Sulfur Mustard and Cancer

Sulfur mustard is a vesicant. That means it blisters whatever it touches: skin, eyes, and the lining of the airways. Its CAS number is 505-60-2. Chemists call it bis(2-chloroethyl) sulfide. Most people know it as mustard gas.

The name is misleading. At room temperature it is not a gas. It is a thick, oily liquid, clear to yellow, with a sweet smell. As a vapor it looks yellow-brown. It smells of mustard, garlic, or horseradish. It melts at 13 to 14 degrees Celsius and boils near 216. It barely dissolves in water. It dissolves very well in fats, which is part of why it soaks into skin.

Lewisite is in the same broad class of blistering agents. It holds arsenic instead. Nitrogen mustard is in that family too.

The history is short and specific:

• World War I, its first use in chemical warfare.
• Ethiopia, in 1936.
• The Iran-Iraq war, from 1984 to 1988.
• Production plants, where workers made and handled it.
• Weapons testing programs run by governments on their own troops.
• Operation Iraqi Freedom. Some service members who handled or blew up explosive ordnance may have met mustard agents.

Sulfur mustard was even tested as a cancer drug. It failed, because it was far too toxic to use.

This part is often left out, and it matters for veterans and their families.

In the 1940s, the Department of Defense recruited "volunteer soldier" subjects. The goal was to test clothing, ointments, and gear. All of it was meant to protect American troops from mustard attacks. Nearly 60,000 military personnel took part, at a wide range of doses.

Most of those were mild. The common design was a patch test. A drop of agent went on the arm, to see how well an ointment worked. NTP puts the patch and drop test group at 15,000 to 60,000 soldiers and sailors.

Some were not mild at all. About 4,000 soldiers went through severe, full-body exposures. Those happened in field exercises over contaminated ground. In chamber tests, volunteers wore masks and gear inside a gas chamber. Sessions ran an hour or more, daily or every other day, until the gear failed. Failure was measured by moderate to intense chemical burns on the skin.

The scale in the earlier war was larger still. As many as 28,000 members of the American Expeditionary Forces were exposed in World War I. Levels were seldom lethal, because the agent scattered on the battlefield.

Since the early 1990s, the VA has run outreach to find these veterans. The Department of Defense identified them as test participants. The Army also provides medical care to veterans who volunteered for chemical and biological testing between 1942 and 1975. That care covers an injury or disease caused by taking part. The number for questions is 1-800-984-8523.

NTP lists mustard gas as known to be a human carcinogen. The basis is sufficient evidence in humans, not an inference from animals. It has been on the list since the first Report on Carcinogens in 1980.

Several studies found raised rates of lung and other airway cancer. Those studies covered people exposed in military use or at work. The dose pattern is there too. Among production workers, airway cancer risk was higher in people exposed for longer.

The clearest single study followed workers who made mustard gas in England in World War II. It found significant excesses of four cancers. Those were laryngeal, pharyngeal, upper-airway, and lung cancer. Our page on lung cancer covers how that cancer is found and staged.

Animal work fits the human pattern. Mustard gas caused cancer in mice of both sexes. Breathed in or injected into a vein, it caused lung tumors. Injected under the skin, it caused tumors at the injection site.

The mechanism is blunt. Mustard gas is an alkylating agent. That means it sticks chemical groups straight onto DNA. NTP reports genetic damage in every system where it was tested. In mice, it bound tightly to DNA, RNA, and protein. Tissue that takes the highest dose takes the most damage. When the agent is breathed in, that tissue is the airway lining.

The VA may grant service connection by presumption. That means a veteran does not have to prove the link case by case. The eligibility rule is specific. It requires full-body exposure during active military service.

Cancers on the presumptive list for mustard gas or lewisite exposure:

• Lung cancer, except mesothelioma.
• Laryngeal cancer.
• Nasopharyngeal cancer.
• Squamous cell carcinoma of the skin.
• Acute nonlymphocytic leukemia.

Non-cancer conditions on the same list:

• Chronic conjunctivitis, keratitis, corneal opacities, and scar formation.
• Chronic laryngitis, bronchitis, emphysema, asthma, and chronic obstructive pulmonary disease.

Conditions not on the list are decided case by case. Note the mesothelioma exclusion. That cancer is tied to asbestos, and the VA handles it under a different exposure.

NTP's profile still describes aging US stockpiles. It places them at eight Army bases, waiting for destruction. That description is now out of date. The update is worth stating plainly.

On July 7, 2023, the Organisation for the Prohibition of Chemical Weapons made an announcement. The last chemical munition in the United States' declared stockpile had been destroyed for good. It happened at the Blue Grass Chemical Agent-Destruction Pilot Plant in Kentucky. It was also the last from any declared stockpile in the treaty.

The running totals are worth knowing. The Chemical Weapons Convention took effect in 1997. Since then the OPCW has verified destruction of 72,304.34 metric tonnes. The treaty has 193 member states. Four countries have not joined.

The scale was large. By the end of World War I, US daily output reached about 18,000 kilograms. The country kept making and storing it until 1968. The total passed 34 million pounds.

Declared stockpiles are gone. But the OPCW named the loose ends itself. Old and abandoned chemical weapons still need to be found and destroyed. That is the realistic exposure route now. Buried or dumped munitions can be disturbed by building work, dredging, or farming, decades later. Small research amounts also still exist, used as a model alkylating agent in labs.

For anyone with a documented exposure, two steps are practical. One is a VA Environmental Health Coordinator evaluation. The other is attention to airway symptoms, such as hoarseness or a cough that will not settle. For how these hazard labels work in general, see our explainers on what a carcinogen is and hazard versus risk.

• National Toxicology Program, Report on Carcinogens, Fifteenth Edition: Mustard Gas, accessed August 6, 2026
• U.S. Department of Veterans Affairs, Mustard Gas — Public Health, last updated April 16, 2026, accessed August 6, 2026
• Organisation for the Prohibition of Chemical Weapons, OPCW confirms: All declared chemical weapons stockpiles verified as irreversibly destroyed, July 7, 2023, accessed August 6, 2026