Introduction

Thallium poisoning is a serious condition that results from exposure to Thallium and its salts. Thallium is a highly toxic heavy metal. Its excessive exposure in humans can result in its accumulation in the body, ultimately leading to Thallium poisoning. Since its discovery in the 19th century, Thallium has been reported in many cases of poisoning, both accidental and intentional. Thallium poisoning is particularly dangerous because Thallium is odourless and tasteless, which makes it difficult to be detected in contaminated food, water, or the environment.¹

It can either occur accidentally or intentionally:

• Accidental Thallium poisoning happens when a person is exposed to Thallium unintentionally. This occurs in industrial exposure, through environmental contamination, or by unknowingly ingesting Thallium
• Intentional Thallium poisoning involves deliberate exposure, usually for malicious purposes, such as criminal poisoning

Thallium poisoning results in adverse health effects, including neurological damage, gastrointestinal distress, skin changes, and even death.¹

What Is Thallium?

Chemical and Physical Properties

Thallium is a chemical element. It has the atomic number 81 and the symbol Tl. It was discovered in 1861 by English chemist Sir William Crookes. It is a soft, malleable, ductile metal found in trace amounts in the earth's crust. It is a bluish-white metal that is tasteless, odourless, and water-soluble. It is highly toxic and can cause severe poisoning if ingested or inhaled.²

Uses of Thallium

Thallium and its compounds are widely used in the manufacturing, agriculture, and medicine industries. It is used in glass manufacturing, semiconductors, electronic equipment, camera lenses, laser equipment, low-temperature thermometers, etc. Thallium isotopes are used in medical imaging, specifically Thallium-201, as a radiological contrast agent in scintigraphy imaging. It was used in rat poisons and insecticides, but these uses have been largely discontinued due to safety concerns. Due to its high toxicity, most of its applications have been banned or highly regulated in many countries.²

Routes of Exposure

Different routes of exposure include:¹

• Direct contact with skin: Thallium is easily absorbed through the skin. It mainly occurs from direct contact with protective gloves used for Thallium handling in industries
• Inhalation: Breathing contaminated air, vapours, or dust is a common route of exposure. Industrial emissions contribute to the contamination of air with Thallium
• Ingestion: Thallium is readily absorbed through the gastrointestinal tract. It occurs when Thallium is accidentally ingested or naturally consumed through contaminated food, vegetables, or water

Causes of Thallium Poisoning

Thallium poisoning can occur from acute exposure (a large dose in a short period) or from chronic exposure (smaller amounts over a longer time). Various causes include: 

Environmental Exposure

Emissions from the coal combustion in electric power, petroleum-refining, cement production, and the ores smelting industries lead to the release of Thallium in the air. Breathing contaminated air, vapours, or dust exposes the individuals to Thallium. Studies have shown that releasing industrial waste into environment has led to increased levels of Thallium in water and soils. This accumulation of Thallium in soil and water results in its absorption by crops grown in contaminated soils. As a result, the consumption of contaminated fish, vegetables, fruits, and water exposes humans to Thallium unknowingly.²

Occupational Exposure

Workers in industries dealing with Thallium are at higher risk of Thallium poisoning. Handling Thallium requires strict adherence to safety guidelines. The threshold limit value of Thallium in workplace air is 0.1 mg per cubic meter. Levels above 15 mg per cubic meter are considered severely hazardous. The maximum allowable working hours for workers in such industrial settings are eight hours per day.¹

Accidental Exposure

Accidental exposure to Thallium from rat poison was once a significant concern because Thallium was used as a key ingredient in many rodenticides and insecticides. Although it has been banned in many countries for decades due to its high toxicity, accidental exposure can still occur in areas where it is still used. Individuals, especially children, can accidentally ingest rat poison or contaminate their hands with it and subsequently ingest it.^1,2

Intentional Poisoning

Thallium has been used in criminal poisonings. There have been cases of deliberate Thallium poisoning because Thallium is odourless and tasteless, making it difficult to detect.¹

Mechanism of Poisoning

Thallium is structurally similar to potassium. Therefore, it disrupts normal cellular functions by interfering with potassium-dependent biological processes, leading to adverse health effects. These include effects on cellular metabolism, the central and peripheral nervous systems, protein synthesis, keratin production, and glucose metabolism.¹

Thallium goes through three main phases in the body after exposure. First, during the initial 4 hours, Thallium spreads to different organs through the bloodstream. During the next 4 to 48 hours, it moves into the central nervous system. Finally, in the Elimination Phase, which starts about 24 hours after exposure, Thallium is mainly removed from the body through urine and faeces. The process of elimination is slow and can take up to a month.¹

Symptoms of Thallium Poisoning

Symptoms of Thallium poisoning include:¹

Gastrointestinal Symptoms

These symptoms appear early within 3-4 hours. These include:

• Abdominal pain
• Nausea
• Vomiting
• Diarrhoea or constipation

Neurologic Symptoms

These symptoms appear later, around 2-5 days post-exposure. These include:

• Peripheral neuropathies
• Distal motor weakness- weakness in the muscles of hands, feet, arms, and legs
• Loss of muscle control
• Tremors- Involuntary shaking or trembling movements
• Cranial nerve palsies
• Headache
• Seizures
• Insomnia
• Coma
• Death in severe cases

 Ocular Symptoms

• Double vision
• Drooping of the upper eyelid (Ptosis)
• Involuntary repetitive eye movement (Nystagmus)
• Optic neuropathy
• Lens opacities- Loss of transparency in the lens of the eye

Dermatologic Symptoms

• Peeling or flaking of outer layer of skin
• Pus or fluid containing eruptions on skin
• Hair loss
• Mees lines on the nails (appearance of white lines on nails)
• Reduced sweating /or inability to sweat
• Painful swelling of the tongue

Other Symptoms

• Tachycardia (rapid heartbeat)
• Hypertension (increased blood pressure)

Diagnosis of Thallium Poisoning

Various methods to diagnose Thallium poisoning include:¹

Laboratory Tests

• A definite clinical diagnosis can only be made by detecting elevated levels of Thallium in hair, nails, blood, urine, faeces, and saliva
• The standard method for diagnosing Thallium poisoning is the 24-hour urine Thallium test, which measures Thallium levels in the urine over 24 hours using atomic absorption spectrometry
• Although spot urine tests are faster, they have higher chances of giving false positives; therefore, the 24-hour urine test is the trusted method
• A blood Thallium test is a useful method, but blood tests can sometimes produce false negatives, so urine testing is preferred
• Other tests that are often done are complete blood count with differentials, electrolyte levels, glucose, blood urea nitrogen, liver function, and calcium tests

Radiographic Imaging

Since Thallium is radiopaque, X-rays help detect Thallium in the gastrointestinal tract if ingestion is suspected.

Other Tests

• Electrocardiogram (ECG): To monitor the heart for arrhythmias and tachycardia
• Electroencephalogram (EEG): To detect any seizure-like brain activity

Treatment and Management

Treatment for Thallium poisoning includes:¹

Supportive Care

Stabilising the patient’s airway, breathing, and circulation if required.

First-aid measures

• In case of skin exposure, wash the skin with soap and water
• In case of eye exposure, rinse the eyes with normal water

Decontamination

• Remove contaminated clothing
• Activated charcoal is indicated if the patient presents within an hour of ingestion

Chelation Therapy

This involves the administration of a chelating agent, such as Prussian blue, that binds to Thallium in the body and facilitates its removal. It is considered the most effective treatment and is approved by the FDA for treating Thallium poisoning.

Prevention and Public Health Measures

Various strategies that can be/are implemented to reduce the risk of poisoning include:^1,2

Regulatory Control

• Enforcing strict regulations to limit the use of Thallium in the industrial applications, agriculture, and other products
• Clear labelling on the products that contain Thallium, along with safety warnings and the instructions for safe use and disposal

Public Awareness and Education

• Conducting campaigns to raise awareness about the dangers of Thallium exposure. Educating at-risk communities, such as those living near industries that use Thallium or in areas where Thallium is used as a pesticide or rodenticide
• Providing training for the employees in industries where Thallium is used, focusing on safe handling and use of personal protective equipment (PPE)

Environmental Monitoring

Regularly monitoring environmental levels of Thallium by conducting soil and water tests. This will help in identifying contamination sources and protect public health.

Safe Handling and Disposal Practices

Establishing proper disposal guidelines for the safe disposal of Thallium-containing products and waste to prevent environmental contamination.

Summary

Thallium poisoning is a significant health hazard resulting from exposure to Thallium, a highly toxic heavy metal. Symptoms of Thallium poisoning can range from gastrointestinal distress and skin changes to neurological impairment and to even death. The diagnosis involves laboratory testing and radiographic imaging to confirm Thallium levels in the body, with treatment primarily relying on chelation therapy, particularly the use of Prussian blue. Continued research and vigilance are necessary to address this environmental and occupational health issue, ensuring that both individuals and communities are protected from the risks associated with Thallium exposure.