Oxygen is a poison, as well as a necessity, for most animals and
plants. Organisms exposed to excessive oxygen can suffer severe
damage or even death. Even in the course of normal metabolism
involving ordinary amounts of oxygen, extremely promiscuous and
reactive chemical compounds called oxygen free radicals. These
are formed not only min normal metabolism, but also in the
breakdown of peroxidised lipids (rancid fats), by radiation, and
by white cells.

Oxygen free radicals are of particular importance in the body.
They take part in chemical reactions that may be beneficial -
killing bacteria, production of local hormones that regulate
metabolism, or harmful - destruction of polyunsaturated fatty
acids in membranes and damage to proteins, DNA and whole tissues.

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