NFE2L2 and pulmonary emphysema: The Nrf2 knockout mice and Nrf2 silenced cell used in our study allowed us to fully investigate the role of Nrf2 in the protective efforts of H2S. As only PM2.5 can get into bottom of the lung and deposit on alveoli, we used the PM2.5 instead of PM to stimulate alveolar epithelial cells, which was more likely to reveal the mechanism of emphysema pathogenesis in real world.