Incidence of asthma and respiratory symptoms by sex, age and smoking in a community study.

The purpose of this study was to establish incidence rates for a wide range of respiratory symptoms and asthma, and relate them to sex, age, and smoking habits. A cohort established in 1985 as a random sample from the population of Western Norway, aged 15-70 yrs, was followed-up in 1996-1997. Of the initial cohort of 3,786 subjects, a total of 2,819 replied to mailed questionnaires at both baseline and follow-up. The 11-yrs cumulative incidence of asthma was 4.0% in males and 3.5% in females. For respiratory symptoms, the cumulative incidences for both sexes varied between 2.0% (dyspnoea grade 4) and 25.8% (wheezing), being higher in females than males for most symptoms. For calculation of odds ratios (ORs) multivariate logistic regression analyses were used. The sex and smoking-adjusted incidences increased by age for all symptoms except wheezing and attacks of dyspnoea. Those starting to smoke within the follow-up had ORs of 1.9-2.2 for the cough symptoms compared to never-smokers, after adjusting for sex, age, and pack-yrs. To conclude, the 11-yrs incidence of dyspnoea increased with increasing pack-yrs, after adjusting for sex, age, and changes in smoking habits. This indicated that when analysing other risk factors, adjustment has to be made for the risks posed by smoking, sex and age.