It wasn't actually a gas.
That’s usually the first thing that trips people up when they ask what was in mustard gas. If you were standing on a battlefield in Flanders in 1917, you wouldn't be looking for a floating cloud of vapor like you see in the movies. You’d be looking for a fine, oily mist. This stuff behaved more like a heavy spray that coated everything it touched—the mud, the wooden supports of the trenches, and, most horrifcally, the skin of the soldiers.
It smelled like horseradish. Or garlic. Sometimes onions.
Technically, we’re talking about bis(2-chloroethyl) sulfide. It’s a blistering agent, or a vesicant. While the "mustard" name comes from that distinct, pungent odor and the yellow-brown hue of the industrial-grade version, the chemical reality is much more clinical and much more devastating than a kitchen spice.
The chemistry behind the sting
So, what exactly made up this compound?
At its core, sulfur mustard is an organic compound. The chemical formula is $(ClCH_2CH_2)_2S$. It’s basically a sulfur atom sitting in the middle, flanked by two ethyl groups that have been "chlorinated." When you put it all together, you get a liquid that is relatively stable at room temperature but evaporates just enough to become a lethal inhalation hazard.
It’s heavy.
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Because it’s denser than air, it didn't just blow away. It sank. It crept into the lowest parts of the trenches and stayed there for days, sometimes weeks. If the weather was cold, it could even freeze, lying dormant in the soil like a chemical landmine until the morning sun warmed it back up into a toxic vapor.
The synthesis process during the Great War usually involved the Levinstein Process. This was the "quick and dirty" way to make it. Chemists would react ethylene with sulfur dichloride. The result wasn't pure; it contained various sulfur impurities, which is actually why the wartime stuff looked dark and smelled so foul. Pure sulfur mustard is actually colorless and almost odorless. Imagine how much more terrifying that would have been—a silent, invisible killer that you couldn't even smell coming.
Why it didn't kill you right away
Mustard gas is a "delayed" weapon.
Unlike chlorine or phosgene, which make you choke and gasp for air immediately, sulfur mustard is a sneaky bastard. You could be exposed to a lethal dose and not feel a single thing for two to twenty-four hours. This was a psychological nightmare for soldiers. You’d finish your shift, eat your rations, maybe even try to sleep, and then—slowly—your eyes would start to burn. Your skin would turn red.
Then came the blisters.
These weren't just small heat rashes. We’re talking about massive, fluid-filled sacs that could be the size of a grapefruit. Because the chemical is "lipophilic" (it loves fats), it soaks right through clothing and into the fatty layers of your skin. Once it’s in, it starts a process called alkylation. It literally attacks your DNA. It breaks the chains of your genetic code, which causes the cells to liquefy and die.
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It wasn't just about the skin
While the blisters are the most famous symptom, the real "what was in mustard gas" story involves the internal damage. If you breathed it in, the same blistering happened to the lining of your bronchial tubes. The tissue would slough off, essentially causing the victim to drown in their own fluids or die later from massive secondary infections.
According to records from the Imperial War Museum, the mortality rate for mustard gas was actually quite low—only about 2% to 3%. But that’s a deceptive stat. The goal wasn't necessarily to kill; it was to incapacitate. A soldier with his eyes swollen shut and 50% of his body covered in chemical burns requires a massive amount of medical resources. It took four men to carry one gassed soldier. It clogged the field hospitals. It broke the logistics of the opposing army.
The dark irony of chemotherapy
It sounds insane, but the same stuff that blinded men in the trenches actually gave us the first real breakthrough in treating cancer.
During World War II, a ship carrying mustard gas was bombed in the port of Bari, Italy. The Allied soldiers who survived the explosion showed a strange side effect: their white blood cell counts had plummeted. Doctors Louis Goodman and Alfred Gilman at Yale looked at this and had a "lightbulb" moment. If mustard gas could kill white blood cells, could it be used to kill the rapidly dividing cells of leukemia or lymphoma?
They tested a derivative called Mustine (nitrogen mustard). It worked. It was the birth of modern chemotherapy. We took a weapon of mass destruction and tweaked its molecular structure to create a weapon against cancer. Life is weird like that.
What most people get wrong about "gas"
People often conflate all WWI gases into one big category. They aren't the same.
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- Chlorine: Greenish-yellow. Smells like bleach. Forms hydrochloric acid in your lungs.
- Phosgene: Smells like musty hay. Much more lethal than chlorine. It’s a "choking agent."
- Mustard Gas: The persistent one. The oily one. The one that stayed on your boots and burned you when you took them off three hours later.
The persistence is the key. You could fire a phosgene shell and, once the wind shifted, you could charge the enemy trench. If you fired sulfur mustard, you couldn't occupy that trench for days without poisoning your own troops. It changed the very nature of how territory was held and contested.
The lingering legacy in our oceans
You might think this is all ancient history, but the ingredients of mustard gas are still very much with us. After both World Wars, the easiest way to get rid of leftover chemical stockpiles was to just dump them in the ocean.
Thousands of tons of mustard gas shells were dumped into the Baltic Sea and off the coast of the United States. Today, those metal casings are rusting through. Every few years, a fisherman in New Jersey or Denmark pulls up a "strange-looking rock" in their net, only to end up in the emergency room with chemical burns. The sulfur mustard inside has thickened into a jelly-like substance that remains potent even after 80 years underwater.
How to handle the history (and the risk)
If you are a history buff or a "magnet fisher," you need to be aware that this stuff is still out there. It’s not just in textbooks.
- Identify the danger: If you find old unexploded ordnance (UXO), do not touch it. Mustard gas shells often have specific markings, but after decades of rust, those are gone.
- Understand the symptoms: Delayed redness and itching that turns into blistering is the hallmark of sulfur mustard exposure.
- Decontamination: In a historical exposure scenario, the only way to stop the damage was immediate washing with a bleach solution or specialized decontaminants like M258 kits, though for the average person today, flushing with massive amounts of water and soap is the only immediate option before hitting the ER.
The reality of what was in mustard gas is a mixture of brilliant chemistry and absolute moral depravity. It was a weapon designed to turn a man’s own skin against him. It remains one of the most persistent environmental hazards left over from the 20th century, proving that once you let the "yellow cross" out of the bottle, it’s almost impossible to put it back in.
If you're researching this for a project or family history, look into the Chemical Weapons Convention (CWC) records. They provide the most detailed modern breakdowns of how these stockpiles are being neutralized today. It’s a slow, expensive process involving high-heat incineration or chemical neutralization, ensuring that these molecules never get the chance to react with human DNA again.
Next time you see a photo of a WWI soldier in a mask, remember it wasn't just about breathing. They were fighting a liquid that wanted to soak into their very pores. That is the true, oily legacy of sulfur mustard.