New Findings What is the central question of this study?The aim was to explore the effects and underlying mechanisms of H-2 on bleomycin-induced pulmonary fibrosis. What are the main findings and its importance?Our results indicate that, in bleomycin-induced pulmonary fibrosis, H-2 inhalation attenuated oxidative stress and reversed the pulmonary epithelial-to-mesenchymal transition process by reducing reactive oxygen species production and inhibiting the expression of transforming growth factor-beta 1, alpha-smooth muscle actin and collagen I to improve fibrotic injury and exert anti-fibrogenic effects. Thus, H-2 inhalation has promising therapeutic potential as a useful adjuvant treatment for patients with idiopathic pulmonary fibrosis, which deserves further study and evaluation. center dot center dot Hydrogen (H-2) can protect against tissue damage. The effect of H-2 inhalation therapy on the pathogenesis of pulmonary fibrosis remains unknown. This study was designed to explore the effects and underlying mechanisms of H-2 inhalation on bleomycin (BLM)-induced pulmonary fibrosis. A rat model of pulmonary fibrosis was established with BLM. Rats were randomly divided into control and H-2 inhalation groups. Haematoxylin and Eosin staining and Mason's Trichrome staining were performed to evaluate pulmonary fibrosis injury, inflammatory cell infiltration, structural disorder and collagen deposition. qRT-PCR and western blot assays were used to determine the expression of TNF-alpha, TGF-beta 1, alpha-SMA, E-cadherin, N-cadherin, vimentin, VEGF and collagen type I at both mRNA and protein levels. The contents of reactive oxygen species, TGF-beta 1, TNF-alpha, malondialdehyde and hydroxyproline were determined with biochemical test kits or ELISA kits. Bleomycin-stimulated rats exhibited typical symptoms of pulmonary fibrosis, which featured an increase in collagen deposition, alveolitis, fibrosis and parenchymal structural disorder in the lung. However, BLM-induced oxidative stress was attenuated by H-2 inhalation therapy, which reduced the contents of reactive oxygen species, malondialdehyde and hydroxyproline, enhanced the activity of glutathione peroxidase and decreased the expression of TGF-beta 1 and TNF-alpha. In addition, H-2 inhalation also inhibited BLM-induced epithelial-to-mesenchymal transition by inhibiting TGF-beta 1, increasing the expression level of the epithelial cell marker E-cadherin, and decreasing the expression level of the mesenchymal cell marker vimentin in a time-dependent manner. In addition, H-2 inhalation downregulated alpha-SMA expression and suppressed collagen I generation, exerting anti-fibrogenic effects. Hydrogen inhalation therapy attenuates BLM-induced pulmonary fibrosis by inhibiting TGF-beta 1, relevant oxidative stress and epithelial-to-mesenchymal transition.