Mustard gas is made primarily from the chemical compound bis(2-chloroethyl) sulfide, known as sulfur mustard. It is a viscous, oily liquid that emits an odor resembling mustard plants, garlic, or horseradish. It is produced chemically by reacting substances such as dry ethylene with disulfur dichloride in controlled conditions. The compound is not actually a gas under normal conditions but forms a vapor or fine mist that is highly toxic and causes blistering on contact with skin or mucous membranes. Mustard gas also includes related nitrogen mustard compounds that share similar alkylating and toxic properties. The molecular structure of sulfur mustard allows it to alkylate DNA, disrupting cellular function and leading to cell death, which underlies its toxicity and use as a chemical weapon.

In short, mustard gas is chemically synthesized from ethylene and sulfur-based chemicals to produce bis(2-chloroethyl) sulfide, a potent blistering and alkylating agent with devastating effects on human tissue and DNA.