When asked to name a poison, people may well think of cyanide, arsenic or strychnine. But these are not the most toxic substances known. More poisonous than these, but still not near the top of the tree, is tetrodotoxin, the pufferfish toxin that poisons around 50 Japanese people every year. The fish is a delicacy in Japan, but can be lethal if prepared incorrectly. Incidentally, this was the poison favoured by evil assassin Rosa Klebb in James Bond film From Russia With Love. It also crops up in the blue-ringed octopus and was more recently discovered in tiny frogs in Brazil.
The LD50 (Lethal Dose, 50%) – the amount needed to kill 50% of the test population – is how toxicity is most often assessed, and is usually quoted per kilogram of body weight. On this scale, for example, sodium cyanide comes out at around 6 milligrams per kg. The LD50 of tetrodoxotin, by comparison, is around 300 micrograms per kg if orally ingested, and as little as 10 micrograms per kg if injected.
Assessing toxicity is not easy. The chemical state of a substance is important, as is how we ingest it. If we swallowed liquid mercury metal (as distinct from inhaling the vapour), it would very likely pass straight through us harmlessly. And yet when in 1996 an American professor got just a drop or two of the compound dimethyl mercury on her rubber gloves, it penetrated the gloves and her skin, sending her into a fatal coma some months later.
Nevertheless, here is a representative selection, in ascending order, of five truly deadly poisons, all at least a hundred times more toxic than cyanide, arsenic or strychnine.
This extremely toxic plant poison was famously used to kill the Bulgarian dissident Georgi Markov, exiled in London. On September 7 1978, he was waiting for a bus near Waterloo Bridge, when he felt an impact on the back of his right thigh. Looking round he saw a man bending down to pick up an umbrella. Markov was soon taken to hospital with a high fever – and died three days later. An autopsy revealed a tiny sphere made of a platinum-iridium alloy in Markov’s thigh. The sphere had been drilled to take a small amount of ricin and may have been fired from an air gun hidden in the umbrella.
Ricin is obtained from the beans of the castor oil plant (Ricinus communis), which is cultivated to extract the oil – the ricin remains in the solid fibre. It is a glycoprotein that interferes with protein synthesis in the cell, causing cell death. It has an LD50 of 1-20 milligrams per kg if orally ingested, but far less is required to kill if inhaled or injected (as in Markov’s case).
The only synthetic compound in our top five, VX is a nerve agent with the consistency of engine oil. It emerged from ICI’s research into new insecticides in the early 1950s but proved too toxic to use in agriculture. VX kills by interfering with the transmission of nerve messages between cells; this requires a molecule called acetylcholine. After acetylcholine has passed on its message, it needs to be broken down (otherwise it will keep sending the message) by an enzyme catalyst called acetylcholinesterase. VX and other nerve agents stop this enzyme from working, so muscle contractions go out of control and you die of asphyxiation.
Nerve agents explained.
Nerve agents were made by both sides during the Cold War, but VX became particularly well-known after featuring in Hollywood blockbuster film The Rock. Only one person is known to have been killed by VX, a former member of the Aum Shinrikyo cult, though some 4,000 sheep were killed by it in an accident in Skull Valley, Utah in 1968. It has an LD50 of as little as 3 micrograms per kg (although some reports suggest the figure is a little higher).