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Conversely, a very small percentage of oxygen interactions may create an unstable reactive molecule. That instability occurs when a molecule is left with an unpaired electron—known as a “free radical”—during the electron exchange with oxygen. These unstable free radicals steal electrons from the body's healthy molecules in order to balance their electrons and stabilize themselves. These interactions are also called oxidation, but this kind of oxidation is not a balanced, simultaneous exchange of electrons that creates a stable molecule. Instead, the unstable free radical may start a chain reaction, because the molecule from which it stole the electron becomes a new free radical. That new free radical steals an electron from another molecule, and so the chain reaction proceeds. Unfortunately, during the time the molecules are free radicals searching for an electron with which to pair, they cause a great deal of damage to different parts of one or many cells.

Especially hard hit are the cell's lipid membranes. This damage is dangerous because the cell's membrane is its barrier to the extracellular environment. If the oxidation happens inside the cell, its DNA, RNA, lipids, and proteins may also be damaged, having enormous implications for the everyday functioning of the cell. The body does have a defense against these free radicals, and they are antioxidants. Antioxidants give the free radicals the electrons they need in order to stop the radical oxidants from causing more damage and generating more free radicals. “Oxidative stress” is defined as the unbalance between free radical oxidants and antioxidants, favoring the oxidant to the point that exceeds the body's ability to defend itself against such damage. Oxidative stress promotes a host of chronic diseases. Nutrition has a major role to play in oxidative stress, as well.