Rationale: Individuals exposed to second-hand smoking may be more susceptible to asthma and chronic obstructive pulmonary disease (COPD). We investigated the risk of respiratory symptoms, asthma, and COPD in adults exposed to second-hand smoking at different stages of life in the general population.
Methods: We identified individuals who had been exposed to second-hand smoking in childhood only, adulthood only, or lifelong in a cohort of 20,421 adults from the Danish General Suburban Population Study and recorded respiratory symptoms, lung function, asthma, and COPD as outcomes.
Results: Among 20,421 adults from the general population, 2,551 (12%) had been lifelong exposed to second-hand smoking, 459 (2%) had been exposed in adulthood only, and 13,998 (69%) had been exposed in childhood only; the mean ages of the three groups were 54 years, 55 years, and 57 years, respectably, compared with 56 years in non-exposed individuals (P<0.001). Equivalent values for the prevalence of current smoking were 25%, 20%, and 18% versus 12% (P<0.001). After adjustment for age, smoking, and sex, the odds ratios for wheezing, severe dyspnoea, cough on exertion, and asthma increased as a function of second-hand smoke exposure (Ps≤0.004); individuals who had been exposed to second-hand smoking lifelong, in adulthood only, or in childhood only versus non-exposed had increased odds ratios for wheezing of 1.62 (95% CI=1.41-1.87), 1.50 (1.15-1.94), and 1.16 (1.04-1.30). Corresponding values were 2.08 (1.52-2.85), 2.05 (1.22-3-44), and 1.23 (0.95-1.59) for severe dyspnoea, 1.56 (1.33-1.83), 1.53 (1.15-2.02), and 1.19 (1.05-1.35) for cough on exertion, 1.36 (1.14-1.63), 1.49 (1.09-2.05), and 1.13 (0.99-1.30) for asthma, and 1.24 (1.03-1.48), 1.25 (0.90-1.74), and 1.09 (0.96-1.24) for COPD. The population attributable fractions of asthma and COPD due to lifelong second-hand smoke exposure were 4.3% and 2.9%.
Conclusion: Individuals exposed to lifelong second-hand smoking have increased risks of respiratory symptoms, asthma, and COPD, and may account for 4.3% and 2.9% of people with asthma and COPD in the general population.