Mustard gas or sulfur mustard is any of several chemical compounds that contain the chemical structure SCH2CH2Cl. In the wider sense, compounds with the substituent are known as sulfur mustards and nitrogen mustards (X = Cl, Br), respectively. Such compounds are potent alkylating agents, which can interfere with several biological processes. Also known as mustard agents, this family of compounds are infamous cytotoxic and blister agents. The name mustard gas is technically incorrect: the substances, when dispersed, are often not gases but a fine mist of liquid droplets. Mustard gases form blisters on exposed skin and in the lungs, often resulting in prolonged illness ending in death. The typical mustard gas is the organosulfur compound called bis(2-chloroethyl) sulfide. History as chemical weapons Sulfur mustard is a type of chemical warfare agent. As a chemical weapon, mustard gas was first used in World War I, and has been used in several armed conflicts since then, including the Iran–Iraq War, resulting in more than 100,000 casualties. Today, sulfur-based and 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 (though since then, mustard gas has been found to be useful in cancer chemotherapy). Mustard agents could be deployed by means of artillery shells, aerial bombs, rockets, or by spraying from aircraft. Mechanism of cellular toxicity Sulfur mustards readily eliminate chloride ions by intramolecular nucleophilic substitution to form cyclic sulfonium ions. These very reactive intermediates tend to permanently alkylate nucleotides in DNA strands, which can prevent cellular division, leading to programmed cell death. Alternatively, if cell death is not immediate, the damaged DNA can lead to the development of cancer. Oxidative stress would be another pathology involved in mustard gas toxicity. In the wider sense, compounds with the structural element BC2H4X, where X is any leaving group and B is a Lewis base, are known as mustards. Such compounds can form cyclic "onium" ions (sulfonium, ammonium, etc.) that are good alkylating agents. Other such compounds are bis(2-haloethyl)ethers (oxygen mustards), the (2-haloethyl)amines (nitrogen mustards), and sesquimustard, which has two α-chloroethyl thioether groups (ClC2H4S−) connected by an ethylene bridge (−C2H4−). These compounds have a similar ability to alkylate DNA, but their physical properties vary. Physiological effects Mustard gases react with DNA, which interferes with cellular division and can lead to mutations. Mustard gases are extremely toxic and have powerful blistering effects