The park in Salisbury where former Russian double agent Sergei Skripal and his daughter Yulia were found critically ill. Photo: Andrew Matthews/PA Wire/PA Images Now it is official that Sergei Skripal, the former Russian intelligence officer, and his daughter Yulia were poisoned in Salisbury last Sunday (4th March) by a nerve agent, there is something of a scramble among commentators to unpack what that scary-sounding term means. Of course, the first thing it means is that Skripal is not a victim of food poisoning or some other accidentally incurred chemical or biological poisoning. As was suspected, given that he is a former spy jailed in Russia in 2006 for counter-espionage and then released in the kind of tit-for-tat swap most of us imagined went out of fashion after the Cold War ended, the poisoning is evidently deliberate. Counter-terrorist officials in the Metropolitan Police say they have identified the nerve agent used but have not yet disclosed what it was. The BBC was told by an anonymous source that it is rare, however, and so seems unlikely to be the best known agents: sarin—which was used in the Tokyo underground attack by members of the Aum Shinrikyo sect in 1995, and last year by Syrian government forces to attack rebel groups—and VX, which was used to kill Kim Jong-nam, the half-brother of North Korean leader Kim Jong-un, in Malaysia, also last year. It’s the kind of substance, then, that few have access to. The Russian foreign minister Sergei Lavrov has been quoted as denying involvement of the Russian government, although naturally comparisons have been made with the murder by poison (in that case the rare element polonium-210) of the Russian dissident Alexander Litvinenko in 2006. An enquiry into that event concluded that it was carried out by Russian agents, probably ordered by President Putin. Skripal and his daughter were found slumped unconscious on a bench by a shopping centre in Salisbury, and one eye-witness described him as performing “strange hand movements” and looking “out of it”—symptoms expected in victims of a nerve agent. So what are these chemicals? There are plenty of substances that disrupt our nervous system, but not all are classed as “nerve agents.” Nature abounds in neurotoxins such as the nasty compounds in puffer fish, some shellfish and snakes, and the poison dart frog of the Colombian rain forests. They exist to do damage: to deter predators or to stun and paralyse prey. But nerve agents, as defined for military and scientific purposes, are generally synthetic chemicals designed to do bad things to the nervous system, and they usually have particular types of chemical composition and mode of biological action. Specifically, they are generally compounds called organophosphates, a class that includes sarin and VX as well as some pesticides. They work by disabling an enzyme in our bodies called (brace yourself) acetylcholinesterase, which plays a vital role in nerve signaling. Nerves “talk” to one another by exchanging small molecules called neurotransmitters at the junctions (synapses) where they meet. The enzyme breaks down a neurotransmitter called acetylcholine, and so is essential to turning the nerve signals off. If the enzyme is blocked by one of these nerve agents, nerve signaling gets out of control. The result is that muscles can go into spasm or contract involuntarily, causing vomiting, jerky movements and convulsions, breathing problems, bowel movements—and, in sufficiently high dosage, death. Nerve agents can exert their horrible effects if they are inhaled as aerosol droplets, swallowed in food or drink, absorbed through the skin or eyes, or administered via a wound. It’s not yet clear how the Skripals were given the poison, but VX was smeared on the face of Kim Jong-nam by his attacker. A police officer who first approached the Skripals is also in hospital in a serious condition, perhaps because he touched or breathed in the nerve agent left on their skin or clothes. It is presumably from such tiny traces that the agent has now apparently been identified. There is thought to be no danger now to the public in the location where the pair were found, however. It’s not clear how the Skripals will fare. There are drug treatments that will stop acetylcholine from continuing its action on nerve signals, and others that will restore the enzyme to proper function. But even if the two survive, they might suffer medium- or long-term consequences: nerve agents can leave lingering blurred vision, tiredness and headaches. Identifying the nerve agent is a first step in figuring out where it came from. It was the rarity, as well as the detectable remnants, of polonium-210, which kills by radiation damage to cells, that helped to trace the killing of Litvinenko to the Russian politician and former KGB bodyguard Andrey Lugovoy, who remains living in Russia in defiance of a British demand for extradition to face trial. The puzzle here is the same as there: why carry out an assassination with a rare yet detectable poison that seems likely to be accessible only by state agents? Unless perhaps you do want to signal on whose authority the deed was done?