Thallium occurs in the environment naturally in very small amounts. It is not applied very widely by humans, merely as rat poison and as a substance in electro-technical and chemical industries. These applications can cause human exposure to thallium substances.

The human body absorbs thallium very effectively, especially through the skin, the breathing organs and the digestive tract.

Thallium poisoning is mainly caused by accidental uptake of rat poison, which contains large amounts of thallium sulphate. Consequently, stomach-aches will appear and the nervous system will be damaged. In some cases the damage is so irreversible that death will soon follow. When a human survives thallium poisoning often consequences of disturbances of the nervous system, such as trembling, paralyses and behavioural changes will remain. With unborn children thallium poisoning can cause congenital disorders.

Due to accumulation of thallium in the bodies of humans, chronic effects are known, such as tiredness, headaches, depressions, lack of appetite, leg pains, hair loss and disturbances of the sight.

Further effects that can be related to thallium poisoning are nerve pains and joint pains. These are consequences of thallium uptake through food. However, thallium toxicity from foods is very rare and nearly always caused due to environmental pollution.