After the atom bomb, nerve agents are possibly some of the most dangerous things that humans have ever made.

They represent a group of chemicals that all generally work in the same way, by inhibiting an enzyme called ‘acetylcholinesterase’ (ACHase) at nerve junctions, or ‘synapses’. Under normal circumstances, ACHase controls the amount of acetylcholine (ACH) – a chemical signal or ‘neurotransmitter’- that is crossing nerve synapses, by breaking it down. So it effectively works as an ‘off-switch’. ACH normally has a vital role in the body’s autonomic nervous system — the body’s involuntary nervous system — which controls things such as heart rate, respiratory rate, salivation, digestion, pupil dilation, and urination.

Without one of the major ‘off-switches’ of the body, all the lights are turned ‘on’ all of the time, and the body will very rapidly run into trouble. With an extremely rapid build up of acetylcholine in the synapse, things like secretions, respiratory problems, and muscular dysfunction can go on unattenuated.
 
By effectively killing one of the body’s major off-switches, it also makes the diagnosis of nerve agent exposure relatively straightforward. If the levels of ACHase are very low in the blood of a possible victim, a nerve agent exposure is likely. Identifying the actual agent can be more difficult- but in this particular case, the event occurred practically on the front door of Porton Down, the UK Government’s top secret analytical laboratory, which has vast experience in this area (they identified the agents used by the FSB to finish the Moscow Theatre siege in 2002).

Commonly-used chemicals of this type are available to civilians in the form of organophosphate pesticides – we use them in agriculture, but obviously these are designed to attack the nervous system of agricultural pests, not people. Human exposure to these is incredibly dangerous, and kills thousands of people every year. The weaponized versions, developed by the Germans but never deployed, in the Second World War included agents like tabun. Another nerve agent, Sarin, was deployed in dilute form in the Tokyo gas attacks, and VX on Kim Jong Nam in Malaysia. That last example showed that with some ingenuity, versions of these chemicals can be used as personalised weapons and not just weapons of mass destruction.
 
The fact that a nerve agent, presumably military in origin, (given its apparent potency) has been used against a person who has been found guilty of treason against a country with an apparent track record of poisoning it’s opponents in exile leaves very little to the imagination as to who the perpetrators might be. The former Soviet Union, and now Russia, has extensive expertise, and probable experience, in the use of exotic poisons. From Ukrainian presidential candidate Viktor Yushchenko in 2004, who was found to have the second highest levels ever recorded of 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) in his blood stream, to the dissident Alexandr Litvinenko, who died in horrible circumstances in 2006 from an exposure to polonium-210, outspoken opponents to Vladimir Putin’s Russia seem to have a curiously high rate of exposure to some of the most exotic, lethal agents ever known.