Imagine trying desperately to catch your breath, and no matter how hard you try, you just can't manage it. While this is happening, you may experience spastic twitching, drooling, and your head is probably pounding unlike any other headache you've ever had, you're vomiting and maybe even defecating, and all of this is totally out of your control.
That's what nerve toxins like Sarin do.
Journalists at ABC talked to a doctor about the effects:
[Nerve toxins] break down an enzyme that allows nerves to talk to each other, so victims become over-stimulated. In addition to other symptoms, there's chest tightening, rapid breathing, vomiting, diarrhea, confusion, headache, changes in heart rate, loss of consciousness, convulsions and paralysis. Eventually, and depending on dose levels it could only be a matter of minutes, you do die of respiratory failure, Adalja said.
Though the victims look peaceful and generally intact in death, don't be deceived, the painful, terrifying symptoms from the exposure to chemical weapons set in almost instantly, often with deadly results.
Here's a more bookish definition from the Federation of American Scientists:
After exposure to small to moderate amounts of vapor, there are usually miosis and conjunctival injection, rhinorrhea, and pulmonary signs, although the latter may be absent even in the face of mild to moderate pulmonary complaints. In addition to these signs, an exposure to a high Ct may precipitate copious secretions from the nose and mouth, generalized muscular fasciculations, twitching or seizure activity, loss of consciousness, and apnea. Cyanosis, hypotension, and bradycardia may be present just before death.
"Conjunctival injection" and "rhinorrhea" basically mean sudden onset of fluids running out of your mouth, nose and eyes. Pulmonary signs mean heart arrhythmia (lack of regular rhythm) and tightness in the chest.
Add that to "fasciculations, twitching, or seizure activity" and the victim's sheer terror must be at an all time high.
Cyanosis is like when someone's lips turn blue, except in this case it happens around all the mucous membranes because of a lack of oxygen. That's right, your cardio pulmonary system starts to malfunction, eventually leaving victims unable to breath and/or pump blood.
Bradycardia basically means an incredibly slow heart rate, which goes against what would normally be associated with high amounts of stress and fear — of course because it's chemically induced.
Victims usually expire as a result of the heart stopping.
Death can occur in as quickly as a few minutes, but victims can linger for upward of 30 minutes to 18 hours.