That leaves UV-b, smack in the middle. The big deal is we, as people, have been stabbing ozone in the back. While oxygen atoms are happy to recombine into ozone, they're also fickle and will just as easily combine with other elements that don't make ozone. Our modern world has been sending these infiltrators into the atmosphere since the turn of the century, and ozone is losing the battle. CFS were developed in the 1920's as a safe alternative to ammonia and other nasty substances in refrigerants and spray propellants.
Chlorine is rare in the atmosphere.
The chlorine is then free to combine with those single atoms of oxygen. But that's not all - the way chlorine works in the atmosphere, the production of ozone is virtually stopped because the free oxygen atoms are constantly being attached in various ways to the chlorine molecules or are forming ordinary oxygen molecules. They're stopped from joining with oxygen to form ozone. You can add it up. With more CFCs in the atmosphere comes more chlorine.