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INDOOR NITROGEN DIOXIDE, GAS COOKING AND ASTHMA 1731 <br />Table I Meta-analysis results of studies restricted to unselected children and of studies with confounder adjustment <br />Number of <br />Variable studies included Summary odds ratio (95% CI) I' (heterogeneity P -value) <br />Unselected children' <br />Gas cooking <br />Current wheeze 21 1.06 (1.01-1.10) 45.1% (0.008) <br />All wheeze" 27 1.05 (1.01-1.09) 36.5% (0.024) <br />Indoor NO,' <br />Current wheeze <br />5 <br />1.12 (1.01-1.23) <br />0.0% (0.530) <br />All wheeze 1, <br />6 <br />1.09 (1.01-1.17) <br />0.0% (0.547) <br />Studies with confounder adjustment <br />6 <br />1.02 (0.90-1,16) <br />0.0% (0.654) <br />Asthma <br />25 <br />1.05 (1.01-1.09) <br />30.5% (0.065) <br />Gas cooking <br />Current asthma <br />8 <br />1.49 (1.28-1.73) <br />0.0% (0.548) <br />Lifetime asthma <br />7 <br />1.29 (1.09-1.52) <br />23.6% (0.249) <br />All asthma e <br />15 <br />1.37 (1.22-1.53) <br />14.4% (0.288) <br />Indoor NO2 <br />Current asthma <br />l <br />1.36 (0.57-3.29) <br />- <br />Lifetime asthma <br />3 <br />1.08 (0.95-1,23) <br />0.0% (0.367) <br />All asthma a <br />4 <br />1.09 (0.91-1,31) <br />35.5% (0.185) <br />Wheeze <br />Gas cooking <br />Current wheeze <br />19 <br />1.05 (1.01-1.10) <br />40.6% (0.026) <br />Lifetime wheeze <br />6 <br />1.02 (0.90-1,16) <br />0.0% (0.654) <br />All wheezer' <br />25 <br />1.05 (1.01-1.09) <br />30.5% (0.065) <br />Indoor NO2 <br />Current wheeze <br />7 <br />1.15 (1.06-1,24) <br />5.0% (0.395) <br />Lifetime wheeze <br />1 <br />1.04 (0.92-1.17) <br />- <br />All wheezed <br />8 <br />1.11 (1.04-1.19) <br />11.3% (0.337) <br />Without two studies performed in asthmatics only tBelanger et aL 2006 (gas cooking) and Belanger et aL 2013 (NOi)1. The health <br />outcome in these two studies was 'current wheeze'. Results for 'lifetime wheeze' are the same as in Figure 3, as all studies were <br />performed in unselected children and are therefore not presented here. <br />°Current+lifetime. <br />`Per 15 -ppb increase in NO2. <br />having current and lifetime asthma; per 15 ppb <br />increase in indoor NO, level, children have a 15% <br />increased risk of having current wheeze. The meta- <br />analyses found no increase in the risk of asthma in <br />relation to indoor NO2 exposure and no increase in <br />the risk of wheeze in relation to gas cooking exposure. <br />The risk estimates for asthma were somewhat higher <br />in studies which had <30% of participants using gas <br />cooking compared with those >30%. The results did <br />not vary much between age groups (<6 years, 6-10 <br />years and >10 years) or among regions (Europe, <br />Asian -Pacific region and North America). There was <br />no indication of publication bias when considering all <br />the evidence. <br />The present study extends the previous meta- <br />analysis of indoor NO2 by Hasselblad et al.' which <br />reported that indoor NOz increased lower respiratory <br />tract illnesses (LRI) by 18% (OR -1.18, 95% Cl, <br />1.11-1.25) in children for each 15 -ppb increase in <br />indoor NO2. The LRI definitions used in the reviewed <br />studies in the Hasselblad meta-analysis' often <br />included relatively minor symptoms probably related <br />to transient respiratory tract infections. The results of <br />this and our study are therefore not directly compar- <br />able. Our meta-analysis did not focus on LRI but on <br />asthma and wheeze (without colds), included data <br />from only those studies with gas cooking without <br />other combustion sources as exposure variable, <br />and indoor NO, only when dominated by indoor <br />sources. The definitions of 'asthma' and 'wheeze' dif- <br />fered in various studies; we categorized them into <br />current and lifetime symptoms to standardize the <br />3 <br />8 <br />