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1726 INTERNATIONAL JOURNAL OF EPIDEMIOLOGY respiratory outcomes in children, that could not isolate indoor gas appliances from other combustion/ energy sources (that is studies where gas, coal, wood, kerosene or fireplace cooking/heating were combined into one exposure group), that compared gas cooking with biomass burning or use of fossil fuels and that included indoor and/or personal NO2 concentrations that were mainly affected by outdoor pollution from traffic (that is studies with personal monitoring of NO, where the sampling period covered both indoor and outdoor activities; and studies with indoor NO, measurements, in the absence or indoor sources, i.e. studies in populations with low prevalence (<10%) of gas stoves) were excluded (Figure 1). Respiratory outcome selection The respiratory outcomes of the studies that met the inclusion criteria included various symptoms such as rhinitis, phlegm, cough, chest illnesses, asthma and wheeze as well as lung function parameters. We re- stricted our review to the respiratory outcomes of wheeze and asthma, the two outcomes most frequently used in epidemiological studies among children. Both self-reported and doctor -diagnosed (either from self- reported questionnaire or clinical evaluation) asthma and self-reported wheeze were selected, in spite of the fact that the precise definition of such assess- ments might have some variability between studies. Furthermore, according to the occurrence time of asthma and wheeze, we categorized them into 'current asthma', 'lifetime asthma', 'current wheeze' and 'life- time wheeze' to overcome the dilemma of various def- initions of those health outcomes. 'Current'was defined as having incident asthma (or wheeze) with the symp- toms occurring within the 12 months prior to the ques- tionnaire. 'Lifetime asthma' was defined as ever having been diagnosed with asthma by a doctor; 'lifetime wheeze' was defined as wheeze ever. If studies defined wheeze in more than one way,13 we selected wheeze without colds to avoid inclusion of symptoms related primarily to respiratory infections. We acknowledge that respiratory infections could be an interesting out- come by themselves. Data abstraction Studies on gas heating often lacked information on whether the heater was directly vented to the outside, in which case it would not be a source of indoor air pollution. For this reason, we did not include gas heat- ing14-'9 in the meta-analysis; indoor NO, and gas cook- ing were the exposure variables that we focused on. Ideally, meta-analysis would combine estimates only from studies with exactly the same exposure vari- ables; we included studies for meta-analysis that were as similar as practicable with respect to these. One study about unvented kitchen geysersZO was excluded because the reference category included gas cooking. One study' that compared the risk effect of gas cooking vs other cooking fuels was excluded because it compared two sources of combus- tion products. One study 22 that did not distinguish gas cooking from coal cooking was excluded. The con- centrations of indoor NOz in some studies23-26 were clearly dominated by traffic outdoors, because the percentage of study homes with household gas stoves was small; we excluded those studies as well. One panel study? was not included as this study provided insight only into the short-term exposure and its health effects. Two publications by Garrett et al. 21.21 were based on the same study population and data except for different confounder adjustment; we only included one study.29 In this review, we refer to each population as a separate study and used the corresponding effect estimates; thus we excluded the combined risk estimates from Moshammer et al.30 because we had already included the individual studies on which this paper was based. The study by von Maffei3' was excluded because it was unclear whether it was current or lifetime asthma. In the end, 41 studies were selected for further analysis. Selected articles were appraised using a data extrac- tion form. Information on authors, publication year, country of origin, study design, population character- istics (gender and age), exposure definition (including proportion of gas cooking), definitions of respiratory outcomes in each reviewed article and the meta- analysis, risk measure and confounding factors was extracted. If unadjusted and adjusted results were both re- ported, we extracted the one adjusted for potential con- founding factors. Where more than one adjusted result was presented, we chose the one with adjustment of smoking in the family.32 When a study reported only the number of cases and controls among the exposed and unexposed, we calculated the crude odds ratio and its corresponding 95% confidence interval (CI) follow- ing.1z When a multi -city study provided risk estimates for single cities in addition to a combined estimate, we selected the combined estimates. If there were no com- bined estimates, risk estimates for single cities were used. If more than one follow-up analysis had been reported for the same population, we used results where health outcomes and exposure were measured in the same period'"' (e.g. questionnaire and indoor NO2 measured in the same year; results linking child- hood (adolescent) exposure to childhood (adolescent) health outcomes]. Ilresults were presented separately for different locations of indoor NO, (kitchen, living room and bedroom), we extracted the results from living room, which were most frequently reported in other studies .36 In Hoek et al. S16 study, we assumed that the majority of NOz concentration was in the range of 10-100Itg/m3, based on the data that the geo- metric mean of NO, in the living room was 68.4 gg/m3, and recalculated the effect estimates. The 95% confi- dence intervals were either extracted directly from the original articles or calculated by standard error transformation.