Oxygen free radicals are molecules that present unpaired electrons in their outer orbit and can transform other molecules such as proteins, carbohydrates, lipids and deoxyribonucleic acid. Oxygen free radicals are produced in various clinical conditions in which hypoxic microenvironments are generated and reoxygenation follows. Such situations include clinical shock, septicemia, systemic inflammatory response, fulminant hepatitis, organ transplant and respiratory failure. In this review, we discuss the main concepts related to oxygen free radicals: the principal types and their formation, as well as the way in which they affect cellular structures and cause significant tissue damage. We present also the main antioxidants that guard against oxidative stress, including glutathione, glutathione peroxidase, superoxide dismutase, catalase, and N-acetylcysteine. The influence of oxygen free radicals on the principal pulmonary diseases are also discussed, with special emphasis given to oxygen free radicals in cigarette smoke, chronic obstructive pulmonary disease, asthma, sleep apnea syndrome and acute respiratory distress syndrome.

Keywords: Key words: Oxygen free radicals. Antioxidant substances. Oxidative stress. Lung diseases.