on NO 2 concentrations in kitchens, in bedrooms there was a small beneficial effect if there was a gas cooker (Table 10). 470

Table 9. Geometric mean indoor NO 2 (µg m -3 ) by heating system and extract fan Natural gas or oil Individual natural Location Extract fan Electric heating central heating gas heaters Kitchen Bedroom Yes Yes 12.8 8.0 22.2 11.9 45.8 14.3 No No 8.2 5.9 24.0 13.1 50.8 21.2 Table 10. Geometric mean bedroom NO 2 (µg m -3 ) by cooking fuel and extract fan Any extract fans? Gas cooking No fossil fuel cooking Yes 15.0 7.8 No 18.1 8.0 Area type, dwelling type and building age NO 2 in kitchens was significantly related to area type (F=19.1, p<0.001, Table 11). NO 2 levels varied significantly with dwelling type (kitchens F=12.1, p<0.001, bedrooms F=18.4, p<0.001, Table 12), probably due to a combination of variation in outdoor levels and ease of cross-ventilation. Bedroom NO 2 was significantly higher in older homes (F=4.4, p<0.001, Table 13), which are more likely to be in urban areas and to have gas cooking (Coward et al, 2001). Table 11. Geometric mean kitchen and bedroom NO 2 by area type (µg m -3 ) Location Rural Suburban Urban Central urban Kitchen 14.5 25.9 24.5 31.6 Bedroom 8.4 13.4 13.8 17.6 Table 12. Geometric mean indoor NO 2 (µg m -3 ) by dwelling type Dwelling type Bedsit or flat Terrace Semi-detached Detached Bungalow Kitchen NO 2 16.9 29.6 25.3 16.8 15.1 Bedroom NO 2 11.5 15.2 12.6 9.7 8.8 Table 13. Geometric mean bedroom NO 2 (µg m -3 ) by building age Before 1919-1941- 1961-1971- 1981-1986- Since When built 1919 1940 1960 1970 1980 1985 1990 1990 Bedroom NO 2 11.8 14.2 12.4 12.6 11.7 11.4 8.0 8.1 Indoor sources Theoretical values were derived for the contribution of indoor sources to indoor NO 2 levels. This was done by using a mathematical formula relating indoor and outdoor levels (Coward et al, 2001). The results of analysis of the resulting values were, as a rule, the same as those for the measured levels. This implies that the results for measured levels are dominated by sources of NO 2 within the home. COMPARISON WITH HEALTH-BASED GUIDELINES The WHO annual average guideline for exposure to NO 2 of 40 µg m -3 (WHO, 2000) was exceeded in kitchens in 25% of all homes and in 53% of homes with a gas oven. It is exceeded in bedrooms in fewer than 5% of homes. Maximum kitchen levels exceeded the WHO one-hour guideline of 200 µg m -3 in 6 months out of 17, and in bedrooms in 2 months 471

out of 17. It can reasonably be assumed that the guideline was exceeded more frequently for periods of an hour (i.e. the reference period for the guideline). CONCLUSIONS The survey identified factors influencing NO 2 levels in homes, i.e. gas cooking, gas heating and smoking. The provision of ventilation devices had little effect. The outdoor air was also found to be an important factor, as shown in effects of urban location and related variables (dwelling type and age). However, seasonal effects were due largely to indoor sources. This would need to be considered when interpreting time series studies of the effect of outdoor air pollution on health. It is apparent that, in the majority of homes in England, NO 2 levels are low for most of the time. However, average levels of NO 2 in the kitchens of more than 50% of homes in England that have a gas oven exceed the WHO air quality guideline for annual exposure to NO 2 and the implications for health of this exposure deserve further investigation. Consideration should therefore be given to measures to reduce NO 2 levels in kitchens with gas cookers. Extract fans in gas cooking kitchens appear to be only marginally effective in reducing NO 2 levels in the kitchen, although they may limit the spread of the gas to other parts of the home. These findings are in the same range as, and are consistent with, previous findings (IEH, 1996); but the findings of the present study are more representative and, being based on a larger sample, provide better data on reasons for variations in indoor NO 2 concentrations. ACKNOWLEDGEMENTS This work was funded by the Chemicals and Biotechnology Division of the Department of the Environment, Food and Rural Affairs (DEFRA). REFERENCES Atkins DHF, Healy C & Tarrant JB. 1978. The use of simple diffusion tubes for the measurement of nitrogen dioxide levels in homes using gas and electricity for cooking. AERE Report R9184. Oxfordshire, AERE Harwell. Brown VM, Coward SKD, Crump DR et al. 2002a. Indoor air quality in English homes - formaldehyde. Proceedings of Indoor Air 2002 (submitted paper). Brown VM, Coward SKD, Crump DR et al. 2002b. Indoor air quality in English homes - VOCs. Proceedings of Indoor Air 2002 (submitted paper). Coward SKD, Llewellyn JW, Raw GJ et al. 2001. Indoor air quality in homes in England. BRE Report 433. London: CRC. Institute for Environment and Health. 1996. IEH Assessment on Indoor Air Quality in the Home. Medical Research Council. Raw GJ, Coward SKD, Llewellyn JW et al. 2002. Indoor air quality in English homes introduction and carbon monoxide findings. Proceedings of Indoor Air 2002 (submitted paper). World Health Organisation. 2000. Guidelines for Air Quality. WHO Geneva 472