A small part of the oxygen you breath in is converted to very active compounds called free radicals. Some of these are so active they damage tissues and genetic material and may lead to numerous diseases, including heart attack and cancer. The answer is not clear. Some studies report an increase in free radicals during and shortly after exercise. These increases don't seem to be permanent, and resting levels can be lower in folks who work out regularly.
Exercise can also cause an increase in enzymes that defend against free radical reactions.
Consequently, even researchers in the field are tentative in assessing whether exercise really presents oxidative stress problems. The standard technique for measuring free radicals is called electron spin resonance. But this method cannot yet be used on live animals or people. Instead, researchers analyze compounds produced by free radical reactions. For example, measurement of a hydrocarbon called pentane in exhaled breath is a favorite method.