The Pueblo Chemical Depot is a chemical weapons storage site located in Pueblo County, Colorado, United States. The Pueblo Chemical Depot was one of the last two sites in the United States with chemical munitions and chemical materiel. The Pueblo Chemical Agent Destruction Pilot Plant (PCAPP) which is under the Program Executive Office, Assembled Chemical Weapons Alternatives program destroyed its stockpile of 155mm and 105mm artillery shells and 4.2-inch mortars, all of which contained a form of the chemical agent mustard gas.

Mustard gas or sulfur mustard is any of the several chemical compounds that contain the chemical structure S(CH2CH2Cl)2. In the wider sense, compounds with the substituent S(CH2CH2X)2 and N(CH2CH2X)3 are known as sulfur mustards and nitrogen mustards, respectively, where X = Cl or Br. Such compounds are potent alkylating agents, which can interfere with several biological processes. Also known as mustard agents, this family of compounds are infamous cytotoxins and 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. Sulfur mustards are viscous liquids at room temperature and have an odor resembling mustard plants, garlic, or horseradish, hence the name. 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 and in the lungs, often resulting in prolonged illness ending in death. The typical mustard gas is the organosulfur compound bis(2-chloroethyl) sulfide.