A free radical is a molecule with an unpaired electron (Afzal & Armstrong, 2002). The molecule is reactive and seeks another electron to pair. This initiates an uncontrolled chain reaction that can damage the natural function of the living cell, causing various diseases.  Free radicals are involved in the mechanisms of damage associated with disease development (Rice-Evans, 1999). Examples are hydrogen peroxide, hypochlorite, nitric oxide and nitrogen dioxide. 
The need for antioxidants in our daily life is becoming more critical with increased exposure to free radicals (Percival, 1998). Pollution, cigarette smoke, drugs, illness and stress can increase our exposure.
Percival, M. (1998). “Antioxidants”. Clinical Nutrition Insights 1/96 Rev. 10/98. http://acudoc.com/Antioxidants.PDF
- Rice-Evans, C. (1999). “Screening of phenolics and flavonoids for antioxidant activity”. In: Antioxidant Food Supplements in Human Health. Academic Press, p. 239–253.
- Afzal, M., Armstrong, D. (2002). “Fractionation of herbal medicine for identifying antioxidant activity”. In: Armstrong, D. (Ed.) Methods in Molecular Biology, vol. 186: Oxidative Stress Biomarkers and Antioxidant Protocols, Humana Press Inc.