In the human body Iron is found in 2 forms: ferrous iron and ferric iron. The former is soluble at neutral pH and is the main form in which it is found in organisms; however, it is oxidized in muscles, rather than in red blood cells. Iron, in the ²⁺ form, binds O₂, a potent oxidant. Hemoglobin and myoglobin frequently convert to beta-hemoglobin and beta-myoglobin (the forms that contain Fe³⁺). Meta-hemoglobin reductase and meta-myoglobin reductase reduce iron instead, turning it into the ²⁺ form.

As a dead organism stops, these enzymes also stop and, therefore, in the corpse iron is all in 3+ form. So there is a continuous alternation between the 2 forms; however ferrous iron is very toxic: if it reacts with oxygen, it can form superoxide anion, which turns into peroxide. In Fenton's reaction there is the reaction of ferrous ion with hydrogen peroxide: iron becomes ferric, giving an electron to hydrogen peroxide and breaking the bond, which forms a normal hydroxide (OH radical). A similar reaction occurs if instead of iron there is a superoxide anion: Haber-Weiss reaction. 

These are the 2 main reactions by which the production of very toxic radicals is triggered.