Poison gas
Poison gas is any gas that is also a poison. Poison gases can kill or hurt a person if there is lots of it. There is a wide range of different poison gases and each has own properties. Many toxic liquids are also evaporate easily at normal temperatures and their vapors are poison gas. "Poison gas" can refer to poisons in chemical weapons, but most of them are in fact liquids, for example mustard gas and VX are sticky liquids that are spread out into fine mists.