Top on the list of the dangers about which HS automechanics students are warned is that when R-12 passes through a flame it becomes phosgene or mustard gas (one of those two WWI poison gases -- I can't remember whether it's the one the British started it all with, or the one with which the Germans responded). People have suffered severe and, in a few cases, fatal lung damage that way -- even by just smoking a cigarette in the presence of a refrigerant leak.

(The other two refrigerant risks are frostbite from being sprayed and the fact that the stuff is heavier than air and will pool on the floor of a garage, causing someone working under a car to suffocate from lack of oxygen.