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1734 INTERNATIONAL JOURNAL OF EPIDEMIOLOGY health effects and thus to reduce the heterogeneity between studies. Although asthma and wheeze are associated, they present distinct entities. In a Dutch birth cohort study, for example, it was found that only 11% of children with symptoms suggestive of asthma, includ- ing wheeze, at preschool age had asthma at age 7-8 years .76 Moreover, one-time wheeze was sufficient to characterize a child as having wheezed in many of the studies included in the meta-analysis and typically no distinction was made between wheeze with and with- out respiratory infections. This may explain why our meta-analysis revealed stronger associations with gas cooking for asthma compared with wheeze. Gas cooking produces NO2 and other pollutants such as ultrafine particles. Our finding of an associ- ation between gas cooking and asthma in the absence of an association between measured NO2 and asthma suggests that gas cooking may act as a surrogate for causal variables other than air pollutants produced by gas combustion. This is supported by an Australian study, where the association between gas cooking and respiratory symptoms remained significant after adjustment for measured NO2 .2' Residual confound- ing by (unmeasured) factors that are associated with gas cooking might be another explanation for our finding of an association between asthma and gas cooking, but not with indoor NO2. However, this is not very likely as we used effect estimates from the included studies which were almost always adjusted for known determinants of childhood asthma. It is also possible that no relationship between indoor NO2 and astinna was found because there were fewer studies that had direct NO2 measurements, and study populations were usually smaller in these studies. Point estimates for the association of NO, and gas cooking with current asthma were actually very similar to those for gas cooking and asthma, but confidence intervals were wider for NO2. As gas cooking is a strong determinant of indoor NO2, it has been argued that one is actually more likely to find associations with gas cooking than with NO2 because much larger studies can be (and have been con- ducted using the surrogate exposure variable. Heterogeneity among reviewed studies existed in various factors such as stove type, age of population, size of population exposed to gas cooking, susceptibil- ity of study population, study region, study design, sampling season, other indoor factors and diagnosis of asthma and wheeze. We therefore conducted meta - regression to explore whether the heterogeneity could be explained by age, study region, study design or size of the population exposed to gas cooking. None of these factors appeared to be associated with the mag- nitude of the effect estimates extracted from the study papers. We did note that the association be- tween gas cooking and asthma was somewhat stron- ger in studies published before the year 2000 than in later studies. Possibly, gas cooking in newer studies is associated with lower indoor pollution levels because of the introduction of microwaves displacing some of the meal preparation, changes in stove performance or kitchen ventilation etG53.72 Exposure assessment (questionnaire reports of gas cookers and passive measurements of NO2) and statistical analysis (mostly logistic regression) were mostly rather straightforward and, therefore, they do not seem a likely source of heterogeneity between the reviewed studies. The findings of our meta-analysis on asthma were also not different when we excluded studies where less than 30% of the population used gas for cooking, by restricting the study population to general popula- tion of children, and by excluding studies without adjustment for potential confounders. The exclusion of single studies from the analysis did not change the pooled estimates. Also, P -values from the Egger's and Begg's tests, as well as the absence of funnel plot asymmetry, suggested that no publication bias exists in our results. Our analysis was based on observational studies and we cannot exclude that associations between gas cooking and asthma are in part due to information bias, e.g. because parents may suspect risks are asso- ciated with gas cooking. However, with studies coming from so many different settings, we do not think this is a likely explanation for the observed associations. Although the effects of gas cooking and indoor NO, on asthma and wheeze were found to be relatively small (all random -effects meta -odds ratios were less than 15) the public health impact may still be consid- erable because gas cooking is widespread. A recent large population study found that 60-70% of European children lived in gas -cooking homes .71 It is not clear to what extent the observed associations with gas cooking are attributable to NO2 alone or also to other pollutants associated with the use of gas for cook- ing. In outdoor air pollution studies, NO2 often is used as a marker of a complex, traffic -related air pollution mixture, which makes extrapolation of our results to outdoor air pollution difficult. Indoors, gas cookers can be replaced by electric cookers, and gas cooking fumes can be removed by using ventilation hoods. Conclusion In summary, this meta-analysis provides quantitative evidence that gas cooking increases the risk of asthma in children, and indoor NO, increases the risk of cur- rent wheeze in children. Funding W.L. was supported by a research fellowship of the Institute for Risk Assessment Sciences (IRAS) Foundation. 0 0 a 0 0 3 a