CORRECTION (PUBLISHED 3 AUGUST 1992) APPENDED TO THIS ARTICLE

THALLIUM sulphate is a colourless, tasteless salt that you can dissolve in water and give to someone you wish to hurt or kill. It is not a perfect poison, but it comes close. It takes about a week to start working, and when it does it produces symptoms that are easily confused with diseases such as encephalitis, epilepsy and neuritis.

The metal thallium and its salts are always open to accidental or deliberate abuse. You are unlikely ever to use or be abused by thallium sulphate in the UK, however, as it is prohibited here. But in Iraq the security forces use thallium sulphate to dispose of opponents of the regime.

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Ten years ago New Scientist claimed that dissident scientists in Iraq were being poisoned with it, and in 1988 an opponent of the regime in Britain, Abdullah Ali, was killed in this way. This spring Abdullah Abdelatif Al-Jabouri, a staff colonel in the Iraqi army, and Abdul Karim Masdiwi Al-Jabouri, a university professor, fell from favour and became ill. They escaped to Damascus and were flown to London, where thallium poisoning was diagnosed and treated.

Agatha Christie is often blamed for bringing thallium sulphate to the attention of poisoners. In 1961 she wrote The Pale Horse, in which the effects of thallium poisoning were attributed to black magic. She described the symptoms perfectly - lethargy, numbness, black-outs, slurred speech, general debility - but she was not the first mystery writer to employ this poison.

In Final Curtain, written in 1947, Ngaio Marsh had her villain using it, even though she did not have a very good idea of how it worked. Her victims dropped dead of thallium poisoning within minutes.

Thallium was once readily available, and was even part of the medical pharmacopoeia as a pre- treatment for ringworm of the scalp. It did not kill the ringworm, but it caused the patient's hair to drop out so that the condition could be more easily treated.

This strange effect was discovered by accident about 100 years ago when thallium was tested on tuberculosis patients as a cure for 'night sweats'. It did not work, but their hair fell out. Dr R J A Sabourand, the chief dermatologist at the St Louis Hospital in Paris, reported this depilatory action in 1898, and thallium became the standard treatment for hair removal for 50 years. Women could buy it over the counter in the Thirties as 'Koremlou creme' for removing unwanted hair.

Thallium is not a rare element - it is 10 times as abundant as silver. It is spread widely in the environment and trace amounts find their way into crops such as grapes, sugar beet and tobacco. About 30 tons of thallium a year are produced as a by-product of lead and zinc smelting. Some of this is used for making special glass for lenses, some for chemical research, and some ends up as thallium sulphate destined for Middle East and Third World countries, where it is still used to kill vermin.

In 1976, the year Agatha Christie died, a 19-month-old girl from Qatar was brought to Hammersmith Hospital, London, suffering from a mysterious disease. A nurse noticed that the symptoms resembled those of victims in The Pale Horse. When she reported this, doctors immediately tested for thallium, found it, changed the treatment and saved the girl's life. Inquiries revealed that her parents had been using thallium sulphate to kill cockroaches in their home.

Hundreds of people were affected in Guyana in 1987, and 44 died, after they drank milk from cows that had eaten molasses poisoned with thallium sulphate intended to kill sugar-cane rats.

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Thallium is an insidious poison because the body mistakes it for the essential element potassium. A fatal dose for an adult would be 800mg (less than a quarter of a teaspoon), yet doses of 500mg of thallium salt were prescribed for ringworm. Hair loss is a symptom of severe thallium poisoning.

Investigations with radioactive thallium show it is readily assimilated into the body, and particularly affects potassium-activated enzymes in the brain, muscles and skin. The body is not fooled for long by the thallium it absorbs, and excretes it into the intestines. However, this is not effective, since a little further along it is once again 'recognised' as potassium and is

reabsorbed.

To break this cycle of excretion and reabsorption, the best cure is Prussian blue, the dye used in blue ink. This is a complex salt of potassium, iron and cyanide and was suggested as an antidote 20 years ago by a German pharmacologist, Horst Heydlauf, of Karlsruhe, at a time when thallium poisoning was believed to be incurable.

The discovery of thallium in 1862 caused an international incident at the London International Exhibition that year. William Crookes, a chemist at the Royal College of Science, discovered the metal when he observed a green flame while testing some impure sulphuric acid. This colour led to the name thallium, from the Greek word thallos, meaning a green bud.

Crookes made several salts of thallium, which he exhibited, but he could not produce a sample of the metal itself. However, Claude- Auguste Lamy, a physicist from Lille, France, had isolated the lead-like metal, which he put on show.

Lamy was awarded an exhibition medal and named as the discoverer of the element. Accusations and counter-accusations flew between London and Paris, until Crookes was eventually awarded a medal as well.

The most notorious thallium poisoner was Graham Young, who in 1971 put thallium sulphate into his workmates' coffee at a photographic equipment factory at Bovingdon, Hertfordshire. The firm used thallium to make lenses. Several workers were taken ill and two died of the mysterious 'bug'.

It was only when Young himself suggested to a visiting health expert that the cause might be thallium that the illness was correctly diagnosed. Young, a former Broadmoor patient, was found guilty of murder and sentenced to life imprisonment. He committed suicide in 1990 in Parkhurst prison.

The case was a milestone in forensic detection. At the Metropolitan Police forensic laboratories, the ashes of Robert Egle, one of Young's cremated victims, were analysed by a technique known as atomic absorption spectrometry, which revealed a level of 5 parts per million of thallium - proof that Mr Egle had been poisoned with it.

Dr John Emsley is the author of 'The Elements', published by Clarendon Press, Oxford, at pounds 11.95.

CORRECTION

In 'The poison prescribed by Agatha Christie' (Molecule of the Month, 20 July), Dr John Emsley described how, in 1971, Graham Young used thallium sulphate to poison workmates at a Hertfordshire photographic equipment factory. The article erroneously stated that the photographic company used thallium to make lenses. This is not the case: the company does not and has never used thallium in any form. The thallium that Graham Young used was purchased from a chemist's in London.

(Photographs omitted)