Thallium (whose chemical symbol is Tl) is a natural component of the Earth's surface (earth's crust). Although present in the environment in small quantities, it is 10 times more abundant than silver and other natural metals. Thallium is a soft and malleable metal which, in its pure form, is odorless and tasteless but when exposed to air it rapidly develops a bluish-gray color similar to that of lead. In the presence of water it forms thallium hydroxides. It exists in nature in the form of oxide or salts such as, for example, chlorides and fluorides, dispersed in clays, muds and granites; it is also found in various sulfur minerals. Thallium compounds are volatile at high temperatures.
The presence of thallium in the environment derives above all from activities carried out by man, including the combustion of coal to produce energy and for domestic use, the smelting of ferrous and non-ferrous metals, the extraction of minerals (gold, copper , lead, zinc and uranium) and various industrial processes.
The thallium released into the air is deposited on the ground and in surface waters where it can remain for a long time. The thallium present in the soil is absorbed by plants through the roots and leaves where it can accumulate.
Until the early twentieth century thallium salts were used to treat syphilis and malaria, to reduce night sweats in tuberculosis patients, to induce hair loss in people with a scalp infection known as ringworm, from the name of the mushroom (tinea capitis) which causes it (ringworm).
In the first half of the 1900s some thallium salts (for example, sulfate, acetate and carbonate) with high toxicity were massively used in the preparation of insecticides and poisons for rodents (rodenticides), a use that caused damage to animals and vegetation.
These uses have been abandoned and prohibited (at least in many countries), although thallium sulfate is still sold in some developing countries where its application as a pesticide or pesticide is still permitted.
Currently, thallium in the form of alloys with silver and aluminum is mainly used for high-tech industrial applications such as, for example, in high-temperature superconducting materials, in lasers, in special low-melting point glasses, in photocells and optical systems. and in electronics. In addition, thallium salts are used as reagents in chemical research as well as in the manufacture of fireworks, pigments and dyes.
Sources of exposure
Human exposure to thallium can be environmental or occupational and can occur through:
- inhalation of dust and fumes of thallium compounds
- direct contact of skin and mucous membranes with metal thallium or its salts
- ingestion of contaminated water and food, especially fruit and vegetables and crops grown in the vicinity of coal-fired power plants or industrial plants that release thallium into the air and onto the soil, with subsequent absorption by plants through the roots
In the working environment (industries or professional activities where thallium is used) exposure occurs mainly by direct contact with the skin and by inhalation.
For the general population the main source is the environmental one: the levels of thallium in the air, in the water and in the soil are however generally low, but there may be exceptions.
Effects on health
Thallium is easily absorbed and is rapidly distributed through the circulatory system (already 1 hour after exposure) in the cells and tissues of plants and animals. It interferes with cellular metabolism, because it replaces potassium, an essential element for many reactions necessary for the life of the cell. In the case of mammals, thallium is mainly distributed in the bones, kidneys, liver and nervous system. Furthermore, thallium is able to cross the blood-brain, placental and gonadal barrier.
It can also be found in breast milk, but there are few data available. The time it takes to halve its concentration in the blood (half-life) in humans is between 10 and 30 days. Its elimination occurs mainly through urine and, to a lesser extent, through sweat, milk, hair, nails and feces.
The accidental and rapid intake of high doses of thallium causes toxic effects in a short time (acute) which can even lead to death. The disorders (symptoms) caused by acute intoxication are mainly:
- gastrointestinal, severe abdominal pain, nausea, vomiting, diarrhea, liver toxicity (liver necrosis)
- cardiac, low blood pressure (hypotension) and heart beat (bradycardia), followed by high blood pressure (hypertension) with fast heart beats (tachycardia) and arrhythmias and heart attack
- respirators, acute bronchitis, pulmonary edema
- hair loss, alopecia, atrophy of the hair follicles
- cutaneous, redness of the skin (erythema), acne and peeling of the skin
- neurological, neuropathies of the central and peripheral nervous system, with tingling, ascending paralysis up to respiratory arrest; loss of consciousness up to the stage of coma
Disorders (symptoms) can appear simultaneously or sequentially, at short intervals of time. The fatal dose of thallium in humans is approximately 0.5-1 gram (or 8-16 milligrams per pound of body weight for an adult weighing 60 pounds). Death from acute poisoning caused by a lethal dose of thallium occurs. checkout within 7-10 days.
Generally, prolonged exposure to thallium over time and at low concentrations (chronic exposure) occurs above all in professional settings due to the inhalation of dust and smoke.
Studies on exposed workers have shown that the disorders (symptoms) caused by chronic thallium intoxication can occur even after long periods and are largely similar to acute ones; in particular they include:
- polyneuropathy, tingling and muscle and joint pains, feelings of paralysis, weakness, tremors, headache (headache), insomnia, anorexia, depression, anxiety, apathy
- loss of hair, eyelashes and eyebrows, nail problems
- visual disturbances, reduced vision, paralysis of the eye muscles, altered color perception
In some cases, chronic inhalation of thallium has been associated with a decrease in thyroid hormones (T4 and FT4) and thyroid dysfunction.
Effects caused in humans by exposure to thallium levels usually present in environmental matrices (water, air and soil), which are generally very low, have never been reported.
To date, there is no evidence of gene mutations and cancers caused by thallium in humans, nor of birth defects (congenital) present in children born to mothers who consumed fruit and vegetables contaminated with thallium at low levels. , however, they are very scarce.
Diagnosis and therapy
The assessment (diagnosis) of thallium poisoning is difficult because it is often masked by other disorders present in the potentially exposed person and the metal is not easily detectable with classical analyzes. For this, and for its lack of flavor, it was referred to in the past as the poison of the poisoner, since it is not easy to identify it as a cause of death.
In addition to a careful observation of the disorders (symptoms), the most sensitive way to ascertain (diagnose) thallium intoxication is to measure its presence in urine and hair. Blood levels of thallium are not a good indicator of exposure because the metal remains in circulation for a short time. In the case of ingestion, X-rays of the gastrointestinal tract can also help because thallium is well detected by X-rays. Exposure, however, does not automatically mean that a disease will develop, as it depends on the individual dose and response.
In case of intoxication ascertained by the doctor, the treatment involves a gastric lavage with activated charcoal to which the thallium adheres, decreasing the absorption at the gastrointestinal level, possibly associated with the intake of laxatives and diuretics: the aim is to lower the levels of thallium in the blood as quickly as possible, in order to counteract the disorders and prevent, above all, permanent brain damage.For this reason, in the most serious cases, in the hospital it is possible to resort to repeated hemodialysis treatments or hemodialysis treatments on activated carbon or adsorbent resin, in which the blood circulating extracorporeal, made non-coagulable, is passed through a column containing the adsorbing particles to which the thallium present adheres. Today, due to its safety and efficacy profile, treatment with Prussian blue (ferric ferrocyanide) is considered the most appropriate, always to be carried out after a positive opinion from the doctor and under his control.
Prevention and control
Thallium poisoning can be prevented through careful control of both environmental and occupational exposure levels.
Any presence in water intended for human consumption must therefore be controlled over time (monitored) especially in areas where thallium is naturally present.
In Italy, to prevent health risks, the Istituto Superiore di Sanità (ISS) has recommended the precautionary adoption of a limit value of 0.002 milligrams per liter (equal to 2 micrograms per liter) for water intended for human consumption, and not to allow the use of water with thallium levels higher than 0.04 milligrams per liter, not even for personal hygiene practices.
An identical value has been set for thallium in drinking water in the United States by the US Environmental Protection Agency (US EPA) as early as 1994.
Thallium can be effectively removed from water by appropriate treatments, reducing its concentration to the lowest possible levels. Only a few cases of thallium contamination of drinking water have been documented, one of these was reported in Italy in September 2014 in the waters distributed in the municipality of Pietrasanta (in the province of Lucca), in concentrations ranging from 1.7 to 10, 1 µg / L.the presence of thallium is to be associated with the presence of mines of pyrite, barite and sulphides of lead, silver and zinc active until 1989 in the area. The emergency solution provided for the application of the Water Safety Plans (PSA), which have recently become part of the Italian legislation on the quality of drinking water, described in a dedicated Istisan Report.
Protective measures are also adopted to reduce the inhalation of thallium in the workplace. The OSHA (Occupational Safety and Health Administration) has established 0.1 milligrams per cubic meter as the limit concentration (TLV) of thallium in the air in the workplace. Furthermore, the NIOSH (National Institute for Occupational Safety and Health) has indicated in 15 milligrams per cubic meter the concentration that must be considered immediately harmful to health.
As an effective method of control, periodic checks of the levels of thallium in the urine should be carried out by those employed in processes involving exposure to the mineral.
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