Poison gas is any gas that is also a poison. Poison gases can kill or injure a person if present in a high enough concentration. There is a diverse range of different poison gases and each has unique properties. Many toxic liquids are also volatile 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 viscous liquids that are dispersed into fine mists.