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.23 It has been used in alloys as an anticorrosive, in optical lenses to increase the refractive index, in artists' paints, in lamps to improve tungsten filaments, in extreme cold thermometers, in imitation jewelry, as a catalyst, in fireworks, as a rodenticide, and as a medicinal. Diagnostically, small nontoxic amounts of thallium salts are used as radioactive contrast to image tumors and to permit the visualization of cardiac function.65 Thallium toxicity presents as an uncommon constellation of signs and symptoms that most commonly include gastrointestinal (GI) distress, a painful ascending peripheral neuropathy, and alopecia.
Thallium, 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 and trivalent thallic salts, which are highly toxic.
In the early 1900s, thallium salts were used medicinally to treat syphilis, gonorrhea, tuberculosis, and ringworm of the scalp and as a depilatory.6,65 Although the usual oral dose given for epilation in the treatment of ringworm of the scalp was 7 to 8 mg/kg, fatal doses ranged from 6 to 40 mg/kg.13,54 Many cases of severe thallium poisoning (thallotoxicosis) resulted from the treatment of ringworm, with one author summarizing nearly 700 cases and 46 deaths.67
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,67,68,79 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 where thallium salts are still commonly used as rodenticides.12,77,85,100 Additional cases of thallium poisoning are reported in the United States and other countries as a result of the use of thallium as a homicidal agent19,59,63,71,73,82,89 and through contamination of herbal products86 and illicit drugs such as heroin76 and cocaine.41 Although occupational exposures to consequential amounts of thallium salts are uncommon, toxicity is well described in this setting.35
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 have been reported...