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INTRODUCTION

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HISTORY AND EPIDEMIOLOGY

Thallium (Tl0), a metal with atomic number 81, is located between mercury and lead on the periodic table. Elemental thallium is a soft, pliable metal that melts at 572°F (300°C), boils at 26,99.6°F (1,482°C), and is essentially nontoxic. Thallium forms univalent thallous (Tl1+) and trivalent thallic (Tl3+) 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.28

In the early 1900s, thallium salts were used medicinally to treat syphilis, gonorrhea, tuberculosis, and as a depilatory for ringworm of the scalp.7,73 Although the usual oral dose given for ringworm was 7 to 8 mg/kg, fatal doses ranged from 6 to 40 mg/kg.16,62 Many cases of severe thallium poisoning (thallotoxicosis) resulted from this treatment, with one author summarizing nearly 700 cases and 46 deaths.75

Because thallium sulfate is odorless, tasteless, and highly toxic it became commercially available as a rodenticide and was sold as Thalgrain, Echol’s Roach Powder, Mo-Go, Martin’s Rat Stop liquid, and Senco Corn Mix. As a consequence of numerous case reports of unintentional poisonings,75,87 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.2,15,58,85,89,94,104,113 Additional cases of thallium poisoning are reported in the United States and other countries as a result of the use of thallium for suicide,101 homicide,23,57,66,71,79,81,91,100 through contamination of herbal products,18,97 and illicit drugs such as heroin, opium,1,34,84 and cocaine.48 Although occupational exposures to consequential amounts of thallium salts are uncommon, occupational toxicity is well described.42 Additionally, although small amounts of thallium salts (<1 mcg) are used as radioactive contrast to image tumors and to permit the visualization of cardiac function, the doses used are insignificant and pose no risk for thallium toxicity.73

The following discussion of thallium toxicity is based on exposures to inorganic thallium salts, which represents virtually the entire literature on thallium poisoning. Although exceedingly rare, exposures to organic thallium compounds are reported4 and should be assessed and managed in a fashion similar to that used for patients with inorganic exposures.

TOXICOKINETICS

Exposures usually occur via one of 3 routes: inhalation, ingestion, and absorption through intact skin. Thallium is rapidly absorbed following all routes of exposure. Bioavailability is greatest after ingestion and exceeds 90%.44 Distribution ...

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