Overwhelming evidence indicates that oxidative stress can lead to cell and tissue injury. However, the same free radicals that are generated during oxidative stress are produced during normal metabolism and thus are involved in both human health and disease.
Free radicals are molecules with an odd number of electrons. The odd, or unpaired, electron is highly reactive as it seeks to pair with another free electron.
Free radicals are generated during oxidative metabolism and energy production in the body.

Free radicals are involved in:
Enzyme-catalyzed reactions
Electron transport in mitochondria
Signal transduction and gene expression
Activation of nuclear transcription factors
Oxidative damage to molecules, cells, and tissues
Antimicrobial action of neutrophils and macrophages
Aging and disease