We all have a little thallium in our body, probably no more than half a milligram, and the level in blood is only 0.5 ppb. The average person takes in about 2 ^g of thallium a day as part of his or her diet; this accumulates in the body over time and most ends up in the skeleton. Indeed thallium finds its way into all tissues except fat, and it can even pass the placental barrier. There is no biological role for thallium although some marine organisms appear deliberately to concentrate it, but for what purpose is unclear. Thallium is like lead in that it is a cumulative poison and, like lead, it attacks the nervous system. Thankfully we never accumulate enough to affect our health. Others have not been so lucky and have had to cope with excess thallium, given accidentally, deliberately, or even medicinally. Then it slowly begins to affect those parts of the metabolism that depend on potassium, and these include the brain, nerves, and muscles. Water-soluble thallium salts are readily absorbed by mucous membranes such as the mouth, stomach, and intestines, and they can even penetrate through the skin.
Why does the body accept thallium so easily? The reason is that the positively charged thallium ion (Tl+) is almost the same size as the potassium ion (K+) which is essential to living cells. Tl+ is sufficiently similar to that of K+ for it to get into the cell but once inside the cell the slight differences that there are become apparent and the cell's function is impaired. Thallium mimics potassium so effectively that it can displace it at all sites around the body but most damagingly along the central nervous system which soon begins to malfunction. Thallium eventually affects hair follicles and these become unable to produce more hair. The existing hair drops off, and this occurs all over the body.
Studies using the radioactive isotope thallium-204* can trace the movement of thallium in the body, showing that it accumulates in the bones, kidneys, stomach wall, intestines, pancreas, and salivary glands. The hair, eyes, and tongue also contain considerable amounts, while the muscles and liver have lower concentrations. Excretion is mainly through the faeces and urine with the former predominating. Once it has penetrated the body, thallium tends to form thallium chloride, which is not very soluble and consequently it takes the body a long time to eliminate thallium. The studies with thallium-204 showed that it takes at least a month to remove half of a given dose, but some is released so slowly that it can still be detected in the urine after three months.
The biochemistry of thallium has not yet been completely researched, so its mode of operation within the body is still not fully understood. In addition to mimicking potassium, it interferes with the operation of the B vitamins and with calcium and iron. Comparison of the effects of thallium with those of thiamine (vitamin B1) deficiency show such remarkable similarity that it would seem that the metal is somehow interfering with the body's thiamine metabolism. Another vitamin that thallium disrupts is riboflavin (vitamin B2), which is also involved in energy production within the body.f Thallium upsets the sugar metabolism of the body and produces symptoms of diabetes. In addition to all of this, men are affected sexually and become impotent. Its most dangerous impact is on the central nervous system, particularly damaging organs that have high energy requirements, such as the skin, testicles, and heart.
A fatal dose of thallium for an adult is around 800 mg, which is less than a quarter of a teaspoonful, and yet doses of 500 mg of thallium salts were prescribed medically as a pre-treatment for ringworm. J Only when all the hair had been removed was it thought possible to eradicate the fungus. Hair loss would begin after ten days or so, but such hair loss would today be taken as an indication that a person was suffering from near-lethal
* This has a half-life of 3 years 40 weeks and emits beta-radiation which does not damage the body.
f At one time riboflavin used to be separated from milk by adding thallium acetate, which caused it to precipitate as an insoluble material.
J Ringworm is highly contagious and is often transmitted through contact with farm animals like cows.
thallium poisoning. A lethal dose of a thallium salt produces the following effects:
Day i - No symptoms at all or mild symptoms typical of a cold or flu.
Day 2 - Gastroenteritis, pins and needles in the feet, and possibly diarrhoea.
Day 3 - Band-like pain around the body, joint pains, feet become very sensitive to touch, and sleep is almost impossible.
As the days progress, the nervous system becomes more and more affected, and these symptoms become intensified. Later there is paralysis of the muscles that control talking, swallowing, and movement of the tongue and lips. The eyes become inflamed and blindness may result. Paralysis of the face and mouth produces a mask-like feature and the patient is unable to speak. The skin becomes grey and scaly and a rash may appear on the hands. Profuse sweating occurs and this may lead to an offensive odour from the palms of the hands and soles of the feet. Thallium can lead to excessive urination, although this removes only a little of the poison. Victims suffer degeneration of the heart, liver, and kidneys and may die from respiratory disorders such as paralysis of the lungs, pneumonia, or heart failure. Blood pressure is high due to thallium stimulating the muscles of the arteries. Side by side with the physical manifestations are the mental disturbances which may range from acute depression and a wish to die, through hallucination to epileptic fits.
It is easy to see why thallium poisoning was often mistaken for so many other ailments and especially in societies where thallium poisoning was extremely rare. As we shall see in the final chapter, only i of the 43 doctors who examined the victims of poisoner Graham Young correctly diagnosed thallium poisoning. It is almost impossible to diagnose it from its symptoms alone, not even when a post-mortem is carried out. Widespread degeneration of the peripheral nerve cells is a typical autopsy finding, but heart, intestines, liver, spleen, and pancreas may appear normal.
A case of misdiagnosed poisoning with thallium sulphate was reported in 1977, but the victim was saved thanks to Christie's The Pale Horse. A 19-month-old girl living in Qatar had been taken very ill and the doctors there had been unable to diagnose the cause of her illness, so her parents took her to London for specialist treatment. It was clear she was suffering from something serious - by the time she arrived at the hospital she was semiconscious - although it was unclear what was wrong with her. At the Royal Postgraduate Medical School at Hammersmith Hospital she was referred to Dr T. G. Matthews and Professor Victor Dubowitz of the Department of Paediatrics and they began to examine her in the conventional manner taking blood samples, carrying out a lumbar puncture, and giving her a full body X-ray. They found nothing that would account for the child's condition although an EEG (electroencephalogram) of the brain was clearly abnormal. During the next few days she got progressively worse, her blood pressure was high, her heartbeat rate was 200/minute, and she was breathing irregularly. There seemed no hope for her until a nurse Marsha Maitland heard the doctors talking about the child and she suggested that she might be suffering from thallium poisoning. Maitland drew their attention to the book she had been reading, which was The Pale Horse, and they were soon convinced that they were dealing with a case of thallium poisoning because the child's hair was starting to fall out.
The doctors immediately contacted Scotland Yard and were able to rush a sample of urine to a forensic laboratory where thallium was shown to be present at 3.7 ppb, ten times the normal level. The child was immediately put on a course of the antidote potassium ferric ferrocyanide, and over the next two weeks she stabilized as the level of thallium in her urine fell. After three weeks she began to show clear signs of improvement and a week later she was discharged into the care of her parents who took her home to Qatar. A follow-up assessment four months later showed her almost back to normal. And the source of the thallium that nearly killed her? It was a pesticide that the parents had been using to kill the cockroaches and rats which infested the drains and septic tank of their home. The child was thought to have found it under the kitchen sink and consumed some of it.
The only certain way to prove that someone is suffering from thallium poisoning is to analyse their blood, faeces, or urine, and to prove that death has been caused by thallium it is necessary to perform chemical analysis on body tissues and bone. Thallium may be so widely dispersed in the body that its concentration in tissue is likely to be low. The level of thallium in a person who has been killed by it will vary from organ to organ but typically levels of 8-10 ppm will be found in the liver, muscle, and bone, with slightly lower levels in the heart, kidney, and lungs, and only around 2 ppm in the brain. It affects this last organ quite dramatically in those who have been poisoned with a large dose, but can it also affect those who have been exposed to non-lethal doses? The eccentric behaviour of one of its discoverers would seem to suggest that it might.
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