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Thallium is a soft, malleable gray metal that was previously widely used in rat poisons and insecticides. Thallium itself and compounds containing the element are highly toxic. It is particularly dangerous because compounds containing thallium are colourless, odourless, and tasteless. However, poisons containing thallium are still in use in some parts of the world.
Small amounts of thallium are normally found in the earth's crust and atmosphere. It is also present in small amounts in cigarette smoke. Thallium can be absorbed from the skin as well as be ingested through food and drink or inhaled. If a significant amount (significant poisoning is usually defined as ingesting over 8 milligrams per kilogram of body weight) of thallium enters the body, symptoms of thallium poisoning develop.
In the first 48 hours after serious thallium exposure, the affected individual will usually experience nausea, vomiting, and diarrhoea. Within a few days, symptoms of nervous system damage become apparent. These symptoms can include pain, loss of reflexes, convulsions, muscle wasting, headaches, numbness of fingers and toes, dementia, psychosis, and even coma. After two to three weeks, characteristic changes are seen in the bases of hair shafts, and there is hair loss (alopecia). Finally, after around three weeks post-exposure, heart rhythm disturbances may occur.
Thallium was serendipitously discovered by Sir William Crookes in 1861 while trying to extract selenium from the by-products of sulfuric acid production. Crookes named the new element "thallium" from the Greek thallos, meaning "green shoot or twig" after the bright green spectral emission lines that identified the element. In 1862, Claude-Auguste Lamy independently isolated thallium, studying both its chemical and physical properties.
In the past, thallium was used as a therapeutic agent to treat syphilis, gonorrhea, tuberculosis, and ringworm, and it was also used as a depilatory for excess hair. In the early part of the last century, a product known as Koremlu was marketed in the United States for the treatment of ringworm as well as a depilatory agent.Â By 1934, 692 cases of thallium poisoning were reportedÂ with at least 31 deaths. Thallium was also widely usedÂ as a rodenticide.Â Its use as a household rodenticide was banned in the United States in 1965 after multiple unintentional poisonings. Commercial use was banned a decade later. Unfortunately, unintentional poisonings are still reported in other countries where thallium is used as a rodenticide and ant killer.
Currently,Â thalliumÂ is used in the manufacture of electronic components, optical lenses, semiconductor materials, alloys, gamma radiation detection equipment, imitation jewellery, artist's paints, low temperature thermometers, and green fireworks. Trace amounts of thallium are used as a contrast agent in the visualization of cardiac function and tumours. Thallium exposure may occur at smelters in the maintenance and cleaning of ducts and flues and through contamination of cocaine, heroin, and herbal products. Criminal and unintentional thallium poisonings are still reported, some leading to death.
Thallium poisoning can be treated. Effective treatment to prevent absorption of thallium is available if therapy is begun within six hours following ingestion. The antidote against thallium (known as potassium ferrihexacyanoferrate, or Prussian blue or Berlin blue, which is a solid ion exchange material which absorbs thallium and releases potassium) works by sequestering thallium molecules and preventing their absorption by the intestine. Other treatments that may be successful for victims of thallium poisoning include forced diuresis, treatment with potassium chloride (this promotes the renal excretion of thallium), and peritoneal dialysis.
Polonium-210 (is another poison) is a highly radioactive and toxic element that releases alpha particles. Its typical uses include devices that eliminate static charges and dust in textile mills, photographic plates and phonographs/records. It is found in small amounts as a contaminant in cigarette smoke and is likely a significant contributor to lung cancer.
Alpha particles are ionizing radiation that can be stopped simply with a piece of paper, or by the dead superficial layer of skin. The particles release all their energy in a very short distance, so when polonium-210 is placed on the skin, it is not dangerous; however, when taken into the body via inhalation or ingestion, polonium can enter the blood stream and alpha particles can impact organs and vital tissues directly. Polonium-210 is excreted in faeces and urine over a period of several months.
The death of Aleksandr Litvinenko from polonium poisoning is the first case in history; it was also the first case of someone dying from the acute effects of alpha radiation from any source. Polonium-210 has a short half-life, 138.38 days. This means that the material used to poison Litvinenko had to have been produced in the relatively recent past - it could not, for example, have been obtained from an old disused radioisotope source. Such sources using other radioisotopes, like cesium-137 and cobalt-60, have long been a concern for their potential use in terrorism and other criminal actions, and have been the cause of serious accidental exposures in the past, most notably in the Goiânia, Brazil disaster in September 1987 that significantly exposed 244 people and killed 2. Although mathematically it would be possible to have a toxic amount still left from an initial enormous quantity after as long as a few years, it does not make a plausible theory of the case that Litvinenko's attackers acquired their material so long ago.
If someone is poisoned by a minimal lethal dose of a radionuclide they do not become ill right away. Instead they steadily accumulate radiation exposure as long as the material remains in their body. Radiation has relatively small ratios between an exposure that first shows any apparent illness, an exposure that has a significant chance of causing death, and an exposure that is almost certainly lethal. There is thus a latent period while exposure accumulates and finally reaches a level where illness becomes evident. After this point severity of exposure rapidly increases.
Polonium, when absorbed orally, tends to distribute widely through the tissues of the body, approximating whole body radiation exposure. Polonium is excreted fairly quickly, its effective half-life in the body is about 30 days (mostly due to excretion, but also due to radioactive decay). This results in a maximum total exposure equal to about 43 times the first day exposure. Most of this is accumulated in the first 30 days, but significant amounts occur of the next several weeks following.
Because of its short half-life and rapid decay, a small quantity of polonium-210 is intensely active. Its "specific activity" is 4490 curies/g (166 TBq/g). Thus 4 millicuries (a minimal lethal dose for 80 kg person) is 0.89 micrograms, an almost invisible speck of matter.
In the case of absorbing a minimal lethal dose no initial illness would result. The decay of the polonium would deliver an accumulating exposure of 16 cGy a day initially. No visible effect would be expected for about a week, with mild symptoms developing over the next week. Serious illness would not be expected for a month or more. This delay is partly due to the time is takes to accumulate dangerous internal exposures, and partly due to the time delay of 1-2 weeks after dangerous exposure before serious illness becomes evident. The exposure rises eventually to 700 cGy after a few months. The actual toxic effect of this exposure would be lower than what the table below indicates, because radiation received over a period of time is somewhat less harmful than a dose received all at once. Nonetheless death would be expected from the poisoning within two or three months.
The polonium-210 dose that will kill 50 percent of persons who internalize it is about 100,000th of a milligram, one-million times more toxic than cyanide. Some commercially manufactured antistatic devices may contain as much as 500 micro-curies of polonium-210, theoretically enough radioactive material to kill 5,000 persons. However, this polonium is affixed in a gold foil amalgam; extremely sophisticated techniques and advanced technical knowledge would be required to weaponize such polonium.
In its purest form, the amount of polonium-210 that would fit on the tip of a pen (0.5 mm3), if properly dispersed, could kill 500 persons. In the U.S., access to pure radioactive material such as polonium-210 is heavily government-regulated and requires a license for its handling and use.
It has been suggested that Dimercaprol, also known as British Anti-Lewisite (can be used to decontaminate humans), is a chelating agent that may be effective as a treatment for Polonium-210 poisoning.