The Free Radical Theory of aging was first put forth by Dr. Denham Harman in his thesis in 1954:
"Aging is caused by free radical reactions, which may be caused by the environment, from disease, and intrinsic reactions within the aging process."

The idea that dangerous free radicals were produced by normal human biology was considered unlikely until 1969. It was eventually recognised that superoxide and hydroxyl free radicals were causative factors in many degenerative diseases and also in aging. Consequently, aging is not simply the wearing out of body parts, but results from the accumulated damage done to individual cells by free radicals, resulting in decreased function throughout the body.

The Free Radical Theory of aging involves a bit of chemistry. If we hark back to our high school lessons we will recall how atoms, or ions, carry an electrical charge. Each of these ions seeks to neutralize the charge, and may do so by combining with an atom or ion with an opposite charge. In the case of the positively charged sodium ion and the negatively charged chlorine ion, each is able to neutralize the other's opposite charge by combining to form sodium chloride, common table salt. Until they come together and neutralize one another each is very active, looking for something to react with.

Free radicals are atoms, or groups of atoms, that have an imbalance of electrons. This means that the free radicals need another atom or group of atoms to bond with to become neutralized, and stabilized. This makes a free radical extremely active. As free radicals move through the body, they try to achieve chemical and electrical stability by combining with atoms of various body tissues. The effect of these free radicals on body tissue might be likened to rust on metal or the browning of a freshly cut apple - a sort of biorust.