A 55-year-old male comes to your office complaining of increasing dyspnea on exertion and chronic cough with sputum production, slight at first but increasing over time. On exploring his past history he has been experiencing a persistent cough with sputum production lasting the entire spring season in the past two years. Spirometry analysis shows an FEV1/FVC ratio of 0.6.

Which of the following factors was likely the greatest contributor to this patient’s condition?