Lifelong farm exposure may strongly reduce the risk of asthma in adults

Background: Farm exposures may protect against childhood asthma, hay fever and eczema. Whether farm exposures also confer protection in adult farmers remains unclear. Moreover, little is known about the role of timing of exposure. We assessed the effects of current and childhood farm exposures on asthma, hay fever and eczema in farmers and a rural nonfarming control population. Methods: We conducted a cross-sectional questionnaire survey in 2509 farming families (response rate 78%) and 1001 nonfarming families (response rate 67%), which included 4288 farmers and 1328 nonfarmers. Results: Farmers were less likely to have asthma symptoms, hay fever and eczema; no significant differences were observed among dairy, sheep and beef, and horticulture farmers. A combination of current and childhood exposure was more strongly associated with shortness of breath (OR 0.50, CL 0.39-0.66), wheeze (OR 0.60, CL 0.49-0.73), asthma medication (OR 0.48, CL 0.37-0.63); and asthma ever (OR 0.56, CL 0.46-0.68) than current exposure alone (OR 0.63, CL 0.47-0.84; OR 0.80, CL 0.65-0.99; OR 0.68, CL 0.51-0.9; OR 0.69, CL 0.56-0.85 respectively) or childhood exposure alone (OR 0.97, CL0.65-1.44; OR 1.01, CL 0.75-1.34; OR 0.78, CL 0.51-1.19; OR 0.87, CL 0.63-1.19 respectively). Moreover, the combined number of years of farm exposure in childhood and adulthood showed a dose-dependent inverse association with symptom prevalence. Conclusions: Although both current and childhood farm exposures may play a role in the observed low prevalence of asthma symptoms in adult farmers, continued long-term exposure may be required to maintain optimal protection.