When my wife worked reviewing WHMIS sheets, and combing the clinical and research literature for the latest info on hazardous chemicals, she would read about workplace incidents like this on a regular basis. If it wasn't ammona, it was something else where appropriate precautions were not taken.
Thankfully, she now works in food safety, where the worst things she has to read about are how many mice or rats died when exposed to something.
But the Fernie thing is a damn shame. People trying to maintain a rink so kids can skate and play hockey shouldn't have to die for that sort of thing.