Blister agents, or vesicants, are one of the most common CW agents. These oily substances act via inhalation and contact with skin. They affect the eyes, respiratory tract, and skin, first as an irritant and then as a cell poison. As the name suggests, blister agents cause large and often life-threatening skin blisters which resemble severe burns.

Examples include:

• sulfur mustard (H, HD)
• nitrogen mustard (HN)
• lewisite (L) and phosgene oxime (CX).

Mustard agents and lewisite are the best known.

Blister agents were first tested in combat in 1917 by Germany and have been used in several conflicts since, notably in the Iran-Iraq War (1980–88). They are primarily dispersed in liquid or vapour (aerosol) form and may persist for days. Like phosgene, mustard agents have a delayed effect. Deaths typically only represent a small percentage of the casualties they cause.

Exposure to blister agents often results in blindness and permanent damage to the respiratory system.