No case series were included. There were 62 studies from Europe (including inhibitor Wortmannin 3 meta-analyses), 32 from North America, 13 studies from Australia or New Zealand, 3 from Japan and single remaining papers from UAE, India, Qatar, South Korea, Mexico, Taiwan and Brazil. There were 84 (63%) studies published in the past 5 years, that is, from 2009. Box 1 in the online supplementary file presents details

of the included studies, including number and mean age of children included, the respiratory outcome reported and the effect size. No studies were identified for industrial combustion, fireworks, bonfires, vacuuming, air conditioning or air humidifiers. Table 1 presents the effect size of the exposures on asthma risk from the studies identified. Table 2 presents results from studies where interactions between exposures were associated with altered asthma risk Table 1 Magnitude of effect of environmental exposure on respiratory symptoms Table 2 Magnitude of effect of main effect on asthma aetiology and magnitude of interaction with other factor Figure 1 QUOROM statement flow chart. Secondhand smoke Antenatal exposure One meta-analysis and five cohort studies were identified and most found exposure was associated with increased risk for asthma. The meta-analysis12 identified 735 exposed children and concluded that exposure was associated with an increased risk for asthma at 6 years (OR 1.7). The cohort

studies found that risk was increased by 1.1313 and 2.114 at 2 years, and 1.4 at 7 years.15 One study of infants born 3–4 weeks prematurely found increased risk for wheeze at 3 years only among those exposed to secondhand smoke (SHS; OR 4.0, table 2).16 One study found no association between antenatal exposure and risk for symptoms.17 Postnatal exposure One systematic review and six cohort studies were identified and all reported that exposure was associated with increased asthma risk. The systematic review concluded that exposure to tobacco smoke was associated with an increased risk of 1.3 among children aged 6–18 years.5 Postnatal exposure was associated

with increased risk for wheeze between 1.218 and 2.9,17 and 1.7 for asthma at 5 years (table Entinostat 2).19 The study from Japan17 found a link between postnatal but not antenatal maternal smoking and wheeze at 16–24 months. One study18 found that postnatal paternal smoking was a risk factor for wheeze (RR 1.14 (1.04 to 1.24)) independent of maternal smoking. Another study reported an interaction between short duration of maternal education and SHS exposure.19 A final study found that increasing exposure to fine particulates (PM2.5) and urinary cotinine, products of tobacco combustion, was positively linked to risk for infant wheeze.20 Domestic combustion Two cohort, one cross-sectional and two case–control studies were identified and there was inconsistent evidence between exposure and asthma risk.