Mustard gas is a vesicant chemical weapon that was outlawed in 1925 due to the grizzly nature of its effects. What is a vesicant? It’s a chemical that causes severe blisters to form as a result of severe chemical burns.
How does it do this? The compound is readily absorbed into your cells where it attaches to the guanine nucleotide in your DNA. Once it binds to the DNA it doesn’t ever come off, rendering the DNA in that cell unusable and triggering cell death.
What does this look like? People exposed to mustard gas get massive, fluid filled blisters wherever they were exposed. Because the gas can diffuse through cotton and wool it could attack any part of your body. Countless blisters all over the body left victims wide open to bacterial infection, and many died from the sepsis alone. More importantly, if it came in contact with someones eyes the resulting blisters usually resulted in permanent blindness. But it doesn’t stop there: if the gas was inhaled it would result in tissue damage to your lungs, causing them to fill up with blood and fluid. That’s how most people exposed to the stuff die, drowning in their own fluids.
Pure mustard gas is colourless and odourless, but when its produced on a large scale like it was during World War 1 the contaminants gave it a yellow-brown colour (which is why it was called mustard gas), and a smell like garlic. The effects of the gas weren’t immediate. Most victims don’t begin showing skin irritation until hours after exposure. They found out that you could neutralize the gas with bleach, but by the time the victim felt the initial symptoms it was too late.
Given the volatile effects on living organisms, and the difficulty of controlling a cloud of gas the world decided to ban the use of mustard gas in 1925. Diplomats didn’t need to have a chemical or medical understanding of its mechanisms to condemn its use. They could see the wounded returning home from war, testifying first hand to its terror.
That didn’t keep countries from using it however, military use has occurred as recently as the late 1990’s, and many countries continue to maintain stockpiles of it (including the US)
Source: Compound Fractur