Consistent with the observation of reduced systemic SCGB1A1 protein concentration among COPD patients and asymptomatic smokers in this study, the normal and chronic bronchitis-like bronchial mucosa models exhibited reduction in secreted SCGB1A1 protein levels following exposure to only the lowest (0.25%) CSC concentration. Here, SCGB1A1 is linked to chronic obstructive pulmonary disease.