The thallium sulfate was once widely used as rat poison and ant killer. Since 1975, this use has been prohibited in many countries due to safety concerns. Current man-made sources of thallium pollution include emission of cement factories, coal burning power plants, and metal sewers. The main source of elevated thallium concentrations in water is the leaching of thallium from ore processing operations.
Thallium and its compound are extremely toxic. They are also highly soluble and are readily absorbed through the skin. Among the distinctive effects of thallium poisoning are loss of hair and damage to peripheral nerves (some victims experience a sensation of walking on hot coals). Thallium has been known to accumulate in the kidney, causing sleep disorders and fatigue. It is also a suspected human carcinogen.
Interestingly, Thallium was once used as an effective murder weapon before its effects became understood and an antidote (Prussian blue) discovered.