Prevalence of asthma symptoms among adults aged 20-44 years in Canada

Background: Reported prevalence rates of asthma vary within and between cou ntries around the world. These differences suggest environmental factors in addition to genetic factors in the cause of the disease and may provide cl ues for preventive strategies. We examined the variability of asthma-relate d symptoms and medication use among adults in 6 sites across Canada (Vancou ver, Winnipeg, Hamilton, Montreal, Halifax and Prince Edward Island) and co mpared our findings with those from sites that had participated in a recent European survey. Methods: We used the same sampling strategy and standardized questionnaire as those used in the European Community Respiratory Health Survey (ECRHS). The 6 Canadian sites were selected to represent different environments with respect to climate, air pollution and occupational exposure. Community-bas ed samples of 3000 to 4000 people aged 20-44 years were randomly selected i n each site. Subjects were asked to complete the questionnaire by mail betw een March 1993 and November 1994. Prevalence rates land 95% confidence inte rvals (Clsl) of asthma symptoms, self-reported asthma attacks and use of as thma medication were compared across the Canadian sites and with sites that had participated in the ECRHS. Results: The overall response rate of those selected to receive the questio nnaire was 86.5% (range 74.5%-92.8%). The prevalence rates of most asthma s ymptoms varied significantly among the Canadian sites. For instance, 21.9% (Montreal) to 30.4% (Halifax) of the men and 24.0% (Vancouver) to 35.2% (Ha lifax) of the women reported wheezing in the year before the survey. Depend ing on the site, 4.4% to 6.3% of the men and 5.2% to 9.5% of the women repo rted an asthma attack in the last year, and 4.0% to 6.1% of the men and 4.9 % to 9.7% of the women were currently using asthma medication. Prevalence r ates of symptoms, asthma attacks and medication use did not change with age , but they were higher among women than among men. Compared with the result s from the ECRHS sites, those from the Canadian sites were among the highes t. Interpretation: Significant variation in the prevalence of asthma symptoms, asthma attacks and use of asthma medication between Canadian sites and int ernational sites suggests environmental influences. Different combinations of factors in different sites may be responsible for the high prevalence ra tes and should be the subject of further research to guide clinical managem ent and public health intervention.