Setting: Our knowledge about asthma incidence in an adult population is limited. The aim of the present investigation was to estimate the incidence rate of adult-onset asthma in relation to age, sex, atopy and smoking in a random population sample.
Methods: A random sample of 20000 subjects 20 to 50 years of age was investigated using a short respiratory questionnaire. It was answered by 15813 persons. Adult-onset asthma was defined as a positive response to 'physician-diagnosed' asthma from 16 years of age. Subjects were also asked to report the year of asthma diagnosis, and also, when relevant, the year of smoke-start and smoke-stop. Incidence rates of adult-onset asthma and incidence rate ratios (IRR) were calculated.
Results: The incidence rate of adult-onset asthma among females was 1.3 cases/1000 person-years compared with 1.0/1000 person-years for males (IRR 1.3, 95% confidence interval [CI] 1.0-1.6). The incidence rate was high (3.0/1000 person-years) among females aged 16-20 years. There was a strong association between the incidence rate of adult-onset asthma and hay fever, atopic dermatitis and family history of atopy. Compared with never-smokers, the IRR for female smokers was 1.6 (95% CI 1.1-2.2), while for male smokers it was unity. Both male and female ex-smokers had moderately increased rate ratios, of 1.5 and 1.1, respectively.
Conclusions: In this retrospective study, reported atopic symptoms and family history of atopy were strongly associated with incidence of adult-onset asthma. Tobacco smoking may be associated with an increased incidence rate of adult-onset asthma, especially among women.