MMP9 and chronic obstructive pulmonary disease: Yan Jiang et al. have reported that PM2.5 induced airway inflammation, compromised lung function, emphysematous lesions, and deleterious airway remodeling in chronic obstructive pulmonary disease (COPD) through inducing the M2 polarization of AMs and upregulating transforming growth factor-β, matrix metallopeptidase (MMP)-9, and MMP-12 [19].