UK police announced they believe a nerve agent was used to deliberately poison a former Russian double agent and his daughter.
Nerve agents have been used in chemical warfare since WWII and have been linked to a number of high-profile assassinations, including in the case of North Korean ruler Kim Jong-un's exiled half brother last year, who died 20 minutes after being given a highly toxic VX nerve agent.
Here's the effect they have on the body and whether it's possible to recover from a nerve agent attack.
What are nerve agents?
Nerve agents are highly poisonous chemicals that work by disrupting and preventing the nervous system from working properly, according to the US Centre for Disease Control.
They do this by inhibiting the enzyme which orders the muscle or organ to relax, meaning the muscle is repeatedly receiving a signal to contract.
While they are commonly referred to as nerve gases, in pure form they are colourless — and mostly odourless — liquids.
The Organisation for the Prohibition of Chemical Weapons says they are easily dispersed, highly toxic and have rapid effects when absorbed through the skin or through the respiratory system.
G series agents are known as non-persistent (degrade easily), while V series agents are persistent, meaning they do not degrade or wash away easily and are considered among the most toxic substances ever synthesised.
There are four main types of nerve agents, including G and V series:
- Sarin (GB),
- Soman (GD)
- Tabun (GA)
How are they administered?
Nerve agents — either as a gas, aerosol or liquid — can be inhaled, absorbed through the skin or ingested (through contaminated food or liquid).
Targeting the respiratory system is usually the fastest way to poison someone, with nerve agents administered taking longer to reach the central nervous system.
For example, VX is a persistent nerve agent usually absorbed through the skin, but can also be ingested.
It does not wash away easily and can remain on clothes and surfaces for long periods, which poses a risk to first responders who may also come into contact with the poison.
What effect do they have on the body?
People exposed to nerve agents can find it increasingly hard to breathe. Even brief exposure can kill you within minutes.
They usually affect muscles and the central nervous system, particularly respiratory muscles, which is what causes people to die.
"Consequently, death caused by nerve agents is a kind of death by suffocation," the Organisation for the Prohibition of Chemical Weapons says.
A person exposed to a nerve agent would suffer poisoning and experience the following symptoms:
- Increased production of saliva
- A running nose
- Involuntary urination and defecation
- Impaired night vision and short-range vision deteriorates
- Also may be accompanied by a headache, tiredness, slurred speech, hallucinations and nausea
Why are nerve agents a weapon of choice?
One of the reasons why nerve agents, such as VX, may be used in assassinations is because they are persistent.
Senior lecturer of Medicinal and Pharmaceutical Chemistry at Charles Darwin University Martin Boland writes this means they can be used in relatively confined spaces, such as an airport terminal, with less risk of obvious adverse effects on bystanders or the perpetrators.
The victim needs to be treated almost as soon as they are exposed to prevent long-term damage, and nerve agents can be administered fairly easily and in public without sparking too much attention
As well as being used in the attempted murder of Sergei Skripal, nerve agents were also used to kill Kim Jong-nam in Malaysia and in the assassination of a suspected traitor to Aum Shinrikyo, a Japanese cult.
Britain has also drawn parallels between the Skripal case and the poisoning of ex-KGB agent Alexander Litvinenko, who was killed with radioactive polonium-210 in London in 2006.
How are they made?
The process to manufacture nerve agents involves fairly straightforward chemical techniques, but they are usually made in labs given how lethal the substance can be.
The Organisation for the Prohibition of Chemical Weapons says the "raw materials are inexpensive and generally readily available", with the same types of compounds used in insecticides.
Can you recover from a nerve agent attack?
There are antidotes, but the likelihood of recovery depends on how quickly a person is able to access it.
The armed forces in a number of countries usually have access to auto-injectors containing antidotes to nerve agents.