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.
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 7 September 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.
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.
We’ve all heard of South American Indians using venom-tipped blowpipes to hunt their prey. Curare is the best known, and comes from a plant. The most toxic, however, come from the skins of tiny frogs – and the deadliest of all is Batrachotoxin.
There are a number of potent marine toxins, such as Saxitoxin, which are often the cause of poisoning after eating contaminated shellfish. These are often associated with harmful algal blooms in the sea.
1. Botulinum toxin
Scientists differ about the relative toxicities of substances, but they seem to agree that botulinum toxin, produced by anaerobic bacteria, is the most toxic substance known. Its LD50 is tiny – at most 1 nanogram per kilogram can kill a human. Extrapolating from its effect on mice, an intravenous dose of just 10-7g would be fatal to a 70kg person.
It was first identified as a cause of food poisoning due to incorrectly prepared sausage (Latin, botulus) in late-18th century Germany. There are several botulinum toxins, with type A being the most potent. These are polypeptides, consisting of over 1,000 amino acid molecules joined together. They cause muscle paralysis by preventing the release of the signalling molecule (neurotransmitter) acetylcholine.