But if you are going to have a heart attack, you are more likely to have it during or shortly after working out. However, this doesn't change the fact that moderate and vigorous exercise reduces risk of heart disease in healthy folks. 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.