Interactions between domestic water hardness, infant swimming and atopy in the development of childhood eczema

AIM: Recent studies suggest that domestic water hardness and swimming in chlorinated pools may increase the prevalence of childhood eczema. The combined influence of these two factors as well as their interaction with atopic status has not been investigated. METHODS: We conducted a cross-sectional study on 358 children aged 5–6 years (54% of boys) in 30 kindergarten schools. Parents completed a questionnaire about the child's health, chlorinated pool attendance and potential confounders. Data about tap water quality were provided by water companies. Atopy was defined as a sensitization to at least one aeroallergen or as a medication for allergy. The effect of water hardness and infant swimming practice were assessed by multivariate logistic models. In addition, the effects of these risk factors combined with atopy were evaluated using two measures of biological interaction: the attributable proportion of interaction (AP) and the synergy index (S). AP>0 and S>1 indicate biological interaction between the two risk factors. RESULTS: Water hardness was linearly associated to the prevalence of eczema whereas the relationship of eczema with infant swimming was not linear. We observed a biological interaction between hard water (>150mg/L CaCO₃L⁻¹) and atopic status that increases the prevalence of eczema with an odds ratio (OR) of 3.30 and a 95% confidence interval (CI) of 1.34–8.15 (AP, 0.41; 95% CI 0.15–0.66 and S, 2.4; 95% CI 0.96–6.01). Infant swimming practice combined with atopy also increased the prevalence of eczema (OR, 2.72; 95% CI 1.29–5.74) although none of the interaction measures was significant. However, when water hardness and infant swimming were combined, there was no further increase of the eczema prevalence due to some form of antagonistic interaction between these two factors (AP, −0.56; 95% CI −1.12 to −0.01 and S, 0.54; 95% CI 0.33–0.87). CONCLUSIONS: Our study shows that exposure to hard water and infant swimming interact with atopic status to increase the prevalence of childhood eczema. A breaching of the epidermal barrier by detergents or salts in hard water and by chlorine-based oxidants in swimming pool water might explain these interactions.