Human being survives only in presence of oxygen. During the utilization of this life supporting element, oxygen we generate by-products as ‘reactive oxygen species’ (ROS), commonly known as free radicals. These free radicals are produced as the part of the body's normal stress response. But when these reactive species formed in larger amount and with chronic exposure, it can damage healthy cells, especially by attacking the membrane fats resulting degenerative cellular function. Like rusting of iron takes place due to long exposure of oxygen, our body cells also get degenerated due to long exposure to these free radicals, resulting chronic non communicable diseases. Excessive production of reactive oxygen species is reflected by increased peroxidation of lipid and DNA, leading to oxidative stress.
Free radicals are also produced from exposure to cigarette smoke, excess exposure to the sun, drinking alcohol, from exposure to large amounts of heavy metals and during any inflammatory response  and become the cause for many lifestyle related non-communicable diseases. The reactive oxygen species (ROS) has now been identified to play a fundamental role in the pathogenesis of cellular function including endothelial dysfunction and atherosclerosis  or glucose auto- oxidation, monocyte dysfunction and nonenzymatic glycation etc .