Incidence of chronic obstructive pulmonary disease, and the relationship between age and smoking in a Japanese population

Background: Accurately evaluating a risk of chronic obstructive pulmonary disease (COPD) requires a large-scale longitudinal study using a standard criterion for diagnosing COPD. There have been only a few such follow-up studies in Europe and no reports in Asia. We estimated the incidence rate and incidence rate ratio (IRR) of age and smoking for COPD in a Japanese population using the diagnosis criterion of the Global Initiative for Chronic Obstructive Lung Disease guidelines. Methods: Subjects were 17,106 participants aged 25-74 years during health check-ups including spirometry from April 1997 through March 2005 in Japan. Total follow-up of participants were 47,652 person-years in males and 25,224 person-years in females. The IRR of age and smoking was estimated using Cox proportional hazard models with both variables. Results: We identified 466 incidence cases of COPD. The incidence rate per 100 person-years was 0.81 (95% confidence interval [CI], 0.73-0.89) in males and 0.31 (0.24-0.38) in females, and significantly increased with age in both sexes. The incidence rate for current smokers was significantly higher than that for male non-smokers but not significantly for females. Among males, the IRR for current smokers with Brinkman Index < 400, 400-799, and 800+ was 1.2 (0.8-1.9), 2.7 (1.9-3.8), and 4.6 (3.3-6.5), respectively. Conclusion: These results indicated that the COPD risk gradually increased with aging, and that there was a dose-response relationship between smoking and COPD risk.