A scientific explanation: In essence, a free radical is any molecular species capable of independent existence, that contains one or more unpaired electrons not contributing to intermolecular bonding, and is, in that sense, "free". They are produced by oxidation/reduction reactions, in which there is a transfer of only one electron at a time, or when a covalent bond is broken and one electron from each pair remains with each atom. Thus, a free radical has an unpaired electron.
Many free radicals are highly reactive, owing to the tendency of electrons to pair; that is, to pair by the receipt of an electron from an appropriate donor or to donate an electron to an appropriate acceptor. Whenever a free radical reacts with a non-radical, a chain reaction is initiated until two free radicals react and then terminate the propagation with a 2-electron bond, with each radical contributing its single unpaired electron. The free radicals of special interest in aging are the oxygen free radicals (OH., H., O2.-). These free radicals often take an electron away from a "target" molecule to pair with their single free electron; this is what is commonly termed “oxidation.?The term "reactive oxygen species" is used to refer to these oxidants and the oxygen free radicals.
In the human body, oxidized free radicals are believed to cause tissue damage at the cellular level, causing damage to our DNA, mitochondria (the powerhouse of the cell), and cell membrane, and have often been referred to as one of the causes attributed to aging, cancer, heart disease, and other human ailments. While the production of free radicals is a normal part of metabolism at the cellular level, things such as excessive alcohol intake, smoking, and various chemical exposures only serve to increase the amount of free radicals present in the body. To prevent free radical damage the body has a defense system of antioxidants.
Antioxidants are molecules which can safely interact with free radicals and terminate the chain reaction before vital molecules are damaged (as in Vitamin C), or seek out and scavenge free radicals (as in Vitamin E). This is where it can be noted that research has indicated that one of the main antioxidants found in green tea (epigalloctechin 3-gallate a.k.a. EGCG) has been found to be much more powerful than both Vitamins C and E. Compared to other known antioxidants, EGCG was found to be 100 times more effective than vitamin C, 25 times more effective than Vitamin E and twice as powerful as resveratrol at neutralizing free radicals.