Mustard gas is a chemical warfare agent that was first used during World War I. It is a yellowish-brown liquid at room temperature and can form a vapor that is harmful when inhaled or absorbed through the skin. Mustard gas causes severe blistering and damage to the respiratory system, skin, and eyes. The chemical name for mustard gas is sulfur mustard, and it belongs to a class of compounds known as blister agents. Its use is banned under the Chemical Weapons Convention, which prohibits the development, production, and stockpiling of chemical weapons.