Occurrence and Uses
Thallium (Tl) is fairly widely distributed in the earth’s crust in very low concentrations; it is also found as an accompanying substance of other heavy metals in pyrites and blendes, and in the manganese nodules on the ocean floor.
Thallium is used in the manufacture of thallium salts, mercury alloys, low-melting glasses, photoelectric cells, lamps and electronics. It is used in an alloy with mercury in low-range glass thermometers and in some switches. It has also been used in semiconductor research and in myocardial imaging. Thallium is a catalyst in organic synthesis.
Thallium compounds are used in infrared spectrometers, crystals and other optical systems. They are useful for colouring glass. While many thallium salts have been prepared, few are of commercial significance.
Thallium hydroxide (TlOH), or thallous hydroxide, is produced by dissolving thallium oxide in water, or by treating thallium sulphate with barium hydroxide solution. It can be used in the preparation of thallium oxide, thallium sulphate or thallium carbonate.
Thallium sulphate (Tl2SO4), or thallous sulphate, is produced by dissolving thallium in hot concentrated sulphuric acid or by neutralizing thallium hydroxide with dilute sulphuric acid, followed by crystallization. Because of its outstanding efficacy in the destruction of vermin, particularly rats and mice, thallium sulphate is one of the most important of the thallium salts. However, some western European countries and the United States have prohibited the use of thallium on the grounds that it is inadvisable that such a toxic substance should be easily obtainable. In other countries, following the development of warfarin resistance in rats, the use of thallium sulphate has increased. Thallium sulphate is also used in semiconductor research, optical systems and in photoelectric cells.
Thallium is a skin sensitizer and cumulative poison which is toxic by ingestion, inhalation or skin absorption. Occupational exposure may occur during the extraction of the metal from thallium-bearing ores. Inhalation of thallium has resulted from the handling of flue dusts and the dusts from roasting of pyrites. Exposure may also occur during the manufacture and use of thallium-salt vermin exterminators, the manufacture of thallium-containing lenses and separation of industrial diamonds. The toxic action of thallium and its salts is well documented from reports of cases of acute non-occupational poisoning (not infrequently fatal) and from instances of suicidal and homicidal use.
Occupational thallium poisoning is normally the result of moderate, long-term exposure, and the symptoms are usually far less marked than those observed in acute accidental, suicidal or homicidal intoxication. The course is usually unremarkable and characterized by subjective symptoms such as asthenia, irritability, pains in the legs, some nervous system disorders. Objective symptoms of polyneuritis may not be demonstrable for quite some time. The early neurologic findings include changes in the superficially provoked tendon reflexes and a pronounced weakness and fall-off in the speed of pupil reflexes.
The victim’s occupational history will usually give the first clue to the diagnosis of thallium poisoning since a considerable time may elapse before the rather vague initial symptoms are replaced by the polyneuritis followed by loss of hair. Where massive hair loss occurs, the likelihood of thallium poisoning is readily suspected. However, in occupational poisoning, where exposure is usually moderate but protracted, the loss of hair may be a late symptom and often noticeable only after the appearance of polyneuritis; in cases of slight poisoning, it may not occur at all.
The two principal criteria for the diagnosis of occupational thallium poisoning are:
- occupational history which shows that the patient has or may have been exposed to thallium in such work as rodenticide handling, thallium, lead, zinc or cadmium production, or the production or use of various thallium salts
- neurological symptoms, dominated initially by subjective changes in the form of paraesthesia (both hyperaesthesia and hypoaesthesia) and, subsequently, by reflex changes.
Concentrations of Tl in urine above 500 µg/l have been associated with clinical poisoning. At concentrations of 5 to 500 µg/l the magnitude of risk and severity of adverse effects on humans are uncertain.
Long-term experiments with radioactive thallium have shown marked excretion of thallium in both urine and faeces. On autopsy, the highest thallium concentrations are found in the kidneys, but moderate concentrations may also be present in the liver, other internal organs, muscles and bones. It is striking that, although the principal signs and symptoms of thallium poisoning originate from the central nervous system, only very low concentrations of thallium are retained there. This may be due to extreme sensitivity to even very small amounts of the thallium acting on the enzymes, the transmission substances, or directly on the brain cells.
Safety and Health Measures
The most effective measure against the dangers associated with the manufacture and use of this group of extremely toxic substances is the substitution of a less harmful material. This measure should be adopted wherever possible. When thallium or its compounds must be used, the strictest safety precautions should be taken to ensure that the concentration in the workplace air is kept below permissible limits and that skin contact is prevented. Continuous inhalation of such concentrations of thallium during normal working days of 8 hours may cause the urine level to exceed the above permissible levels.
Persons involved in work with thallium and its compounds should wear personal protective equipment, and respiratory protective equipment is essential where there is the possibility of dangerous inhalation of airborne dust. A complete set of working clothes is essential; these clothes should be washed regularly and kept in accommodation separate from that employed for ordinary clothes. Washing and shower facilities should be provided and scrupulous personal hygiene encouraged. Workrooms must be kept scrupulously clean, and eating, drinking or smoking at the workplace prohibited.