From Longman Dictionary of Contemporary Englishpoison gasˌpoison ˈgas noun [uncountable] PMWgas that causes death or serious injury, used especially against an enemy in a war There are reports that poison gas is being used against the rebels.
Examples from the Corpus
poison gas• A heavy overcast like a poison gas cloud was stationary over the battered city.• Whether it's against poison gas or pollution or radiation he can't say.• Bombs and poison gas would rain down on undefended cities.• At the time, however, I feared poison gas.• It's a land where doom hangs upon the air like poison gas.• There was fear of poison gas, in a nation where Zyklon-B was part of the collective memory.• If this emission takes place it triggers the breaking of a vial of poison gas which instantaneously kills the cat. 