Objectives: Chronic bronchitis was studied in relation to work time and years of exposure in farming, as well as to production type, dusty occupation outside farming, and the combination of work exposure and smoking, in a population of farmers.
Methods: In 1989 a representative cohort of 10,792 farmers and spouses was selected from a government register and invited to participate in a cross-sectional study in 1991. The total response rate was 80%. There were 33% part-time farmers, and among the men 32% of the full-time and 42% of the part-time farmers had worked in dusty occupations outside farming. Bronchitis symptoms were recorded on a self-administered questionnaire, spirometric data were obtained, and internal reference equations were calculated for forced expiratory volume in 1 s (FEV1.0).
Results: The exposure factors of importance for chronic bronchitis were full-time farming versus part-time farming, livestock production types (poultry, dairy, swine, horse and combinations), and occupational dust exposure outside agriculture. The combinations of the work exposure factors were significant and showed a 2- to 3-fold increase in risk for chronic bronchitis. Combinations with smoking showed up to a 6-fold increase in risk. Over the age of 50 years, chronic bronchitis was a risk factor for airway obstruction defined as the standardized residuals for FEV1.0 less than -2 for both nonsmokers (OR 2.8, 95% CI 1.1-6.8) and smokers (OR 8.5, 95% CI 5.1-14.3).
Conclusions: Work exposure factors in farming and other dusty occupations enhance the risk for chronic bronchitis from 2- to 3-fold for farmers. In combination with smoking the risk increases to up to 6-fold.