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To many people, thallium is synonymous with rat poison. It is more toxic to mammals than mercury, cadmium, and lead and has been responsible for many deliberate, accidental, occupational, and therapeutic poisonings of people since its discovery in 1861. For the past 30 years or so, I have been fascinated by the risks associated with the ongoing release of this highly toxic and unpredictable metal into our environment.

Public angst and concern were first drawn in the late 1960s to reports of widespread contamination of the Great Lakes' ecosystems with toxic metals. At the time, I was a graduate student at the University of Toronto. Of the various groundbreaking studies on the hazards of heavy metals in the Great Lakes basin, I was captivated by reports that alluded to the fact that symptoms typical of thallium poisoning were being observed in many wildlife populations in the Great Lakes basin, and especially one report that claimed that nine out of the 34 bald eagles found sick or dying in 1971-72 in parts of the basin in the U.S. were poisoned by thallium. For some inexplicable reasons, these early reports on poisoning of wildlife by thallium were ignored by the scientific community and no efforts were made to conduct necessary follow-up studies. Nevertheless, they aroused my curiosity and served as my initiation into the literature on the environmental chemistry and toxicity of thallium.

Thallium is an enigmatic element to study because of its highly divergent properties predicated upon its oxidation status. Its paradoxical nature became obvious after its discovery as attempts were made to place the new element in the periodic classification of the elements. The physical properties of elemental thallium, including specific gravity, hardness, appearance, melting point, and electrical conductivity, were found to be similar to those of lead. The chemical properties of many Tl(I) (thallous) salts resemble those of lead, but the valency and other features of thallium were found to be too divergent from the lead family of elements.

CHECKUP Scan of a normal human heart after injection of radioactive thallium-201. The test can reveal defects in blood supply.
On the other hand, Tl(I) resembles the alkali metals in flame spectra; solubility of the hydroxide, sulfate, and carbonate in water; ready oxidation of the metal in air; existence of thallium alums; and isomorphism of some of its salts with those of potassium, cesium, and rubidium. But the absence of isomorphism and divergent properties of many of its common salts precluded thallium from the family of alkali metals. In his periodic classification of the elements, Dmitry Mendeleyev offered the clinching argument in favor of placing thallium among the aluminum group of elements based on the chemical properties of thallic salts. Variations in chemical personality and nonconforming behavior of thallium make for a fertile field for research but provide a minefield in any attempts to adequately assess its risks in the environment.

Thallium elicits some of the most complex and serious toxicities in living things, involving a wide range of organs and tissues. A recent study with planktonic communities showed that Tl3+ ions, a common form in aquatic environments, are about 34,000 times more toxic than cadmium ions [Environ. Sci. Technol., 37, 2720 (2003)]. This is serious, considering that organisms (especially mammals) at the top of the food chain are the most susceptible to thallium toxicosis. Recently, we have found elevated levels of thallium in Great Lakes fish [Bull. Environ. Contam. Toxicol., 67, 921 (2001)], which has raised the issue of bioaccumulation and biomagnification of this element in aquatic food webs and the potential risks to fish-eating members of the food chain. I keep wondering whether the dismissal of the early reports of thallium poisoning of bald eagles and other wildlife should, in fact, be regarded as a major scientific oversight of our time.

Compared with many heavy metals, thallium has a short history and what appears to be a rosy future. Its traditional uses (in rodenticides and insecticides, pigments, wood preservatives, and ore separation; in mercury lamps to increase the intensity and spectrum of the light; as catalysts in chemical syntheses; and so forth) are being phased out in deference to its toxicity. At the same time, there is increasing demand for thallium in the high-technology and future-technology fields.

Among the growing uses for thallium are in the semiconductor and laser industry, in fiber (optical) glass, in scintillographic imaging, in superconductivity, and as a molecular probe to emulate the biological function of alkali-metal ions. Recycling of thallium in commercial products is not yet a serious business, and one has to be concerned about the long-term environmental effects of the growing technological applications of one of the most toxic and eccentric metals known to humankind.

Jerome O. Nriagu is a professor in the School of Public Health at the University of Michigan, Ann Arbor. He is the editor of "Thallium in the Environment" as well as many other volumes on heavy metals in the environment.

Chemical & Engineering News
Copyright © 2003 American Chemical Society

Name: From the Greek thallos, green twig or shoot, after the color of its spectral lines.
Atomic mass: 204.38.
History: Discovered in 1861 by Sir William Crookes.
Occurrence: Thallium can be obtained as a by-product of producing sulfuric acid or refining zinc or lead.
Appearance: Silvery white metal.
Behavior: Thallium compounds are extremely
Uses: TI is used in fiber and low-melting glass. Many traditional uses have been phased out because of its toxicity.

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