Polonium | biology and toxicity

Biology and toxicity

Overview

Polonium is highly dangerous and has no biological role.[14] By mass, polonium-210 is around 250,000 times more toxic than hydrogen cyanide (the 50 for 210Po is less than 1 microgram for an average adult (see below) compared with about 250 milligrams for hydrogen cyanide[73]). The main hazard is its intense radioactivity (as an alpha emitter), which makes it difficult to handle safely. Even in microgram amounts, handling 210Po is extremely dangerous, requiring specialized equipment (a negative pressure alpha glove box equipped with high-performance filters), adequate monitoring, and strict handling procedures to avoid any contamination. Alpha particles emitted by polonium will damage organic tissue easily if polonium is ingested, inhaled, or absorbed, although they do not penetrate the epidermis and hence are not hazardous as long as the alpha particles remain outside the body. Wearing chemically resistant and intact gloves is a mandatory precaution to avoid transcutaneous diffusion of polonium directly through the skin. Polonium delivered in concentrated nitric acid can easily diffuse through inadequate gloves (e.g., latex gloves) or the acid may damage the gloves.[74]

Polonium does not have toxic chemical properties.[75]

It has been reported that some microbes can methylate polonium by the action of methylcobalamin.[76][77] This is similar to the way in which mercury, selenium, and tellurium are methylated in living things to create organometallic compounds. Studies investigating the metabolism of polonium-210 in rats have shown that only 0.002 to 0.009% of polonium-210 ingested is excreted as volatile polonium-210.[78]

Acute effects

The median lethal dose (LD50) for acute radiation exposure is about 4.5 Sv.[79] The committed effective dose equivalent 210Po is 0.51 µSv/Bq if ingested, and 2.5 µSv/Bq if inhaled.[80] A fatal 4.5 Sv dose can be caused by ingesting 8.8 MBq (240 μCi), about 50 nanograms (ng), or inhaling 1.8 MBq (49 μCi), about 10 ng. One gram of 210Po could thus in theory poison 20 million people, of whom 10 million would die. The actual toxicity of 210Po is lower than these estimates because radiation exposure that is spread out over several weeks (the biological half-life of polonium in humans is 30 to 50 days[81]) is somewhat less damaging than an instantaneous dose. It has been estimated that a median lethal dose of 210Po is 15 megabecquerels (0.41 mCi), or 0.089 micrograms (μg), still an extremely small amount.[82][83] For comparison, one grain of table salt is about 0.06 mg = 60 μg.[84]

Long term (chronic) effects

In addition to the acute effects, radiation exposure (both internal and external) carries a long-term risk of death from cancer of 5–10% per Sv.[79] The general population is exposed to small amounts of polonium as a radon daughter in indoor air; the isotopes 214Po and 218Po are thought to cause the majority[85] of the estimated 15,000–22,000 lung cancer deaths in the US every year that have been attributed to indoor radon.[86] Tobacco smoking causes additional exposure to polonium.[87]

Regulatory exposure limits and handling

The maximum allowable body burden for ingested 210Po is only 1.1 kBq (30 nCi), which is equivalent to a particle massing only 6.8 picograms. The maximum permissible workplace concentration of airborne 210Po is about 10 Bq/m3 (3×10−10 µCi/cm3).[88] The target organs for polonium in humans are the spleen and liver.[89] As the spleen (150 g) and the liver (1.3 to 3 kg) are much smaller than the rest of the body, if the polonium is concentrated in these vital organs, it is a greater threat to life than the dose which would be suffered (on average) by the whole body if it were spread evenly throughout the body, in the same way as caesium or tritium (as T2O).

210Po is widely used in industry, and readily available with little regulation or restriction.[citation needed][90] In the US, a tracking system run by the Nuclear Regulatory Commission was implemented in 2007 to register purchases of more than 16 curies (590 GBq) of polonium-210 (enough to make up 5,000 lethal doses). The IAEA "is said to be considering tighter regulations ... There is talk that it might tighten the polonium reporting requirement by a factor of 10, to 1.6 curies (59 GBq)."[91] As of 2013, this is still the only alpha emitting byproduct material available, as a NRC Exempt Quantity, which may be held without a radioactive material license.[citation needed]

Polonium and its compounds must be handled in a glove box, which is further enclosed in another box, maintained at a slightly higher pressure than the glove box to prevent the radioactive materials from leaking out. Gloves made of natural rubber do not provide sufficient protection against the radiation from polonium; surgical gloves are necessary. Neoprene gloves shield radiation from polonium better than natural rubber.[92]

Cases of poisoning

20th century

Polonium was administered to humans for experimental purposes from 1943 to 1947; it was injected into four hospitalised patients, and orally given to a fifth. Studies such as this were funded by the Manhattan Project and the AEC and conducted at the University of Rochester. The objective was to obtain data on human excretion of polonium to correlate with more extensive data from rats. Patients selected as subjects were chosen because experimenters wanted persons who had not been exposed to polonium either through work or accident. All subjects had incurable diseases. Excretion of polonium was followed, and an autopsy was conducted at that time on the deceased patient to determine which organs absorbed the polonium. Patients' ages ranged from "early thirties" to "early forties". The experiments were described in Chapter 3 of Biological Studies with Polonium, Radium, and Plutonium, National Nuclear Energy Series, Volume VI-3, McGraw-Hill, New York, 1950. Not specified is the isotope under study, but at the time polonium-210 was the most readily available polonium isotope. The DoE factsheet submitted for this experiment reported no follow up on these subjects.[93]

It has also been suggested that Irène Joliot-Curie was the first person to die from the radiation effects of polonium. She was accidentally exposed to polonium in 1946 when a sealed capsule of the element exploded on her laboratory bench. In 1956, she died from leukemia.[94]

According to the 2008 book The Bomb in the Basement, several deaths in Israel during 1957–1969 were caused by 210Po.[95] A leak was discovered at a Weizmann Institute laboratory in 1957. Traces of 210Po were found on the hands of Professor Dror Sadeh, a physicist who researched radioactive materials. Medical tests indicated no harm, but the tests did not include bone marrow. Sadeh died from cancer. One of his students died of leukemia, and two colleagues died after a few years, both from cancer. The issue was investigated secretly, and there was never any formal admission that a connection between the leak and the deaths had existed.[96]

21st century

The cause of death in the 2006 homicide of Alexander Litvinenko, a Russian FSB agent who had defected to the British MI6 intelligence agency, was determined to be 210Po poisoning.[97][98] According to Prof. Nick Priest of Middlesex University, an environmental toxicologist and radiation expert, speaking on Sky News on December 3, 2006, Litvinenko was probably the first person to die of the acute α-radiation effects of 210Po.[99]

Abnormally high concentrations of 210Po were detected in July 2012 in clothes and personal belongings of the Palestinian leader Yasser Arafat, a heavy smoker, who died on 11 November 2004 of uncertain causes. The spokesman for the Institut de Radiophysique in Lausanne, Switzerland, where those items were analyzed, stressed that the "clinical symptoms described in Arafat's medical reports were not consistent with polonium-210 and that conclusions could not be drawn as to whether the Palestinian leader was poisoned or not", and that "the only way to confirm the findings would be to exhume Arafat's body to test it for polonium-210."[100] On 27 November 2012 Arafat's body was exhumed, and samples were taken for separate analysis by experts from France, Switzerland and Russia.[101] On 12 October 2013, The Lancet published the group's finding that high levels of the element were found in Arafat's blood, urine, and in saliva stains on his clothes and toothbrush.[102] The French tests later found some polonium but stated it was from "natural environmental origin".[103] Following later Russian tests, Vladimir Uiba, the head of the Russian Federal Medical and Biological Agency, stated in December 2013 that Arafat died of natural causes, and they had no plans to conduct further tests.[103]

Treatment

It has been suggested that chelation agents, such as British Anti-Lewisite (dimercaprol), can be used to decontaminate humans.[104] In one experiment, rats were given a fatal dose of 1.45 MBq/kg (8.7 ng/kg) of 210Po; all untreated rats were dead after 44 days, but 90% of the rats treated with the chelation agent HOEtTTC remained alive for 5 months.[105]

Detection in biological specimens

Polonium-210 may be quantified in biological specimens by alpha particle spectrometry to confirm a diagnosis of poisoning in hospitalized patients or to provide evidence in a medicolegal death investigation. The baseline urinary excretion of polonium-210 in healthy persons due to routine exposure to environmental sources is normally in a range of 5–15 mBq/day. Levels in excess of 30 mBq/day are suggestive of excessive exposure to the radionuclide.[106]

Occurrence in humans and the biosphere

Polonium-210 is widespread in the biosphere, including in human tissues, because of its position in the uranium-238 decay chain. Natural uranium-238 in the Earth's crust decays through a series of solid radioactive intermediates including radium-226 to the radioactive noble gas radon-222, some of which, during its 3.8-day half-life, diffuses into the atmosphere. There it decays through several more steps to polonium-210, much of which, during its 138-day half-life, is washed back down to the Earth's surface, thus entering the biosphere, before finally decaying to stable lead-206.[107][108][109]

As early as the 1920s, French biologist Antoine Lacassagne [fr], using polonium provided by his colleague Marie Curie, showed that the element has a specific pattern of uptake in rabbit tissues, with high concentrations, particularly in liver, kidney, and testes.[110] More recent evidence suggests that this behavior results from polonium substituting for its congener sulfur, also in group 16 of the periodic table, in sulfur-containing amino-acids or related molecules[111] and that similar patterns of distribution occur in human tissues.[112] Polonium is indeed an element naturally present in all humans, contributing appreciably to natural background dose, with wide geographical and cultural variations, and particularly high levels in arctic residents, for example.[113]

Tobacco

Polonium-210 in tobacco contributes to many of the cases of lung cancer worldwide. Most of this polonium is derived from lead-210 deposited on tobacco leaves from the atmosphere; the lead-210 is a product of radon-222 gas, much of which appears to originate from the decay of radium-226 from fertilizers applied to the tobacco soils.[48][114][115][116][117]

The presence of polonium in tobacco smoke has been known since the early 1960s.[118][119] Some of the world's biggest tobacco firms researched ways to remove the substance—to no avail—over a 40-year period. The results were never published.[48]

Food

Polonium is found in the food chain, especially in seafood.[120][121]