The Vapor Density comparisons are only valid when the gas escaping is at precisely the same temperature as the encompassing air by itself. In case the chemical is escaping from a container where it was pressurized or refrigerated, it could to start with escape and behave like a heavy fuel https://potassium-cyanide-sds70257.blogtov.com/10096573/the-basic-principles-of-potassium-cyanide-symptoms