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Author: Vanessa Ngan, Staff Writer, 2013.
Thallium is a heavy metal that is found in trace amounts in the earth's crust. It was first discovered accidentally in 1861 when trying to extract selenium from the by-products of sulfuric acid production. It was named after the Greek word “thallos” meaning “green shoot or twig” because of its bright green spectral emission.
Thallium is mainly obtained as a by-product from the smelting of copper, zinc and lead ores. Its main entry into the environment is from coal-burning and smelting where it stays in the air, water and soil for a long time. It is absorbed by plants and can build up in fish and shellfish.
In the past, thallium salts were used to treat syphilis, gonorrhoea, tuberculosis and ringworm (tinea corporis). Other uses were as a hair removal agent for excess hair and in formulations as rat poison and ant killer. However in the early 1930s numerous cases of thallium poisoning, with some causing death were being reported. By the mid-seventies the commercial use of thallium was banned in the United States.
Some current uses of thallium include:
Thallium and its salts are extremely toxic. Exposure can occur in the workplace or from the environment where there may be higher than normal levels of thallium.
Thallium poisoning is via ingestion or absorption through the skin. The lethal dose for humans is 15-20 mg/kg, although much small doses have also led to death. Non-lethal doses cause toxic effects. Prolonged exposure may lead to build-up and chronic poisoning.
Thallium poisoning is more common in developing countries where thallium rat poisons are still available and where there are less rigorous health and safety workplace practices.
The clinical presentation of thallium toxicity can vary depending on the type, severity, and timeframe of the exposure. It may present as an acute poisoning or as a chronic poisoning.
Where large amounts of thallium have been ingested over a short period of time patients will show signs and symptoms of acute poisoning. The classic symptoms of acute thallium toxicity are:
Chronic thallium poisoning occurs over months or years of exposure to thallium. Thallium can be absorbed through the skin, respiratory, and gastrointestinal tracts and builds up to reach toxic levels. Because the presentation of chronic thallium poisoning appears similar to other diseases, many cases of industrial thallium exposure may go undetected. Signs and symptoms of chronic poisoning include tiredness, headaches, depression, hallucinations, psychosis, dementia, poor appetite, leg pains, hair loss and disturbances of vision.
Sudden hair loss followed by diffuse alopecia is one of the characteristic dermatological signs of thallium poisoning. Hair loss due to atrophy of the hair follicles primarily affects the scalp, temporal parts of the eyebrows, the eyelashes, and the limbs. Axillary regions are less affected. Hair discoloration may also occur.
Other dermatological features include:
All patients with significant signs and symptoms of thallium poisoning should be admitted to hospital. The goals of treating a patient with thallium poisoning are:
Early diagnosis and treatment of thallium poisoning is the key to a favourable recovery. Delayed treatment increases the risk of persistent signs and symptoms of peripheral neuropathy, neurological and visual disabilities. The course of recovery may take many years and in some cases a complete recovery is never achieved.
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