Trends in asthma, allergic rhinitis and eczema among Swedish conscripts from farming and non-farming environments. A nationwide study over three decades.

BACKGROUND: Asthma and allergies are less common in children who have been raised in farming environments.
OBJECTIVES: To assess whether children who grow up in a farming environment have been protected against the general increase in atopic disorders in Sweden and whether other rural environments could also be protective.
METHOD: Linkage at an individual level of three national registers in Sweden: The Swedish Military Service Conscription Register (MSCR), the Register of the Total Population (RTP) and the Population and Housing Censuses (PHC). Asthma, allergic rhinitis and eczema at conscription were analysed in relation to area of residence, parental occupation, maternal age, family size and being the first born for 1 309 652 male conscripts in three successive cohorts born between 1952 and 1981.
RESULTS: Allergic rhinitis and eczema displayed a continuous increase throughout the study period, whereas the rise in asthma mainly occurred in conscripts born after 1961. Farming environments and rural living already provided protection from allergic rhinitis in conscripts born during the 1950s, but the protective effect was greater in later cohorts. An inverse association was observed between farm living and asthma, but mainly in conscripts born after 1970. The adjusted risk ratios for asthma in conscripts from farming vs. non-farming families were 1.00 (95% CI 0.93-1.07), 0.94 (95% CI 0.88-1.01) and 0.85 (95% CI 0.79-0.91) in conscripts born in 1952-1961, 1962-1971 and 1972-1981, respectively. Rural living per se had no effect on the occurrence of asthma. Eczema was less common in rural areas, but the time trend was similar in urban and rural areas.
CONCLUSIONS: Our findings suggest that environmental changes affecting the whole of society have promoted an increase in asthma, allergic rhinitis and eczema in both farming and non-farming environments. A lower risk of allergic rhinitis in conscripts whose parents were involved in farming was observed in all birth cohorts, whereas the protective effect of growing up on a farm on the risk of asthma appears to be a fairly recent phenomenon.