Agatha Christie and Thallium poisoning

Agatha Christie (1890-1976) is often blamed for bringing thallium sulphate (Tl₂SO₄) to the attention of would-be poisoners. In 1961, she published '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, and general debility.

Thallium was once readily available, and was even part of the medical pharmacopoeia as a pre-treatment for ringworm of the scalp[1]. 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.

Thallium sulphate is a colourless, odourless, and tasteless salt that you can dissolve in water. It was historically used as a rodenticide and ant bait. You can also use it to kill your adversary. It is not perfect, but it comes close. It can enter the body by ingestion, inhalation, or even through contact with the skin. 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.

Thallium is an insidious poison because the body mistakes it for the essential element potassium. A fatal dose for an adult can be as low as 800 milligrams (less than a quarter of a teaspoon).

In 1976, the year Agatha Christie died, a 19-month-old girl from Qatar was brought to Hammersmith Hospital (London, UK) suffering from a mysterious disease which left the doctors baffled. Marsha Maitland, a nurse, who was at the bedside of the baby, happened to be reading 'The Pale Horse'. She peered over the top of the book and noticed that the symptoms resembled those of victims described in 'The Pale Horse' and alerted the doctors[2]. The one consistent feature stressed in the book, namely loss of hair, seemed to be developing in the child that morning.

However, the hospital lacked testing for this rare metal, so Scotland Yard was involved, and a urine sample confirmed thallium poisoning. After changing the treatment, the child recovered fully.

Inquiries revealed that her parents had been using thallium sulphate to kill cockroaches in their home. I suppose they overdid it a bit.

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 developed countries, as it is prohibited there.

[1] Ingram,: Thallium Acetate in the Treatment of Ringworm of the Scalp in British Medical Journal - 1932
[2] Matthews, Dubowitz: Diagnostic Mousetrap in British Journal of Hospital Medicine – 1977. See here.