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

BACKGROUND: Atopic dermatitis (AD) appears to be more common in regions with hard domestic water and in children with a fall/winter birth. However, it is unknown whether a synergistic effect exists.

OBJECTIVE: To evaluate the association between domestic water hardness and season of birth, respectively, with onset of AD within the first 18 months of life in a large Danish birth cohort.

METHODS: 52,950 children from the Danish National Birth Cohort were included. History of physician-diagnosed AD and population characteristics were obtained from interviews. Birth data was obtained from the Civil Registration System, and domestic water hardness data from the Geological Survey of Denmark and Greenland. The relative prevalence (RP) of AD was calculated using log-linear binomial regression.

RESULTS: The prevalence of AD was 15.0% (7,942 of 52,950). The RP of AD was 5% (RPtrend 1.05; 95% CI 1.03-1.07) higher for each 5 degrees increase in domestic water hardness (range 6.60-35.90 °dH (118-641 mg/L)). While 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 domestic water hardness. The population attributable risk of hard domestic water on AD was 2%.

CONCLUSION: We observed that early exposure to hard domestic water and a fall/winter birth was associated with an increase in the relative prevalence of AD within the first 18 months of life. While the two exposures did not interact synergistically, a dose-response relationship was observed between domestic water hardness and AD.