What is VX, when has the nerve agent been used and how is it different to Thallium, sarin gas and ricin?
VX gas is a terrifying nerve agent developed specifically to be used in chemical warfare.
Here's everything you need to know about the "extremely toxic" compound which was allegedly used in the assassination of Kim Jong-un's half-brother.
What is VX?
Nerve agents or nerve gasses are chemical weapons which attack a person's nervous system and prevent their body from functioning properly.
VX - full name methylphosphonothioic acid - was first discovered by two chemists in the 1950s while they were working for the British company ICI.
It was patented by Ranajit Ghosh and J.F. Newman in 1952 but once it was discovered how lethal to humans VX was all, commercial research on the agent stopped.
The nerve agent is a class of organophosphate compounds which attacks its victims by making it impossible for muscles to relax.
This causes victims to spasm uncontrollably and involuntarily shut their windpipe, causing them to suffocate.
As little as 10mg of VX, outlawed by the 1997 Chemical Weapons Convention, can kill a human in just a few minutes.
Has VX been used as a weapon before?
Several countries including Russia, North Korea and Syria are known to possess stockpiles of VX.
It was widely suspected Iraqi dictator Saddam Hussein used the nerve gas against Kurds in 1988 though no physical evidence was ever found.
In December 1994 and January 1995, a Japanese man who was part of a doomsday cult, created around 200 grams of VX at home and used it to attack three victims on its underground.
And in February 2017, Kim Jong-nam, the half-brother of North Korean ruler Kim Jong-un, died after being poisoned with VX gas at Kuala Lumpur International Airport in Malaysia.
The UK was rocked in March 2018 after former Russian spy Sergei Skripal and his daughter Yulia were poisoned in Salisbury, Wiltshire.
Counter-terror police believe the pair were "targetted specifically" when they were dosed with an unidentified nerve agent.
What is sarin gas and how is it different to VX?
Sarin is a colourless and odourless agent that was outlawed as of April 1997 by the Chemical Weapons Convention.
In its purest form, sarin is estimated to be 26 times deadlier than cyanide.
Symptoms following exposure to sarin include a runny nose, tightness in the chest and constriction of the pupils.
Victims then continue to lose control of bodily functions and begin drooling, urinating, vomiting and defecating.
Muscle spasms then make breathing incredibly difficult before the victim becomes comatose and suffocates.
Antidotes like atropine and pralidoxime can help stop the deadly muscle convulsions if administered quickly after exposure.
What is ricin?
Ricin is a highly toxic lectin - a protein chain which bonds with carbon - occurring naturally in the seeds of a castor oil plant.
When purified into a powder, the equivalent of a few grains of table salt would be enough to kill a grown human.
It can also be toxic if dust contacts the eyes or if it is absorbed through damaged skin, fatally affecting the body by cutting off its ability to make protein.
It takes between two and five days for the substance to affect the central nervous system, kidneys and liver.
Ingestion leads to severe nausea, vomiting, diarrhea, difficulty swallowing and haemorrhaging which causes bloody feces (melena) and vomiting blood (hematemesis).
Victims can also suffer shock and organ failure which shows up as disorientation, stupor, weakness, drowsiness, excessive thirst, low urine production and bloody urine (hematuria).
What is thallium?
Known as the "poisoner’s poison", Thallium is colourless, odourless and tasteless.
It is easy to deliver, slipped into food or drink in liquid form, or spilt on skin.
Thallium is a metal element widely used in electronics.
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It was once dubbed “Inheritance Powder” due to the ease with which poisoners used it to bump off relatives.
Its wide-ranging symptoms are often suggestive of other illnesses, and it also has the added bonus of being very hard to trace – meaning Porton Down experts may not know for sure what was administered to the colonel for weeks, or possibly ever.
It is as slow-acting and painful.