title: The combined effects of family size and farm exposure on childhood hay fever and atopy.
abstract: BACKGROUND
  Exposure to farming environments and siblings is associated with reduced risks of childhood hay fever and atopy. We explored the independence and interaction of these protective effects in the GABRIELA study.
  
  
  METHODS
  Questionnaire surveys on farming, asthma, and allergies were conducted in four central European areas among 79,888 6-12-yr-old children. Aeroallergen-specific serum IgE was measured in a stratified sample of 8,023 children. Multiple logistic regression was used to compare gradients in allergy prevalence by sibship size across three categories of exposure to farming environments.
  
  
  RESULTS
  The prevalence of hay fever ranged from 2% (95% confidence interval 1.6%; 2.7%) among farmers' children with more than two siblings to 12% (11.2%; 13.0%) among children with no farm exposure and no siblings. Farming families were larger on average. More siblings and exposure to farming environments independently conferred protection from hay fever and atopy. There was no substantial effect modification between family size and exposure to farming environments. The odds ratios for hay fever per additional sibling were 0.79 among unexposed non-farm children, 0.77 among farm-exposed non-farm children, and 0.72 among children from farming families (2df interaction test: p = 0.41).
  
  
  CONCLUSIONS
  The inverse association of exposure to farming environments with hay fever is found in all sizes of family, with no substantial tendency to saturation or synergism. This suggests that different biological mechanisms may underlie these two protective factors. Combinations of a large family and exposure to farming environments markedly reduce the prevalence of hay fever and indicate the strength of its environmental determinants.
authors.affiliation: Institute of Epidemiology and Medical Biometry, Ulm University, Ulm, Germany. jon.genuneit@uni-ulm.de
authors.forename: Jon
authors.initials: J
authors.lastname: Genuneit
authors.forename: David P
authors.initials: DP
authors.lastname: Strachan
authors.forename: Gisela
authors.initials: G
authors.lastname: Büchele
authors.forename: Juliane
authors.initials: J
authors.lastname: Weber
authors.forename: Georg
authors.initials: G
authors.lastname: Loss
authors.forename: Barbara
authors.initials: B
authors.lastname: Sozanska
authors.forename: Andrzej
authors.initials: A
authors.lastname: Boznanski
authors.forename: Elisabeth
authors.initials: E
authors.lastname: Horak
authors.forename: Dick
authors.initials: D
authors.lastname: Heederik
authors.forename: Charlotte
authors.initials: C
authors.lastname: Braun-Fahrländer
authors.forename: Erika
authors.initials: E
authors.lastname: von Mutius
mesh_terms: D000382:Agricultural Workers' Diseases / Q000453:epidemiology* / Q000276:immunology; D000485:Allergens / Q000009:adverse effects / Q000276:immunology; D002648:Child; D004781:Environmental Exposure / Q000009:adverse effects; D005060:Europe; D005191:Family Characteristics; D005260:Female; D006801:Humans; D007073:Immunoglobulin E / Q000097:blood; D008297:Male; D011058:Pollen / Q000276:immunology; D015995:Prevalence; D006255:Rhinitis, Allergic, Seasonal / Q000453:epidemiology* / Q000276:immunology; D012306:Risk; D011795:Surveys and Questionnaires
chemical_list: D000485:Allergens; D007073:Immunoglobulin E