NOX4 and chronic obstructive pulmonary disease: Some mechanism studies may explain the results: According to Fan et al. (26), PM2.5 exposure enhanced mitophagy via triphosphopyridine nucleotide oxidase 4 (NOX4)/ transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) redox imbalance, further increasing susceptibility to acute exacerbation of COPD (AECOPD).