If exercise generates free radicals and free radicals are bad, why is exercise good?

Free radicals, forms of oxygen that are missing an electron, are actually not all bad.

Although they can destroy cell membranes and DNA, scientists have also discovered that they are also “good,” said Dr. John McDonald, a neurologist at the Kennedy Krieger Institute and Johns Hopkins. For instance, they help with cell signaling, the process of chemical communication that tells a cell, among other things, when to activate certain genes.