Mustard gas or sulfur mustard are names commonly used for the organosulfur chemical compound bis(2-chloroethyl) sulfide, which has the chemical structure S(CH2CH2Cl)2, as well as other species. In the wider sense, compounds with the substituents −SCH2CH2X or −N(CH2CH2X)2 are known as sulfur mustards or nitrogen mustards, respectively, where X = Cl or Br. Such compounds are potent alkylating agents, making mustard gas acutely & severely toxic. Mustard gas is a carcinogen. There is no preventative agent against mustard gas, with protection depending entirely on skin & airways protection, & no antidote exists for mustard poisoning.Also known as mustard agents, this family of compounds comprises infamous cytotoxins & blister agents with a long history of use as chemical weapons. The name mustard gas is technically incorrect; the substances, when dispersed, are often not gases but a fine mist of liquid droplets that can be readily absorbed through the skin & by inhalation. The skin can be affected by contact with either the liquid or vapor. The rate of penetration into skin is proportional to dose, temperature & humidity.Sulfur mustards are viscous liquids at room temperature & have an odor resembling mustard plants, garlic, or horseradish. When pure, they are colorless, but when used in impure forms, such as in warfare, they are usually yellow-brown. Mustard gases form blisters on exposed skin & in the lungs, often resulting in prolonged illness ending in death.History as chemical weaponsSulfur mustard is a type of chemical warfare agent. As a chemical weapon, mustard gas was first used in WW1, & has been used in several armed conflicts since then, including the Iran–Iraq War, resulting in more than 100k casualties. Sulfur-based & nitrogen-based mustard agents are regulated under Schedule 1 of the 1993 Chemical Weapons Convention, as substances with few uses other than in chemical warfare. Mustard agents can be deployed by means of artillery shells, aerial bombs, rockets, or by spraying from aircraft.Mustard gases have powerful blistering effects on victims. Their high lipophilicity accelerates their absorption into the body. Because mustard agents often do not elicit immediate symptoms, contaminated areas may appear normal. Within 24 hours of exposure, victims experience intense itching & skin irritation. If this irritation goes untreated, blisters filled with pus can form wherever the agent contacted the skin. As chemical burns, these are severely debilitating.If the victim's eyes were exposed, then they become sore, starting with conjunctivitis (also known as pink eye), after which the eyelids swell, resulting in temporary blindness. Extreme ocular exposure to mustard gas vapors may result in corneal ulceration, anterior chamber scarring, & neovascularization. In these severe & infrequent cases, corneal transplantation has been used as a treatment option. Miosis, when the pupil constricts more than usual, may also occur, which may be the result of the cholinomimetic activity of mustard. If inhaled in high concentrations, mustard agents cause bleeding & blistering within the respiratory system, damaging mucous membranes & causing pulmonary edema. Depending on the level of contamination, mustard agent burns can vary between first & second degree burns. They can also be as severe, disfiguring, & dangerous as third degree burns. Some 80% of sulfur mustard in contact with the skin evaporates, while 10% stays in the skin & 10% is absorbed & circulated in the blood.The carcinogenic & mutagenic effects of exposure to mustard gas increase the risk of developing cancer later in life. In a study of patients 25 years after wartime exposure to chemical weaponry, c-DNA microarray profiling indicated that 122 genes were significantly mutated in the lungs & airways of mustard gas victims. Those genes all correspond to functions commonly affected by mustard gas exposure, including apoptosis, inflammation, & stress responses.