Background: It is well known that smoking is a major risk factor for lung disease and respiratory symptoms. We examined the association between smoking and the risk of chronic bronchitis in a large twin sample.
Methods: In a population-based questionnaire study of 13,649 twins, aged 50-71 years, from the Danish Twin Registry, we identified 1,146 twin pairs, discordant for a lifetime history smoking. We performed co-twin control analysis to examine the impact of smoking on the risk of chronic bronchitis.
Results: The prevalence of chronic bronchitis was 9.7 %. In the total sample, high age, living without a spouse, and smoking remained statistically significant predictors with an up to tenfold increased risk of chronic bronchitis in the heaviest smokers compared with never-smokers, after multivariate adjustment. Among twin pairs discordant for smoking, chronic bronchitis was significantly more common in the smoking twin compared with the nonsmoking co-twin. There was no differential effect of smoking on the risk of chronic bronchitis in monozygotic and dizygotic twins.
Conclusions: The risk of chronic bronchitis increases with age and increasing tobacco consumption. The results indicate a direct relationship between smoking and development of chronic bronchitis, but other environmental factors, such as exposure to household smoking in childhood and living without a spouse, also play a role.