Ammonia fertilizer tanks explode every year. It's not a flammable as propane, and it works well for refrigeration, but it's also more toxic and corrosive...

I suppose they couldn't find a better way of describing it, but ammonia does have a pretty unique smell. The good thing about ammonia is that its smell is repugnant and detectable at concentrations far below when it starts becoming toxic that you'll be given plenty of warning.