Quote Originally Posted by kothari
Almost everyone has smelled the sharp, penetrating odor of ammonia, NH3. A more than a sniff of this toxic, reactive, and corrosive gas can make one very ill indeed. It can, in fact, be fatal. Ammonia is pretty nasty stuff. Nevertheless, it is also an extremely important bulk chemical widely used in fertilizers, plastics, and explosives and also as a refrigerent.
Indeed, one of the nice things about ammonia is the fact that anyone with a sense of smell has a built-in leak detector. This is not the case for HFC, CFC, etc., which lack any sort of strong odor.

Yes, a leak on a large ammonia system in an enclosed space can be fatal to those working on it, but read the Material Safety Data Sheet for all the other refrigerants - under the same or similar conditions just about any of them can be fatal too. At least with ammonia there is a warning.