What is thallium and where does it come from?
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.
What is thallium used for?
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:
- Electronics manufacturing – thallium sulphide and thallium selenide are used in electronic devices, switches, and closures, primarily for the semiconductor industry.
- Optical glass manufacturing – thallium bromide and iodide are used in infrared optical materials.
- Medical imaging – trace amounts of thallium are used as a contrast agent in the visualization of cardiac function and tumors.
- Other uses – high-temperature superconducting materials, gamma radiation detection equipment, low temperature thermometers, and green fireworks.
How do you get thallium poisoning?
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.
- Occupational exposure occurs from the maintenance and cleaning of ducts and flues at smelting plants where thallium particles can be breathed in.
- Accidental ingestion of rat poison – thallium is water soluble and nearly tasteless so any residue from touching rat poison could be ingested or absorbed through the skin.
- Cocaine, heroin and herbal products may be contaminated with thallium.
- Eating food such as fish and shellfish contaminated with thallium.
- Living near hazardous waste sites containing thallium may contaminate the air and soil and may result in higher than normal exposures.
- Deliberate poisoning.
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.
Signs and symptoms of thallium poisoning
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:
- Severe stomach pain, nausea, vomiting, and diarrhea within 3-4 hours of exposure
- Painful and rapidly progressing peripheral neuropathies (numbness, tingling and pain, especially on the soles and palms) about 2-5 days after exposure
- Sudden hair loss that progresses to widespread alopecia about 2-3 weeks after exposure.
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.
Dermatological signs and symptoms of thallium poisoning
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:
- Skin rash, redness and scaling of the palms and soles, acneform or pustular eruptions of the face.
- Transverse white lines on the nails (Mee’s lines) appear in the nail plate about one month after the poisoning.
- Other findings may include crusted eczematous lesions, hypohidrosis (reduced sweating), anhidrosis (absence of sweating), palmar erythema, painful glossitis (sore tongue) with redness of the tip of the tongue, and stomatitis.
- Hair roots may have dark brown or black pigmentation. With chronic exposure, these darker regions appear in bands, demonstrating multiple thallium exposures.
Treatment of thallium poisoning
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:
- Initial stabilization – assess ABCs (Airway, Breathing, Cardiovascular system) and provide oxygen if necessary, if possible remove source of exposure or remove contaminated clothing.
- Decontamination – if ingested in the last 30 minutes, induce vomiting. Consider gastric decontamination with activated charcoal and Prussian blue. With skin exposure, wash exposed skin thoroughly with soap and water.
- Follow-up – measure thallium concentrations in the blood and urine 3 times per week to confirm a decreasing trend. Prussian blue treatment should be continued until the 24-hour urine thallium concentration returns to the reference range (0-5 mcg/d).
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.