Objectives: To study whether asthma and allergic rhinitis had increased from 1977 to 1991 and if so, in which subpopulations; to study if structural changes of the society or change in the genetic susceptibility of the population could explain the increase.
Design: Cross-sectional surveys with mailed questionnaires in 1977, 1979 and 1991, data from 1977 and 1979 combined; national mortality statistics.
Subjects: A nationwide sample of 12-, 14-, 16- and 18- year-olds. Sample sizes were 4335 and 3059, response rates 88% and 77%. Mortality statistics from 1958 to 1990.
Main outcome measures: Point prevalence rate (%) of self-reported, physician-diagnosed asthma and allergic rhinitis, susceptibility of the population measured by probability of respiratory death.
Results: Prevalence of asthma was 1.0% in 1977-1979 and 2.8% in 1991, that of allergic rhinitis 5.0% and 14.9%. Logistic regression analysis showed that the increase did not differ in socio-demographic subgroups or parents' smoking groups and that changes in the distribution of socio-demographic variables did not explain the difference either. Respiratory deaths in ages 0-4 were fewer in the birth cohorts measured in 1991.
Conclusions: Physician-diagnosed asthma and allergic rhinitis increased three-fold among Finnish adolescents in 1977-1991. Factors which explain the increase affected all subgroups similarly. Such factors could be changes in diagnostic practices and indoor air quality. Increased susceptibility could explain only a small part. It is likely that a major part of this considerable increase is real.