Free radicals form inside the body when certain chemicals lose an electron and become unstable. This happens constantly, for example, as oxygen is metabolized. These unstable atoms and molecules roam the body stealing the electrons they need to re-stabilize. This, in turn, creates more free radicals, and the process repeats itself.
Damage to the cellular DNA occurs when the body has insufficient hydrogen ions to counterbalance the normal supply of oxygen ions. Cells that fail to regain their stability begin to oxidize—similar to how rust forms on iron. Most scientists agree that this cellular oxidation is the primary cause of premature aging.