The cells in our body,turn food and oxygen into energy and carbon dioxide by a process called metabolism.Free radical is a by-product of metabolism.It is also produced by the sun's ultraviolet rays.Smokers inhale a great deal of free radicals produced by tobacco as it burns.Other causes of Free Radical formation are:-air pollution,chemicals,radiation,water quality,stress,emotion and mental attitude
It is a molecule with an extra electron.In this unstable state it is highly reactive,seeking other molecules to bind.

These unpaired molecules are dangerous and can have damaging effects on our body tissues.When an oxygen free radical binds it is called oxidation.Anything which prevents such a binding is called antioxidants

Examples of free radicals:-
1.singlet oxygen
2.superoxide
3.hydroxyl
4.hydrogen peroxide

Some of the damaging effects of Free Radicals are:-

It can 'cross link'or hook together cell structures,leading to hardening of blood vessels.
It oxidises LDL-cholesterol,enabling it to stick to the arterial walls and form plaques.
It can alter the genetic material(DNA) of the cell,causing cellular dysfunction and even cancer!
All these damaging effects of the Free Radicals can be checked by antioxidants!

Some of the diseases thought to be caused by free radicals include:-
coronary artery disease
cancer
rheumatoid arthritis
cataract