Reactive oxygen species in living systems: source, biochemistry, and role in human disease. Barry Halliwell American Journal of Medicine Sept 30, 1991 v91 n3C p14S(9) Author's Abstract Reactive oxygen species are constantly formed in the human body and removed by antioxidant defenses. An antioxidant is a substance that, when present at low concentrations compared to that of an oxidizable substrate, significantly delays or prevents oxidation of that substrate. Antioxidants can act by scavenging biologically important reactive oxygenspecies ([O.sub.2].,[H.sub.2][O.sub.2], .OH, HOCl, ferryl, peroxyl, and alkoxyl), by preventing their formation, or by repairing the damage that they do. One problem with scavenging-type antioxidants is that secondary radicals derived from them can often themselves do biologic damage. These various principles will be illustrated by considering several thiol compounds.