Asthma, chronic bronchitis and respiratory symptoms : prevalence and important determinants

Abstract: The Obstructive Lung Disease in Northern Sweden study's (OLIN) overall aim is prevention of obstructive airways diseases; asthma, chronic bronchitis and chronic obstructive pulmonary disease (COPD). The first part of the OLIN study was a cross- sectional study in three phases, which aimed to estimate the prevalence obstructive lung diseases and to collect data on possible determinants of diseases. This thesis is based on the first part of the OLIN study, and on a postal survey mainly performed in order to evaluate the external validity of the first part of the project Aims:' To assess the prevalences of asthma, chronic bronchitis and respiratory symptoms in adults.' To compare the influence of various diagnostic criteria on prevalence.' To identify subjects with obstructive lung diseases, in particular asthma, for case-referentand prospective longitudinal studies.' To examine whether the trend towards an increase in the prevalence of asthma persists.' Study factors that may influence the development of obstructive lung diseases; age,gender, smoking habit, occupation, socio-economic group, population density and area of domicile.The first part of the OLIN study consisted of three phases. A postal questionnaire regarding respiratory symptoms and diseases, smoking habit and profession was sent to all subjects aged 35-36 y, 50-51 y and 65-66 y (n=6,610) living in eight representative areas of Sweden's northernmost province; 86% completed the questionnaire. Those reporting symptoms suspicious of asthma or chronic bronchitis (n=l,340), together with a stratified sample (n=315) of those not suspected of having the diseases according to the postal questionnaire, were invited to structured interviews and lung function tests. The prevalence of asthma, 5- 6% according to both the postal questionnaire and to the structured interview, prompted a validity study, which included bronchial provocation tests. While the prevalence remained unchanged, the validity study better identified the subjects with asthma and chronic bronchitis, thus improving the representativeness of the subjects with the diseases. In 1992, the study base was expanded by a postal questionnaire study which included 20/489 subjects 20-69 y in order to assess whether the prevalence had changed, to create possibilities to estimate the incidence, and to be better able to detect determinants of diseases.The results show that the prevalence of asthma in adults in 1992 was 7-8% according to postal questionnaire and was considerably higher, approximately 10%, in young adults. Further, the prevalence of asthma in 1986-1987 in subjects aged 35-36 y, 50-51 y and 65-66 y was 5% by using a combination of epidemiological and clinical methods. Various operational criteria yielded a prevalence of 4-7%. Between 1986 and 1992 the prevalence of asthma in these age groups increased with 1% according to the postal questionnaire. Chronic bronchitis in subjects aged 35-36 y was 3% in 1986-1987. The prevalence of chronic bronchitis increased with age, particularly in men. The mean prevalence in the three age groups 35-36 y, 50-51 y and 65-66 y was 12% in men and 8% in women. Chronic bronchitis was strongly associated with smoking, age and a family history of obstructive airways disease. Regarding socioeconomic group chronic bronchitis was related to manual workers in industry and to self- employed other than professionals, and it was particularly common in miners and in those employed in agriculture. The strongest risk factor for asthma was a family history of asthma, and asthma was more common in manual workers in service, in non-manual assistant employees as well as in farmers. The results also indicate the presence of an urban factor in asthma in northern Sweden, in spite of the fact that respiratory symptoms in general tended to be more common in the colder interior of the province compared with the coastal area.