Background: Smoking has decreased in Sweden in the last decades but whether passive smoking remains a risk factor for asthma and respiratory symptoms in children is unclear.
Aim: To compare exposure to passive smoking as a risk factor for asthma and respiratory symptoms among 8-year-olds between 1996 and 2017.
Methods: The Obstructive Lung Disease in Northern Sweden (OLIN) studies have recruited two cohorts of 8-year-old children, in 1996 (n=3430; 97% of invited) and 2017 (n=2785; 91%). Their parents completed a questionnaire about asthma and related risk factors including parental smoking. Outcomes: physician-diagnosed asthma and wheeze last 12 months. Unadjusted and adjusted Poisson regression analyses were performed to calculate relative risk (RR).
Results: Asthma increased from 6% to 12% (p<0.001) while wheeze remained similar (10.1% vs 10.3%; p=0.714). Smoking during pregnancy decreased from 25% to 4% and maternal smoking when the child was 8y decreased from 32% to 8% (both p<0.001). In 1996, maternal smoking at 8y was significantly associated with asthma (RR 1.38 95%CI 1.03-1.84), but not after adjustment for sex, family history of asthma and other confounders (RR 1.35 95%CI 0.98-1.85). In 1996, maternal smoking at 8y was associated with wheeze in the adjusted analysis (RR 1.31 95%CI 1.03-1.67). In 2017, smoking during pregnancy was significantly associated with asthma in adjusted analyses (RR 1.65 95%CI 1.03-2.63), while maternal smoking at 8y lost significance (RR 1.31 95%CI 0.91-1.91).
Conclusion: Exposure to passive smoking remains as a risk factor for asthma and wheeze in children.