Exposure to environmental tobacco smoke (ETS) increases the risk of respiratory illness in children but data are inconclusive regarding the risk of IgE sensitisation.

To elucidate whether exposure to smoking prenatally and/or postnatally is related to IgE sensitisation in children at 4 years of age.

As part of a prospective birth cohort study (BAMSE), a total of 4089 families with children answered questionnaires when the child was 2 months, 1, 2 and 4 years old on environmental factors and symptoms of allergic disease. Blood collected at age 4 years from 2614 children was analysed for IgE antibodies to common inhalant and food allergens. Odds ratios (OR) and 95% confidence intervals (CI) were calculated using logistic regression with adjustments for potential confounders.

There was no evident association between maternal smoking during pregnancy and risk of IgE sensitisation. In contrast, a dose–response effect was found for exposure to ETS from parental smoking during the first few months of life and IgE sensitisation. There was an increased risk of sensitisation to inhalant and/or food allergens (OR_adj 1.28 (95% CI 1.01 to 1.62)) among children exposed to ETS at 2 months of age. The risk appeared particularly elevated for indoor inhalant allergens, such as cat (OR_adj 1.96 (95% CI 1.28 to 2.99)) and for food allergens (OR_adj 1.46 (95% CI 1.11 to 1.93)). The IgE sensitising effect of ETS seemed to be confined to infants of parents without allergic diseases and to ETS exposure during early infancy.

Our data indicate that exposure in early infancy to ETS increases the risk of IgE sensitisation to indoor inhalant and food allergens.