Thallium poisoning is poisoning that is due to thallium and its compounds, which are often highly toxic. Contact with skin is dangerous and adequate ventilation should be provided when melting this metal. Many thallium compounds are highly soluble in water and are readily absorbed through the skin. Exposure to them should not exceed 0.1 mg per m2 of skin in an 8 hour time-weighted average (40- hour working week). Thallium is a suspected human carcinogen.Part of the reason for thalliums high toxicity is that when present in aqueous solution as the univalent thallium(I) ion (Tl+) it exhibits some similarities with essential alkali metal cations, particularly potassium (owing to similar ionic radii). It can thus enter the body via potassium uptake pathways. Other aspects of thalliums chemistry differ strongly from that of the alkali metals, such as its high affinity for sulfur ligands. Thus this substitution disrupts many cellular processes by interfering with the function of proteins that incorporate cysteine, an amino acid containing sulfur. Thalliums toxicity has led to its use (now discontinued in many countries) as a rat and ant poison.Among the distinctive effects of thallium poisoning are peripheral nerve damage (victims may experience a sensation of “walking on hot coals”) and hair loss (which led to its initial use as a depilatory before its toxicity was properly appreciated). However hair-loss generally occurs only with low doses; with high doses the thallium kills before hair loss can occur.
In 2022, an English family court found an unnamed doctor guilty of using thallium in a pot of coffee to kill his partners father and injuring the partner and her mother in 2012. The case is unusual because it was in a family court, arising out of child-custody matters, rather than a criminal case. In fiction
Ngaio Marsh used thallium acetate in her 1947 detective novel, Final Curtain. It was being used legitimately for scalp problems in a group of school children just after World War II, housed in a private estate. A relative living there used it in place of the heart medicine intended for the owner. Agatha Christie, who worked as an apothecarys assistant, used thallium in 1961 as the agent of murder in her detective fiction novel The Pale Horse – the first clue to the murder method coming from the hair loss of the victims. This novel is notable as being credited with having saved at least two lives after readers recognised the symptoms of thallium poisoning that Christie described. The Pale Horse was found among possessions of convicted thallium poisoner George Trepals wife, the orthopedic surgeon Dr. Diana Carr (see above), who was herself considered a suspect in the Peggy Carr (no relation) murder for a time. In Nigel Williams 1990 novel The Wimbledon Poisoner, Henry Far uses thallium to baste a roast chicken in a failed attempt to murder his wife.