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INDOOR NITROGEN DIOXIDE, GAS COOKING AND ASTHMA 1725 illness risk. This early quantitative analysis became a benchmark study for the relationship between indoor NO, and respiratory illness in children, and an important reference for the outdoor NO, Air Quality Guideline value established by the World Health Organization (WHO) in 1997' and confirmed in 2005.7 More recently, WHO has reviewed studies on indoor NO, exposure, but without doing a formal meta-analysis.8 Recent journal reviews of the issue""` have also been qualitative. In view of the dearth of quantitative meta-analyses based on recent studies, we decided to review studies on asthma, wheeze, gas cooking and indoor NO, in chil- dren with the purpose of obtaining quantitative effect estimates. Methods Selection criteria We searched for studies from which quantitative effect estimates of the relationship between gas cook- ing, indoor NOz and respiratory health effects in chil- dren could be obtained. We attempted to identify all population studies in relation to this topic. The litera- ture was searched with PubMed and ISI Web of Knowledge from 1977 up to 31 March 2013 with the following search terms: (i) indoor nitrogen 1 1064 articles from databases I dioxide and children; (ii) personal nitrogen dioxide and children; (iii) gas cooking and children; (w) gas appliance and children; (v) unvented and children; (vi) gas heating and children; and (vii) gas heater and children. The seven search results were combined with the Boolean operator 'or'. All of the 34 epidemio- logical studies included in Table 5.2 of the recent WHO guidelines for indoor air quality and citations from previous reviews and identified articles were considered as well. Duplications were removed. To be eligible for inclusion, studies had to: (i) be published in English; (if) be primary study, not re- views; (iii) examine respiratory disease in infancy or in childhood (defined by a maximum age of subjects 18 years) as outcomes; (iv) examine exposure to indoor NO, or household gas cooking or gas heating; (v) be conducted within family houses, not in schools or classrooms; and (vi) report an odds ratio or other effect estimator12 or sufficient data to estimate them. Articles fulfilling all six criteria were included for further review (Figure 1). All studies were reviewed according to the six inclu- sion criteria. Commentaries, and studies not per- formed in children or exposures not relevant or without respiratory outcomes, were excluded. The remaining articles were reviewed independently by the three authors. Articles that did not report on the association between selected exposure variables and 295 duplicates and 34 non-English articles discarded 1 735 articles selected for absa'act review I 571 excluded: Studies did not meet the inclusion criteria (reviews, commentaries, studies not performed in children, exposure not relevant, without respiratory outcomes) 7 additional articles, from search of reference lists 171 articles selected Por review of whole article to determine whether exposure and outcome is relevant 67 excluded: Studies not reporting on respiratory outcomes or selected exposure assessments (including indoor gas heating); not reporting a quantitative effect estimate 63 excluded: Studies not about asthma and wheeze: exposure was gas heating, coal fuel and unvented geysers; gas cooking was compared to biomass burning or use of fossil fuels; indoor nitrogen dioxide was dominated by outdoor source; studies based on same dataset 41 at fides using asthma and wheeze as health outcomes selected for data abstraction, quality assessment and meta-analysis Figure 1 Study selection flow chart