Prospective study of asthma in relation to smoking habits among 14,729 adults.

The prevalence and incidence of asthma in relation to cigarette smoking habits was studied in a population of 14,729 Finnish adult men and women who participated in a postal health survey in 1975. Of those invited to participate in a new survey in 1981, 89.7% replied. Asthma was diagnosed on the basis of self reporting of asthma diagnosed by a physician and by record linkage to a national register of hospital admissions to all general and tuberculosis hospitals during 1972 and 1983. The prevalence of diagnosed asthma in 1975 was significantly higher among male smokers than among male non-smokers (relative risk (RR) = 1.73); no significant difference was observed for women (RR = 1.33). People with asthma were slightly but not significantly more likely to stop smoking during the six year follow up period (RR = 1.23). The incidence of asthma among those who had neither reported asthma in 1975 nor been admitted to hospital for asthma before the 1975 questionnaire study was not significantly higher among smokers than among non-smokers during follow up. Although possible mechanisms exist to explain how smoking could have a role in the aetiology of asthma, this study suggests that smoking is not a strong risk factor for asthma.