Russian spy: What are nerve agents and what do they do?

Police say a nerve agent was used in an attempt to murder the Russian former spy Sergei Skripal and his daughter, Yulia, in Salisbury, Wiltshire. The pair are critically ill, along with a police officer who was among the first to attend the scene. But what are nerve agents and what do they do?

What are nerve agents?

They were first discovered by accident in the 1930s, when scientists were trying to find a more cost-effective pesticide.

They proved to be highly toxic chemicals that posed a risk to mammals and eventually ended up in the hands of the German military.

Russia came across such chemical agents for the first time when it swept into East Germany following World War Two and took control of the plants where they were made.

They take different forms - including powder and gas - but they tend to be a liquid, which can seep through the skin.

What are the different types?

There are five types of nerve agent:

tabun
sarin
soman
VX
cyclosarin or GF

The first three agents are clear, colourless, tasteless liquids.

VX is the most deadly - a drop on the skin can kill a person within minutes.

What was used in this case?

Police have not confirmed what agent was used, but a source has told the BBC it was likely to be rarer than sarin or VX nerve agents - two of the best known.

When have they been used in the past?

The half-brother of North Korean President Kim Jong-un was killed by a nerve agent in an attack in Malaysia last year.

Kim Jong-nam died after two women smeared his face with VX nerve agent in 2017 at Kuala Lumpur airport.

The UN says the nerve agent sarin has been used by the Syrian government - in an attack on Ghouta, near Damascus, in 2013 and again in Khan Sheikhoun in the north-west of the country in April 2017, killing hundreds.

In an attack on Tokyo's subway system in 1995, which killed 13 people, liquid sarin was placed in plastic bags that were pierced by umbrellas with sharpened tips.

When Russian dissident Alexander Litvinenko was poisoned in London in 2006, a radioactive substance called polonium 210 was used in a cup of tea.

What do they do to the body?

Nerve agents block the messages from the nerves to the muscles. This causes a collapse of many bodily functions.

They will act within seconds or minutes if inhaled and slightly slower if exposure is the result of skin contamination.

Symptoms include white eyes, as the pupils become constricted, convulsions, drooling and in the worse cases - coma, respiratory failure and death.

If you have ever sprayed insect repellent at a fly, you might have seen it drop to the ground and lie on its back, legs twitching. This is the result of nerve agents taking hold.

How are they delivered?

The nerve agent needs to be ingested, inhaled or to penetrate through the skin, so it usually requires the person delivering it to get close to the people they are targeting.

Only tiny amounts are required for it to take effect. It is so toxic that it would usually be transported in something tightly sealed and those who apply it will need protective clothing.

Dr Andrea Sella, professor of inorganic chemistry at University College London, said because of the extreme toxicity of the nerve agents it would be "very dangerous" to the person who delivered the poisoning.

Is there a risk to the public?

Despite their extreme toxicity, Dr Sella said the agent posed "very little risk" to the public.

You would need to be very close to these chemicals to be affected by them.

They would also decompose relatively swiftly in damp conditions, Dr Sella said.

Dr Hilary Walker, a former radiation scientist and health emergency planner, said there were "well-rehearsed" emergency procedures to prevent contamination by these chemicals.

These included the hosing down of streets that took place in the immediate aftermath of the incident in Salisbury.

What is the treatment?

There are antidotes to help reverse the effect of nerve agents, which the emergency services hold.

But the sooner the treatment is delivered the better the chance of recovery.

Knowing the exact substance enables more targeted treatment.

Alastair Hay, emeritus professor of environmental toxicology at the University of Leeds, said the people affected may well have been treated with atropine, which is one of the chemicals you use to treat individuals with this type of poisoning.

"The problem occurs if the treatment is not provided quickly, there is paralysis of the nerves and the muscles, and that inhibits breathing and leads to some damage in the brain. At this stage we don't know if that is a problem.

"Many people appear to recover without too many long-term problems but we won't know until we know the severity of their symptoms."

Where are they made?

Nerve agents are not materials that can be made at home.

Their level of toxicity is such that they are only to be manufactured in specialised facilities, such as a university or industrial laboratory.

Prof Hay said it was likely to have come from a state.

But Dr Simon Cotton, a senior lecturer in chemistry at the University of Birmingham, said it could also have been made by a group with access to such facilities.

"You can't tell from what we know at the moment, he said.

"It could be a state making it but it doesn't have to be. People shouldn't jump to conclusions."