Reminder of events
On November 23, 2006, Alexander Litvinenko died of heart failure at London's University College Hospital (UCH). His health problems began in early November, a few days after he had met with several of his contacts at his hotel and in a Japanese restaurant in London. He was admitted to UCH on November 17, 2006 with recurrent bouts of vomiting, irregular heartbeat, kidney problems, a very significant drop in his white blood cell count. He also suffered total hair loss within a week. His throat was also inflamed, with the result that he had stopped eating 18 days earlier. Roger Cox, a British professor at the Health Protection Agency (HPA), declared that polonium-210 had been detected in Mr. Litvinenko's urine. Furthermore, the authorities in charge of investigations announced that radioactive traces had been found in various places where the deceased had been seen.
Polonium-210 : characteristics and origins
Discovered by Marie Curie in 1898, polonium-210 (210Po), a decay product of uranium-238, is a naturally-occurring radionuclide found in trace form in every environmental compartment, along with radon-222 and lead-210, its precursor nuclides.
It is the most abundant of polonium's 29 isotopes and has a radioactive half-life of 138.4 days.
Its specific activity is very high: 1.66 x 1014 Bq per gram.
It emits 99.999% alpha particles, with an energy level of 5.304 MeV, and 0.001% gamma rays, representing an energy level of 0.80 MeV. It transforms into stable lead-206.
Radon-222, a radioactive gas and precursor of 210Po, is the reason why 210Po is found permanently suspended in the air in aerosol form. It is generally found in the air at a concentration of some 50 mBq/m3, although this can vary according to the degree of local radon exhalation and the existence of industrial activities favoring its emission (mining, phosphate industry, etc.). Volcanic activity is also responsible for a considerable amount of emissions.
Polonium-210 is also found in the top few centimeters of the soil. This is the result of radon-222 decay in the top layers of the soil and atmospheric fallout of the 210Po suspended in the air. Polonium-210 then binds almost irreversibly to particles of soil by coprecipitating with metal hydroxides or as a sulfide. It is not a very mobile element. Concentration levels in the soil vary from 10 to 200 Bq/kg (dry soil). Significantly higher concentrations (from 15,000 to 22,000 Bq/kg) can be found in uranium mine tailings.
It is also found in the world's oceans, either through direct emission into the water or via exchange mechanisms with volatile compounds in the air. It is generally insoluble and associated with a particulate or colloid phase. For this reason, it is found trapped in sediments, bound to other mineral phases, or as a sulfide precipitate.
Polonium-210 also has an artificial source, as it can be produced in a nuclear reactor by bombarding bismuth-209 with neutrons to obtain bismuth-210, which then turns into polonium-210 over a physical half-life of 5 days as a result of beta emission. Polonium can be used in industrial air ionizers to prevent the buildup of electrostatic charges that are generated, for example, when rolling paper, wire or metal foil.
Environmental behavior of polonium-210
Terrestrial plants take up 210Po mostly through their leaves. Little or no incorporation or translocation occurs after that and most of the polonium remains concentrated in the leaves(*).
In the case of animals, ingestion is the main route of entry. Concentration factors are relatively high but vary with the animals' habits (quantity ingested) and the target organs. For example, the concentration of 210Po ranges from 3.7 x 10-2 Bq/kg (fresh product) in ox muscle to 332 Bq/kg in caribou liver.
Aquatic organisms, especially plankton and invertebrates, can concentrate the 210Po contained in water in their soft tissues or, in the case of animals, in their internal organs. Fish have less ability to concentrate polonium, the lowest specific activity levels being observed in the flesh, i.e. the part destined for human consumption.
In seawater, 210Po has a strong affinity for suspended matter. Ingestion is the main route of entry in animals. Significant concentrations of polonium-210 are observed in marine animals, with certain organs, including the digestive gland in mollusks, the hepatopancreas in crustaceans and the liver in fish, exhibiting activity levels far above those observed in the surrounding environment. To illustrate this point, the 210Po concentration generally observed in mussels is between 150 and 600 Bq/kg (dry product). At just a few Bq/kg (dry product), concentrations are lower in fish. Polonium-210 therefore makes a large contribution to the radiation doses absorbed by humans through the consumption of fish and seafood.
(*) Polonium-210 is found particularly in tobacco leaves, which explains why it is found in greater quantities in smokers.
Behavior of polonium-210 in the human body
The alpha particles emitted by 210Po only travel a very short distance in the air (no more than a few centimeters). This radionuclide therefore only presents a health risk in the event of internal contamination (intake by ingestion, inhalation or injection) or direct contact with the skin.
As polonium-210 is found everywhere in the environment, humans are constantly exposed to contamination by inhalation and ingestion. The effective dose due to 210Po in the adult is approximately 0.07 mSv per year. Polonium-210 intake in humans is naturally excreted in varying quantities in the urine and feces, with uranium miners and smokers eliminating greater quantities than nonsmokers. Considering the balance between intake and excretion, the total activity found continuously in the average adult is estimated at some 30 Bq (or 0.18 picograms).
Polonium-210 is closely related to sulfur and selenium and its biological behavior resembles that of the rare earths. Following bodily intake, polonium-210 is carried rapidly to the soft tissues via the blood stream. The quantity absorbed following ingestion varies between 10% and 15% depending on the physical-chemical form of administration. In the blood and plasma, it exhibits a very strong affinity for red blood cells (90% of polonium-210 contained in the formed elements of the blood is related to the red corpuscles) and plasma proteins. About 30% of ingested or injected polonium-210 goes to the liver, spleen and kidneys. The bone marrow, lymph nodes and lungs – the lungs are the target organ for inhaled polonium-210 – display higher-than-average concentrations compared with other tissues (except for the liver, spleen and kidneys). The radionuclide is then eliminated in the feces and urine over a biological half-life of about 50 days (at the end of which, the body has excreted half the 210Po). Measurements have shown that, on average, the feces contain nine times more 210Po than the urine. Several studies have demonstrated that polonium-210 could also be found in exposed subjects' hair.
Throughout the period 1940 to 1970, many experiments were performed to study the fate and toxicity of polonium-210 in mice, rats, rabbits, cats, dogs, tamarins and baboons. The studies showed that after polonium-210 had been injected or ingested in citrate, chloride, colloidal hydroxide or nitrate form (polonium-210 seemed to be absorbed most effectively in nitrate form), the animals developed a number of clinical and biological signs, including weight loss, asthenia, lethargy, as well as liver, kidney, spleen, lung, pancreas and hematopoiesis dysfunctions, atrophy of the lymph nodes and hardening of the blood vessels (particularly in the kidneys and testicles). In addition to these symptoms, by the time of death – generally due to cardio-vascular collapse – almost all lymphocytes had disappeared. Within 20 days of intoxication, the median lethal dose (or LD50) was approximately 18 nanogram/kg body weight in the mouse, rabbit, cat and dog and 9 nanogram/kg body weight in the rat. This experimental data would suggest that acute toxic effects in human beings could be triggered by very slight quantities of polonium-210 (a few micrograms).
Several studies of rats, contaminated with an injection of a 210Po nitrate solution, showed that faster excretion could be achieved by rapid intramuscular or subcutaneous administration of chelating agents with thiol groups, such as HOEtTTC (N,N'-di(2-hydroxyethyl)ethylenediamine-N,N'-bisdithiocarbamate) and BAL (British Anti Lewisite or 2,3-dimercaptopropanol). On average, rats treated with the second compound (the reference treatment in France) survived 82 days, as opposed to 21 days for untreated rats. However, a study validating this data in humans has yet to be published.
There is little data available concerning accidental human exposure to polonium-210. The only familiar cases to date concern persons exposed following contact with a leaking Po-Be source. The only symptoms observed in these persons were some temporary functional disturbances – though with no clinical impact – to the liver (increased concentration of bilirubin in the plasma) and kidneys (reduced perfusion). The whole-body activity due to polonium-120 intake in these persons ranged from 0.034 to 2.41 nanograms.
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