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Rationale: Chronic obstructive pulmonary disease (COPD) includes respiratory symptoms and chronic airflow limitation (CAL). In some cases, emphysema and impaired diffusing capacity of the lung for carbon monoxide (DL_CO) are present, but characteristics and symptoms vary with smoking exposure.

Objective: To study the prevalence of CAL, emphysema, and impaired DL_CO in relation to smoking and respiratory symptoms in a middle-aged population.

Methods: We investigated 28,746 randomly invited individuals (52% women) aged 50-64 years across six Swedish sites. We performed spirometry, DL_CO testing, and high-resolution computed tomography and asked for smoking habits and respiratory symptoms. CAL was defined as post-bronchodilator forced expiratory volume in 1 second divided by forced vital capacity (FEV₁/FVC) < 0.7.

Results: The overall prevalence was 8.8% for CAL, 5.7% for impaired DL_CO (DL_CO < LLN), and 8.8% for emphysema, with a higher prevalence in current smokers than in ex-smokers and never-smokers. The proportion of never-smokers among those with CAL, emphysema, and impaired DL_CO was 32%, 19%, and 31%, respectively. Regardless of smoking habits, the prevalence of respiratory symptoms was higher among people with CAL and impaired DL_CO than those with normal lung function. Asthma prevalence in never-smokers with CAL was 14%. In this group, asthma was associated with lower FEV₁ and more respiratory symptoms.

Conclusions: In this large population-based study of middle-aged people, CAL and impaired DL_CO were associated with common respiratory symptoms. Self-reported asthma was not associated with CAL in never-smokers. Our findings suggest that CAL in never-smokers signifies a separate clinical phenotype that may be monitored and, possibly, treated differently from smoking-related COPD.