CAS No. 7440-28-0
Elemental thallium is a blue-white metal found in small amounts in soil and in sulfide-based minerals. In the past, thallium was obtained as a by-product of smelting other metals; however, it has not been specifically mined or refined in the United States since 1984. It is still used in relatively small amounts in pharmaceutical and electronics manufacturing, the latter being the current major industrial consumer of thallium in this country. In the United States, thallium has been restricted from use in rodenticides and depilatory cosmetics.
Thallium exposure occurs primarily from industrial processes such as coal-burning and smelting. From these and other sources, thallium is produced in a fine particulate form that can be absorbed through inhalation or ingestion. Thallium disappears from the blood with a half-life of several days, representing distribution into other tissues. In addition, thallium readily crosses the placenta and also distributes into breast milk. Elimination from the body tissues occurs slowly through urine and feces (Blanchardon et al., 2005).
Human health effects from thallium at low environmental doses or at biomonitored levels from low environmental exposures are unknown. Thallium produces toxicity by replacing intracellular potassium in the body, although additional mechanisms of action are possible. Since thallium salts are colorless, odorless, and tasteless, the potential for undetected malevolent use exists. Severe accidental thallium poisonings from ingesting of rat poisons that contained water-soluble thallium salt have occurred. Relatively high-dose intentional or accidental ingestion can result in gastrointestinal symptoms followed by multi-organ failure, neurologic injury, and death. Peripheral neuropathy and alopecia are well-documented effects of acute and chronic exposures. Chronic high-level exposures have been associated with weight loss, arthralgias, and polyneuropathy (ATSDR, 1992).
Workplace air standards and guidelines for external exposure are established by OSHA and ACGIH, respectively, and a drinking water standard has been established by U.S. EPA. IARC and NTP consider the evidence for the carcinogenicity of thallium as inadequate or unclassifiable. Information about external exposure (i.e., environmental levels) and health effects is available from ATSDR at http://www.atsdr.cdc.gov/toxprofiles/index.asp.
Urinary thallium levels reflect recent exposure. Levels of thallium in urine for the U.S. population have been well characterized in NHANES since1999-2000 (CDC, 2012). These urine levels are generally comparable to levels observed in earlier studies of general populations (Brockhaus et al., 1981; Minoia et al., 1990; Paschal et al., 1998; Schaller et al., 1980; White and Sabbioni, 1998). Urinary concentrations of 100 µg/L in asymptomatic workers (500 times higher than median levels in the U.S. population) are thought to correspond to workplace exposures at the threshold limit value of 0.1 mg/m3 (Marcus, 1985). Brockhaus et al. (1981) studied 1,265 people living near a thallium-emitting cement plant in Germany. Nearby residents were exposed by eating garden plants that had been contaminated by the thallium. Seventy-eight percent of the urine specimens in that study contained greater than 1 µg/L, with concentrations ranging up to 76.5 µg/L. There was no increase in the prevalence of symptoms at levels less than 20 µg/L and only a slight increase in nonspecific symptoms greater than 20 µg/L.
Finding a measurable amount of thallium in urine does not imply that the level of thallium causes an adverse health effect. Biomonitoring studies on levels of thallium provide physicians and public health officials with reference values so that they can determine whether people have been exposed to higher levels of thallium than are found in the general population. Biomonitoring data can also help scientists plan and conduct research on exposure and health effects.
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