I to have worked in HVAC for many years, chillers to window units, even did a morgue for body storage once.
A freeze drier application was interesting, that was some very exoctic and expensive gas for that unit! I never worked with ammonia, now THAT stuff is dangerous.
Freon, nitrogen, other gases, just part of the hazard. I am reminded of a news story about a fellow that filled his air mattress with freon, he never woke up...