Prevalence of atopic dermatitis in infants by domestic water hardness and season of birth: Cohort study

Background Atopic dermatitis (AD) appears to be more common in regions with hard domesticwater and in children with a fall/winter birth. However, it is unknown whether a synergistic effect exists. Objective We sought to evaluate the association between domesticwater hardnessand seasonof birth, respectively, with onset of AD within the first 18 months of life in a large Danish birth cohort. Methods Of children from the Danish National Birth Cohort, 52,950 were included. History of physician-diagnosed AD and population characteristics were obtained from interviews. Birth data were obtained from the Civil RegistrationSystem, and domesticwater hardnessdata were obtained from the Geological Survey of Denmark and Greenland. The relative prevalence (RP) of AD was calculated by using log-linear binomial regression. Results The prevalence of AD was 15.0% (7,942/52,950). The RP of AD was 5% (RP trend , 1.05; 95% CI, 1.03-1.07) higher for each 5° increase in domesticwater hardness(range, 6.60-35.90 German degrees of hardness[118- 641mg/L]). Although the RP of AD was higher in children with a fall (RP, 1.24; 95% CI, 1.17-1.31) or winter (RP, 1.18; 95% CI, 1.11-1.25) birth, no significant interaction was observed with domesticwater hardness. The population attributable riskof hard domesticwater on AD was 2%. Conclusion We observed that early exposure to hard domesticwater and a fall/winter birth was associated with an increase in the relative prevalence of AD within the first 18 months of life. Although the 2 exposures did not interact synergistically, a dose-response relationship was observed between domesticwater hardnessand AD.