The oxidation of polyunsaturated fatty acid side chains, initiated by a free radical such as the hydroxyl radical (OH^•), to form a relatively stable carbon free radical, which reacts further with molecular oxygen to form a peroxy radical, and further yet with another fatty acid side chain to generate another carbon free radical. Such peroxidation causes rancidity in foods, damages membranes and, when it takes place in plasma low-density lipoprotein, can lead to atherosclerosis.