Adult-onset asthma in west Sweden--incidence, sex differences and impact of occupational exposures.

The aim was to estimate the incidence rate of adult-onset asthma in relation to age, sex, smoking and occupational exposures. A random sample of 18,087 subjects aged 16-75 years was investigated using a respiratory questionnaire. Adult-onset asthma was defined as "physician-diagnosed" asthma with onset at or after 16 years of age. The subjects were asked about year of asthma diagnosis and year of starting and stopping smoking. Subjects with onset of asthma before 16 years of age and physician-diagnosed chronic obstructive pulmonary disease were excluded resulting in a study population of 15,761 subjects. Incidence-rates of adult-onset asthma were calculated and relative risks were assessed using Cox-regression models. During the observation period 1990-2008, 359 new cases of asthma occurred and the cumulative incidence for adult-onset asthma was 2.3%. The crude incidence rate was 1.4/1000 person-years (95% confidence interval 1.3/1000-1.6/1000), with significantly higher incidence rate among women than among men. The incidence rate of asthma during never-smoking years was similar to that during smoking years. The rate of asthma incidence decreased with increasing age. Occupational dust and fume exposure and being female were associated with increased risk of asthma. The attributable fraction for occupational exposure to gas, dust and fumes was 9.4% in the total group, 17.3% among men and 5.1% among women. The incidence rate of asthma was higher among women than among men, and the rate declined with increasing age. A substantial proportion of the new-onset asthma cases could be attributed to occupational exposures.