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1728 INTERNATIONAL JOURNAL OF EPIDEMIOLOGY and two studies on asthmatic children;"," the rest were studies on general populations of school-age children. There were 16 cross-sectional, 18 cohort, and 7 case -control studies. However, most of the re- ports from cohort studies were based on cross-sec- tional rather than longitudinal analysis. Three studies included the association between previous gas cooking exposure and the development of respira- tory symptoms: He Bilderling et al.33 and Ponsonby et al.' used a cohort design to link early exposure estimates to subsequent risk of wheeze and asthma, and Wong et al.35 used a survey study with a retro- spective questionnaire. The other reviewed studies focused mainly on whether the presence of respiratory symptoms was associated with current exposure The meta-analysis of findings from 19 studies on the association between gas cooking and asthma ( Figure 2) demonstrates an increased odds of current asthma [random effects meta -odds ratio (OR) 1.42; 95% CI, 1.23-1.64, P=0.000, n=11 studies) and lifetime asthma (1.24; 95% CI, 1.04-1.47, P=0.014, n=8 stu- dies) in children exposed to gas cooking. The overall odds ratio was 1.32 (95% Cl, 1.18-1.48, P=0.000; 12=19.8%, heterogeneity P-value=0.204) for the asso- ciation between asthma and gas cooking (Figure 2a) and 1.09 (95% CI, 0.91-1.31, I2=35.5%, heterogeneity P-value=0.185) per 15 -ppb increase in NO2 exposure (Figure 2b). Indoor NO, was positively associated with the odds of current wheeze (random effects meta -OR 1.15 per 15 ppb, 95% CI, 1.06-1.25, P=0.001) (Figure 3b). There was only one study report- ing lifetime wheeze in children exposed to indoor NO2; combining it into the meta-analysis yielded a pooled random effects OR of 1.12 (95% CI, 1.04-1.21, P=0.002, 12=11.3%, heterogeneity P-value=0.337). The combined analysis of 28 studies, including >11000 children with wheeze, demonstrated no increased risk in children who had ever been exposed to gas cooking (random effects meta -OR= 1.06, 95% Cl, 0.99-1.13, 72=42.8%, heterogeneity P-value=0.006) (Figure 3a). Results for current wheeze (random effects meta-OR=1.07, 95% CI, 0.99-1.15, heterogeneity P -value = 0.002) were similar to results for all wheeze. We observed heterogeneity among those studies, with 72 of 50.4% and 42.8% for current and all wheeze, respectively. Therefore, the combined esti- mates for lifetime wheeze based on the random effects model were likely to represent the effect more accurately. The Forest plots ordered by publication date (Figures 2 and 3) show that there was no obvious trend in risk estimates over time. An influence analysis showed that no single study dominated the combined estimates. Four of the 41 studies compared children exsedto high NO2with children exposed to low NO,. 9,53,65.69 We did not find an increase in asthma49'53 (random effects meta -OR= 1.10, 95% CI, 0.35-3.40, 12=49.5%, heterogeneity P-value=0.159) and in wheezes 3,65.69 (random effects meta-OR=0.81, 95% Cl, 0.59-1.12, 12=0.0%, heterogeneity P-value=0.715) among chil- dren with the highest compared with the lowest NO2 exposure. The results, however, should be interpreted with caution because the number of studies included was small. We performed additional analyses to examine the pooled estimates for wheeze when restricted to gen- eral populations of children, excluding studies based on asthmatic children." 46 Restricting the analysis to general populations of children did not change the effect estimates (Table 1). When we excluded crude effect estimates from five studies38,39,52,59,68 without confounder adjustment, the summary effect of gas cooking exposure on asthma in children became somewhat stronger (Table 1). Risk estimates for asthma were not different in chil- dren awed 56 years ,32,52,6o,7o,72 6-10 years3z49- 51,ss,59,6 or >10 years 29,39,54,56,57,66,69 (Table 2). Stratification by study region showed that the ORs for the association of all asthma with gas cooking exposure tended to be higher in Europe (random ef- fects meta-OR=1.34, 95% Cl, 1.15-1.57) and the Asian -Pacific region (random effects meta -OR = 1.29, 95% CI,1.15-1.45), and lower in North America (random effects meta -OR= 1.12, 95% Cl, 0.73-1.73). However, the ORs did not differ significantly between regions- The trend was similar for all wheeze (Table 3).Taking the proportion of participants using gas for cooking into account (Table 2), there was a tendency for the risk estimates to be higher in the studies which had less than 30% of participants using gas cooking. No stratified analyses by age, study region, proportion of gas stoves or year of publication were performed for indoor NO2 as the numbers of studies in the different strata were too small to obtain enough statistical power. Almost half of the included studies were published before 2000. The estimated effects of gas cooking on asthma were higher in studies that were published before the year 2000; however, the estimates did not differ in the strata of published year (P>0.05) (Table 2). Results of stratified analyses and meta -regressions for current asthma, lifetime asthma, current wheeze and lifetime wheeze are also presented in Tables 2 and 3. The funnel plots (Supplementary Figure 1, available as Supplementary data at IJE online) and P -values from Begg's (Pavh..,R=0.971, Po,, ,=0.975) and Egger's (Pa,1h,,1,,=0.890, Pw4reae=0.644) tests provided no evidence of publication bias. Discussion Our meta-analyses suggest that children living in a home with gas cooking have a 42% increased risk of having current asthma, a 24% increased risk of life- time asthma and an overall 32% increased risk of 0 f a a a 0 3