Our body is made up of cells which require oxygen to subsist.
Processes of cellular oxygenation produce the so-called Reactive Oxygen Species (ROS) which are highly reactive molecules that oxidize other molecules they get in touch with. Certain levels of ROS are necessary to regulate several functions; however, consumed in excess damage cells and may cause death. The metal corroded by rust and the apple that gets dark shortly after it is bitten are visible examples of damage by oxidation.
In optimum conditions, the body has an antioxidant defense system which keeps balance between processes of oxidation and antioxidation. Problems appear when this balance is broken due to an excess of ROS as a result of pollutants in the environment, a reduction in the antioxidant capacity of the defensive system, or the availability of enough antioxidant substances in the body, among other reasons.
The imbalance between oxidation and antioxidation is called oxidative stress.
Oxidative stress appears in 20% to 30% of infertile couples.