title: Prevalence of atopic dermatitis in infants by domestic water hardness and season of birth: Cohort study.
abstract: 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
  We sought 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
  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 Registration System, and domestic water hardness data 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% (RPtrend, 1.05; 95% CI, 1.03-1.07) higher for each 5° increase in domestic water hardness (range, 6.60-35.90 German degrees of hardness [118-641 mg/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 domestic water hardness. The population attributable risk of hard domestic water on AD was 2%.
  
  
  CONCLUSIONS
  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. Although the 2 exposures did not interact synergistically, a dose-response relationship was observed between domestic water hardness and AD.
authors.affiliation: National Allergy Research Centre, Department of Dermatology and Allergy, Herlev and Gentofte Hospital, University of Copenhagen, Hellerup, Denmark; Department of Dermatology and Allergy, Herlev and Gentofte Hospital, University of Copenhagen, Hellerup, Denmark.
authors.forename: Kristiane A
authors.initials: KA
authors.lastname: Engebretsen
authors.affiliation: Department of Epidemiology Research, Statens Serum Institut, Copenhagen, Denmark.
authors.forename: Peter
authors.initials: P
authors.lastname: Bager
authors.affiliation: Department of Epidemiology Research, Statens Serum Institut, Copenhagen, Denmark.
authors.forename: Jan
authors.initials: J
authors.lastname: Wohlfahrt
authors.affiliation: Department of Dermatology and Allergy, Herlev and Gentofte Hospital, University of Copenhagen, Hellerup, Denmark.
authors.forename: Lone
authors.initials: L
authors.lastname: Skov
authors.affiliation: Department of Dermatology and Allergy, Herlev and Gentofte Hospital, University of Copenhagen, Hellerup, Denmark.
authors.forename: Claus
authors.initials: C
authors.lastname: Zachariae
authors.affiliation: Department of Public Health, Section of Social Medicine, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Anne-Marie
authors.initials: AM
authors.lastname: Nybo Andersen
authors.affiliation: Department of Epidemiology Research, Statens Serum Institut, Copenhagen, Denmark.
authors.forename: Mads
authors.initials: M
authors.lastname: Melbye
authors.affiliation: National Allergy Research Centre, Department of Dermatology and Allergy, Herlev and Gentofte Hospital, University of Copenhagen, Hellerup, Denmark; Department of Dermatology and Allergy, Herlev and Gentofte Hospital, University of Copenhagen, Hellerup, Denmark. Electronic address: jacob.pontoppidan.thyssen@regionh.dk.
authors.forename: Jacob P
authors.initials: JP
authors.lastname: Thyssen
mesh_terms: D015331:Cohort Studies; D003718:Denmark / Q000453:epidemiology; D003876:Dermatitis, Atopic / Q000453:epidemiology*; D005260:Female; D006244:Hardness; D006801:Humans; D007223:Infant; D008297:Male; D015995:Prevalence; D012621:Seasons; D014867:Water / Q000737:chemistry*
chemical_list: D014867:Water
keywords: Atopic dermatitis; birth cohort; climate; risk factors; season of birth; water hardness