The association of active and secondhand smoking with airflow obstruction is stronger among individuals with chronic bronchitis

Chronic bronchitis (CB) may act as a "vulnerable pathological background" that modifies the pathogenic effects of other risk factors on airflow obstruction (AO). This study aims to compare the differences in risk factors for AO between individuals with and without CB, and to evaluate the synergistic interaction between CB and tobacco smoke exposure on AO. This study included participants from the 12-month follow-up of the Nationwide COPD Screening Program in China, which enrolled individuals at high risk for COPD based on a COPD-SQ score of ≥ 16. CB was defined by Chronic Airway Assessment Test (CAAT) cough and sputum scores both ≥ 3, and AO was defined by pre-bronchodilator FEV1/FVC below the lower limit of normal (LLN). Multivariable logistic regression models were used to assess AO-associated factors. Interactions between CB and active smoking or secondhand smoke exposure were evaluated on both multiplicative and additive scales. Sensitivity analyses used post-bronchodilator FEV1/FVC < LLN to define the outcome. Among 113,009 participants, 3.6% of them were identified with CB. Most established risk factors showed consistent associations with AO across groups, whereas secondhand smoke exposure, biomass fuel exposure, and tuberculosis history exhibited differential patterns. Significant multiplicative interactions were observed between CB and both active and secondhand smoking on AO risk. The association per 10 pack-year increase in smoking index was stronger in participants with CB (OR = 1.11, 95% CI: 1.07–1.14) than in those without (OR = 1.05, 95% CI: 1.04–1.06; P interaction = 0.001). Similarly, secondhand smoke exposure conferred higher AO risk in the CB group (OR = 1.40, 95% CI: 1.16–1.70) versus the non-CB group (OR = 1.02, 95% CI: 0.97–1.08; P interaction = 0.002), and this interaction varied by smoking status. Additive interactions confirmed positive synergies between CB and smoking index ≥ 10 pack-years (RERI = 0.67, 95% CI: 0.32–1.02) and between CB and secondhand smoke exposure (RERI = 0.70, 95% CI: 0.25–1.15). Sensitivity analyses remained robust. Chronic bronchitis significantly modified the relationship of active and secondhand smoking with airflow obstruction, with stronger associations observed in individuals with CB. These individuals represent a highly susceptible population who should be prioritized for smoking cessation and secondhand smoke protection.