title: Can selection explain the protective effects of farming on asthma?
abstract: OBJECTIVE
  Reduced asthma and allergy risks in farmers have been ascribed to microbial exposures. However, selection may also play a role and this was assessed in two Scandinavian farming populations.
  
  
  METHODS
  Asthma prevalence in 739 Danish farming students was compared to that of 1,105 siblings. 8,482 Norwegian farmers were also compared with 349 early retired farmers.
  
  
  RESULTS
  The prevalence of ever-asthma was 5.4% in farming students and 5.2% in siblings (OR 1.1; 95%CI 0.73-1.7). Current asthma in farmers was 3.0% compared to 6.3% in farmers who had retired early (OR 1.8, 95%CI 1.1-2.9). Adjustments for early retirement increased the asthma prevalence by 0.3-0.6%. Farmers who had changed production were more likely to have asthma (OR 9.8, 95% CI 6.0-16).
  
  
  CONCLUSIONS
  No healthy worker selection into farming was observed and changes in asthma prevalence due to early retirement were small. Selection effects are therefore unlikely to explain the protective effects of farming on asthma.
authors.affiliation: 1. Department of Chemical and Biological Work Environment, STAMI National Institute of Occupational Health, Oslo, Norway2. Centre for Public Health Research, Massey University, Wellington, New Zealand.
authors.forename: Wijnand
authors.initials: W
authors.lastname: Eduard
authors.affiliation: Department of Public Health, Section for Environment, Work and Health, Aarhus University, Aarhus, Denmark.
authors.forename: Vivi
authors.initials: V
authors.lastname: Schlünssen
authors.affiliation: Department of Occupational Medicine, Aalborg University Hospital, Aalborg, Denmark.
authors.forename: Torben
authors.initials: T
authors.lastname: Sigsgaard
authors.affiliation: 1. Department of Public Health, Section for Environment, Work and Health, Aarhus University, Aarhus, Denmark 2. Department of Occupational Medicine, Aalborg University Hospital, Aalborg, Denmark.
authors.forename: Øyvind
authors.initials: Ø
authors.lastname: Omland
authors.affiliation: 1. Centre for Public Health Research, Massey University, Wellington, New Zealand 2. Department of Medical Statistics, London School of Hygiene and Tropical Medicine, London, United Kingdom.
authors.forename: Neil
authors.initials: N
authors.lastname: Pearce
authors.affiliation: Centre for Public Health Research, Massey University, Wellington, New Zealand.
authors.forename: Jeroen
authors.initials: J
authors.lastname: Douwes
mesh_terms: D000293:Adolescent; D000328:Adult; D000383:Agriculture; D001249:Asthma / Q000453:epidemiology* / Q000209:etiology; D003430:Cross-Sectional Studies; D003718:Denmark / Q000453:epidemiology; D000067565:Farmers; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009664:Norway / Q000453:epidemiology; D015995:Prevalence; D012179:Retirement; D012641:Selection, Genetic; D035781:Siblings; D013334:Students; D055815:Young Adult