WHAT ARE NERVE AGENTS?
Nerve agents take different forms. Contrary to popular belief, they are a liquid, rather than a gas, and can seep through the skin.
They were first discovered by accident in the 1930s, when scientists were attempting to find a more cost-effective pesticide.
Russia came across such chemical agents for the first time when they swept into East Germany following World War II and took control of the plants where they were made.
HOW DO THEY WORK?
The toxins interfere with the central nervous system, causing the body to become overstimulated.
Different forms of it have evolved, including sarin, VX and tabun, all of which have very similar structures and appear to work in the same way.
"They interfere with the transmitting of nerve impulses," Dr Simon Cotton of the University of Birmingham said.
WHAT ARE THE SYMPTOMS?
Twitching, spasms, heart failure and respiratory arrest are among the more common side effects.
HOW MUCH IS NEEDED AND HOW LONG DOES IT TAKE?
Part of the allure a nerve agent holds for assassins is that only tiny amounts are required for it to take effect.
It is so toxic it would usually be transported in something tightly sealed and those who apply it would need protective clothing.
"A drop would be needed, that is all, it is that sort of scale - a tiny scale," Dr Cotton said.
HOW IS IT APPLIED?
This can vary. Doses might be turned into an aerosol can spray, for example.
In an attack on the Tokyo subway that left 12 dead in 1995, liquid sarin was placed in plastic bags that were pierced by umbrellas with sharpened tips.
When Kim Jong Nam was killed at Kuala Lumpur airport last year, a cloth doused with VX was smeared on his face.
CAN VICTIMS BE TREATED?
Antidotes do exist, including a medication called atropine, which blocks the receptor that acetylcholine - the neurotransmitter that builds up and overstimulates nerves in the presence of nerve agents - binds to.