TABLE OF CONTENTS Feb 2002 - 0 comments

Chemical Corner: Thallium

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By: Ron Brecher, Ph.D.
2002-02-01
Toxicity summary

In humans, thallium compounds are absorbed following ingestion, inhalation or skin contact, and are widely distributed within the body. Unlike some other metals, thallium does not form complexes with bone. The two main routes of elimination for thallium are the feces and urine.

Acute effects -- Symptoms of thallium toxicity appear within 12 to 24 hours of exposure. Death may result in severe cases. The first symptoms of acute toxicity are typically gastrointestinal hemorrhage and irritation, increased heart rate, and headache. Other symptoms may include abdominal pain, vomiting, and diarrhea. In more severe cases, gastroenteritis, neurological symptoms such as tremors, delirium, convulsions, paralysis, coma and death may occur. Parasthesia, numbness of fingers and toes, and reports of "burning feet" have been noted after low-level exposures. About 2 to 4 weeks after exposure, hair loss, white bands in the fingernails, atrophic skin changes, psychosis, paranoia, hallucinations, liver and kidney damage, or constipation may occur. Recovery from severe thallium toxicity is not immediate, and long-term effects may include intellectual and memory impairment, insomnia, tremors or psychiatric disorders.

Chronic effects -- Symptoms may not appear until several weeks of low-level daily exposure to thallium. After chronic low-level exposure, hair loss (often permanent), skin and nail changes, gastrointestinal changes, neurological and visual disturbances may result. Functional changes in the endocrine system are also possible effects of chronic exposure to thallium. Little information is available for humans chronically exposed to thallium.

Carcinogenicity -- Thallium is classified as Group D: Not classifiable as to Human Carcinogenicity by the U.S. EPA. No studies to date have indicated that thallium is carcinogenic in either humans or animals.

Reproductive and developmental toxicity -- Thallium compounds can cross the placenta, creating a potential for fetal exposure. Chronic thallium exposure during the first trimester of pregnancy can cause birth defects. Exposure after this period can cause damage to the developing nervous system. Low birth weight, rash, hair loss, and white bands on fingernails have been observed in newborn babies exposed in utero to thallium.

In rats, dwarfism and degenerative changes in liver and kidney tissue have been observed in litters exposed in utero to thallium. Sub-chronic studies in rats have indicated that the testes may also be susceptible to the adverse effects of thallium.

Sensitive populations -- Individuals with existing neurological damage, kidney or liver damage may be at greater risk to adverse effects. In particular, thallium may potentiate existing adverse neurological effects. Individuals with potassium deficiency are also at increased risk of toxicity, as potassium increases the elimination of thallium by the kidneys.

Ronald Brecher, Ph.D. is a founding partner of GLOBALTOX International Consultants Inc., with offices in Guelph, Ontario; Seattle, Washington; and, Portland, Oregon. Contributing authors: Karen Phillipps and Beta Montemayor.

Selected regulatory information

Agency Parameter Value

ACGIH 8-hour TWA 0.1 mg/m3

CCME interim soil quality 1 mg/kg

EPA AWQC 13 µg/L

EPA MCLG in drinking water 0.5 ppb

EPA MCL in drinking water 2 ppb

NIOSH/OSHA 8-hour TWA 0.1 mg/m3

Quick reference index

Description -- Thallium in its pure form is a bluish-white metal that is both odourless and tasteless. There are two common valences for thallium: thallous and thallic. The thallous form is the most common and stable. Usually, thallium is found in combination with other elements.

Uses -- Thallium is used in the semiconductor industry, for cardiac imaging and in the manufacturing of refractive glass. Historically, thallium compounds were used as rodenticides and pesticides, though these uses have been banned in the U.S. since 1972. The only registered agricultural use for thallium compounds in Canada was for thallium sulfate-based ant-traps, and this was discontinued in 1974.

Environmental fate -- Thallium is removed from the air by wet and dry deposition. Volatilization from soil and water is not likely. In soil, thallium is usually incorporated into clays and gels, although organic matter can adsorb thallium under reducing conditions. Thallium also bioaccumulates in aquatic organisms.

Sources -- The metal is widely distributed in trace amounts in the earth's crust. Thallium has also been detected in air effluent from cement factories, smelters, coal burning plants and waste incinerators, among other sources.

Exposure -- People may be exposed to thallium through air, food and water. Levels in air and water, however, are relatively low. Eating contaminated food and smoking are both considered routes of human exposure. Occupational exposure may occur in areas where thallium compounds are produced and/or used.

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