Only very small amounts of Sarin, a single milligram coming in contact with the skin is sufficient to kill. In a vaporous form, it takes a concentration of 100 milligrams per cubic meter to be fatal. Their chemical method of killing is to block the enzyme cholinesterase. The body's muscles receive electrical impulses caused by choline. Cholinesterase break down choline, making sure these impulses stop at the proper time.
Cholinesterase attaches itself to choline and breaks it down, thus halting the impulse.
Sarin fools the cholinesterase into acting upon the Sarin as it would choline. When the cholinesterase attaches itself to Sarin, it doesn't break down. Thus, choline is not broken down, and the body goes into convulsions. The first symptoms start in the eyes, where the pupils contract and vision is blurred. It causes breathing problems and chest tightness.