Farm environment in childhood prevents the development of allergies

Background A protective effect of infections in early life might explain th e firmly reported finding of an inverse association between atopic disorder s and large sibships. Objective To study the effect of childhood farm, rural non-farm and urban e nvironment, as well as family size and other factors on the occurrence of a sthma, wheezing and atopic disorders up to young adulthood. Methods Data on lifetime prevalence of physician-diagnosed asthma, allergic rhinitis and/or allergic conjunctivitis, atopic dermatitis, as well as sel f-reported episodic wheezing from 10 667 Finnish first-year university stud ents aged 18-24 years were collected by a postal questionnaire. Association s of lifetime prevalence of the diseases with living on a farm, in a rural non-farm and urban environment during childhood were estimated by logistic regression analysis. Adjustment was made for potential confounding by gende r, parental atopy, parental education, number of older siblings, day care o utside the home and passive smoking. Results The childhood farm environment independently reduced the risk for p hysician-diagnosed allergic rhinitis and/or allergic conjunctivitis (adjust ed odds ratio 0.63, 95% CI 0.50-0.79, P < 0.001), and for diagnosed asthma and episodic wheezing analysed together (OR 0.71, 95% CI 0.54-0.93, P < 0.0 5), but not for atopic dermatitis during lifetime. Urban childhood environm ent did not show independent increased risk when compared with rural non-fa rm residence. The inverse association of sibship size with the occurrence o f allergic rhinitis and/or allergic conjunctivitis was found among subjects with one (OR 0.86, 95% CI 0.77-0.96, P < 0.01) or at least four older sibl ings (OR 0.47, 95% CI 0.26-0.84, P < 0.05). Conclusion Childhood farm environment seems to have a protective effect aga inst allergic rhinitis and/or conjunctivitis, and more weakly against asthm a and wheezing irrespective of family size. Environmental exposure to immun e modulating agents, such as environmental mycobacteria and actinomycetes, favouring manifestation of a nonatopic phenotype could explain the finding.