Maternal and paternal indoor or outdoor smoking and the risk of asthma in their children: A nationwide prospective birth cohort study

• We examine the association between parental indoor smoking and childhood asthma.
• Parental indoor smoking at home increased and exacerbated children's asthma.
• Attributable fraction to both parents smoking indoors was large for childhood asthma.
• Smoking at home, whether it is indoors or outdoors, may increase the risk for childhood asthma.

BackgroundLittle is known about the differential impact of combinations of parental smoking behavior (indoor or outdoor smoking, or not smoking) on preventing childhood asthma. Our objective was to examine the association between parental smoking behavior and children's asthma.MethodsA nationally representative population-based birth cohort of 40,580 babies, aged 0.5 years in 2001 (response rate, 87.8%), was studied to estimate adjusted odds ratios of combinations of maternal and paternal indoor or outdoor smoking at home for physician visits and hospitalization for childhood asthma up to 8-years-old, and population attributable fractions.ResultsOdds of hospitalization for asthma among children whose father alone smokes indoors at home did not largely increase (up to 20%). However, if the mother also smokes indoors at home, the odds strongly increased. After adjusting for demographic, perinatal and socioeconomic factors, the increase in odds for children whose father and mother both smoke indoors compared to children with non-smoking parents was 54% (95% confidence interval: 21–96%), 43% (8–90%) and 72% (22–143%) for children aged 0.5 < −2.5, 2.5 < −4.5 and 4.5 < −8 years-old, respectively. The odds ratios of smoking outdoors did not largely differ from those of smoking indoors. Our estimation of population attributable fractions revealed that if all parents in Japan quit smoking, hospitalization of children for asthma could be reduced by 8.3% (2.2–14.3%), 9.3% (0.9–17.6%) and 18.2% (7.7–28.8%), respectively.ConclusionsParental indoor smoking at home increased and exacerbated children's asthma. Smoking at home, whether it is indoors or outdoors, may increase the risks for asthma attacks of their children.