How are Chlorofluorocarbons dangerous?

Chlorofluorocarbons, or CFC's, get into the upper atmosphere and wreak havoc with the ozone later. High in our atmosphere, oxygen (O2) gets bombarded by solar radiation to produce ozone (O3). This is fine, because ozone has the unique property of being able to block a lot of this harmful solar radiation from reaching the earth's surface, where it would cause crop failure and famine. When CFC's get up there, the solar radiation breaks the chlorine away from the CFC. The chlorine acts as a catalyst, speeding up the conversion of ozone back into diatomic oxygen (O2). You don't want this, because O2 doesn't block solar radiation like O3 can. This results in much more solar radiation reaching the surface, which is bad! Incidentally, this effect was first observed on the planet Venus. It occured to astronomers that it might actually be happening here too, so we took a close look, and sure enough it was. Yet another life-saving benefit of our space program!