Dietary alkyl thiol free radicals (RSS) can be as toxic as reactive oxygen species (ROS).

Harmful free-radicals, such as superoxide anion (a reactive oxygen species: ROS) are produced during aerobic respiration in all tissues because of only partial reduction of some oxygen molecules in mitochondria: this is due to one-electron reduction of each atom of oxygen, instead of four-electron reduction per molecule of oxygen to form water. Similarly, in liver, and many other tissues such as lung and brain, an electron transfer chain from NADPH to water occurs (with insertion of one oxygen atom into xenobiotic substrates) that uses cytochromes P450 (EC 1.14.14.1) as the electron acceptor. Here, futile recycling of electrons, in the absence of substrate produces the superoxide anion (*O2')--see above. Reactive oxygen species (ROS) and reactive sulfur species (RSS) may act in unison to damage biomolecules. For example, damage to biomolecules can occur by attack on phospholipid membranes, and also the targeting of DNA results in mutagenicity and associated carcinogenicity-related mutagenic damage. Free radical injury to low density lipoprotein (LDL) has been identified in the causation of atherosclerosis implicated in arterial disease, which can lead to heart attack and strokes.