MMP9 and chronic obstructive pulmonary disease: Similarly, chronic prolonged exposure to ambient PM2.5 led to pulmonary inflammation, impaired lung function, development of emphysematous lesions, and airway wall remodeling that was indicated by airspace enlargement, increased expression of IL-6 and IL-8, MMP9, MMP12, and TGF-β1 proteins in lungs of COPD mice model.