102: Thallium

HISTORY AND EPIDEMIOLOGY

Thallium (Tl^o), a metal with atomic number 81, is located between mercury and lead on the periodic table. Thallium is a soft, pliable metal that melts at 572°F (300°C), boils at 2699.6°F (1482°C), and is essentially nontoxic. Thallium forms univalent thallous (Tl⁺¹) and trivalent thallic (Tl⁺³) salts, which are highly toxic. Thallium is a commonly found constituent of granite, shale, volcanic rock, and pyrites used to make sulfuric acid and is also recovered as flue dust from iron, lead, cadmium, and copper smelters.²⁵

In the early 1900s, thallium salts were used medicinally to treat syphilis, gonorrhea, tuberculosis, and as a depilatory for ringworm of the scalp.^8,68 Although the usual oral dose given for ringworm was 7 to 8 mg/kg, fatal doses ranged from 6 to 40 mg/kg.^15,57 Many cases of severe thallium poi­soning (thallotoxicosis) resulted from this treatment for ringworm, with one author summarizing nearly 700 cases and 46 deaths.⁷¹

Because thallium sulfate is odorless and tasteless, it was also successfully used as a rodenticide. Commercially available as Thalgrain, Echol’s Roach Powder, Mo-Go, Martin’s Rat Stop liquid, and Senco Corn Mix, thallium sulfate was a very efficient rodenticide. As a consequence of numerous case reports of unintentional poisonings,^71,72,83 the use of thallium salts as a household rodenticide was restricted in the United States in 1965. Ultimately, even the commercial use of thallium salts as a rodenticide was banned in the United States in 1972 because of continued reports of human toxicity.

Life-threatening unintentional poisoning continues to occur in other countries, especially where thallium salts are still commonly used as rodenticides.^{2,14,81,89,97,106} Additional cases of thallium poisoning are reported in the United States and other countries as a result of the use of thallium for homicide^{21,62,67,75,77,86,94} and through contamination of herbal products⁹¹ and illicit drugs such as heroin^1,80 and cocaine.⁴⁴ Although occupational exposures to consequential amounts of thallium salts are uncommon, occupational toxicity is well described.³⁸

The following discussion of thallium toxicity refers to toxicity resulting from exposure to inorganic thallium salts, which represents virtually the entire literature on thallium poisoning. Although exceedingly rare, cases of poisoning with organic thallium compounds are reported⁴ and should be assessed and managed in a fashion similar to that used for patients with inorganic exposures. Although small amounts of thallium salts are used as radioactive contrast to image tumors and to permit the visualization of cardiac function, the doses used are insignificant without potential for thallium toxicity.⁶⁸

TOXICOKINETICS

Exposures usually occur via one of three routes: inhalation of dust, ingestion, and absorption through ...