Objectives This study examined the determinants of asthma in a population of farmers, including as a crude indicator of genetic predisposition "history of asthma in next-of-kin" (family history), and exposure factors such as animal production and smoking.

Methods In a cross-sectional study of 8482 farmers or farmers' spouses in Norway a questionnaire with information on asthma among the subjects and their next-of-kin, production type and farming activities, exposures outside farming, and smoking was applied. Spirometry was performed.

Results The lifetime prevalence of self-reported asthma in the population was 6.3%. Significant risk factors for current asthma were asthma among next-of-kin, asthma as child or adolescent, animal production, and age. In a comparison with subjects with no family history of asthma and no animal production the adjusted odds ratio for current asthma in never smokers was 1.9 [95% confidence interval (95% CI) 0.4-8.9] for subjects with family history only, 2.2 (95% CI 1.1-4.2) for subjects with animal production only, and 6.3 (95% CI 3.1-13.1) for subjects with both factors. A combination of animal production, smoking, and a positive family history gave an odds ratio of 8.1 (95% CI 4.0-16.2) for current asthma.

Conclusion The study can be interpreted as support for the hypothesis of an interaction between genetic factors and exposure factors in the causation of asthma. Since familial associations may be exposure-related, the necessity of considering indicators of both inheritance and exposure in epidemiologic studies of asthma is emphasized.

Key terms airway obstruction; asthma; cross-sectional study; family history; farmer; genetic factor; occupational exposure; smoking