Sulfur dioxide (SO2) is a common air pollutant and is detrimental to many organs. Its toxic effects including oxidative damage, deoxyribonucleic acid (DNA) damage and inflammation have been extensively studied. However, recent studies showed that SO2 can be generated endogenously in mammals. In contrast to the toxic effects of SO2, protective effects have also been found in mammals. Endogenous SO2 has antioxidant, anti-inflammatory, anti-hypertension, and anti-atherogenic effects and regulates vascular tone and cardiac function in mammals. SO2 may have a dual role in regulating physiological and pathophysiological effects in manimals. The biological effects of SO2 in mammals are reviewed in this study. (c) 2014 Elsevier Inc. All rights reserved.