I have my Universal HVAC.
And pure ammonia is hard to detect, has no smell, and in a confined "enough" space can kill a person in under 3 minutes... they simply "pass out" and are dead.
Luckily synthetics are taking over and are HCFC based, so not nearly as bad for you or the environment.
Hell there even is a direct retrofit for old R-12 systems that eliminates the need to vacuum and convert to R-134a.. its just a bit more expensive for the time being.