Polonium-210 is familiar to most people as the deadly poison used to kill Alexander Litvinenko, an MI6 agent and former Soviet and Russian spy who succumbed to radiation sickness in London in the UK in November 2006.
While polonium-210 is highly dangerous, it may surprise some to know that this radioactive material is found naturally in the human body due to low levels in the normal environment.1,2
Tobacco smokers also accumulate polonium isotope 210 in their lungs, and the radioactive substance is also present in the food chain, especially in seafood.
It is alleged that Litvinenko ingested a fatal dose of Po-210 by drinking tea at a business meeting with two other Russians, both of whom have been charged with his murder. 3,4,21
Several locations in London, including a restaurant and nightclub, were closed by authorities during the investigation amid fears of potential public exposure to Po-210.20
Litvinenko's autopsy was a complicated process involving multiple safety measures to avoid contamination with radioactive material. Some 700 or so people were thought to have been exposed to polonium-210 connected to Litvinenko's death, but none experienced serious sickness.21
An inquiry into Litvinenko's death was established by the British government and is ongoing, with public court hearings having commenced at the end of January 2015.5,21 Most recently, a world-leading expert in theoretical physics told the inquiry that the polonium-210 in this case could only have come from a specific closed nuclear facility in Sarov, Russia.22
Use this page to discover what polonium-210 is and how the radioactive material - one of the most toxic substances known to humankind6 - poses its threat as a poison and weapon of terror.
Contents of this article:
Fast facts on polonium-210
Here are some key points about polonium-210. More detail and supporting information is in the body of this article.
- Polonium-210 is a rare radioactive metal discovered by Marie Curie in the late 19th century.
- It has a half-life of 138 days, decaying to lead.
- During its radioactive decay, polonium-210 emits alpha particles.
- The alpha particles are a very high-energy form of radiation but do not travel far - they can be blocked by a few sheets of paper, and lose all their energy after travelling through a few centimeters of air.
- Polonium-210 is poisonous only once it has entered the body, where it is powerful and deadly, even at doses of less than a microgram.
- By mass, polonium-210 is one of the deadliest toxins. It is around 250,000 times more toxic than hydrogen cyanide - the lethal dose of which is 250 milligrams.
- It would be very difficult to obtain polonium for poisoning, necessitating access to a nuclear reactor. But once acquired, polonium is relatively safe to transport and hide from detection.
- Unlike most radioactive substances, polonium-210 emits alpha radiation rather than gamma radiation, which is why it is not detected by Geiger counters.
- Polonium-210 becomes poisonous when it is breathed in, swallowed, or otherwise enters the body, such as through broken skin.
- A high enough dose of polonium by mouth will lead to acute radiation syndrome - the severity of which, and time to illness and death, depends on the amount of radioactive material absorbed.
- The liver, bone marrow and other soft tissues are particularly sensitive to the effects of polonium. Acute radiation syndrome presents as vomiting, hair loss, and reductions in white blood cells.
- Specialist radiation detectors are needed to diagnose contamination with polonium-210.
- Treatment is limited to supportive care against the effects of the radiation, although chelation agents may be able to help eliminate polonium if a case of poisoning is identified early enough.
- The prognosis in a case of polonium-210 poisoning depends largely on the amount of radiation absorbed by the body.
- Former spy Alexander Litvinenko was assassinated by polonium poisoning in London in 2006. British police identified that the polonium-210 had been introduced via a teapot.
What is polonium-210?7-9
Polonium is a radioactive chemical element (atomic number 84) that was discovered in 1898 by Marie Curie.
Polonium is one of the most toxic substances known to humankind.
Being a Polish chemist, Curie named the element after her home country, and her discovery won her the Nobel Prize in Chemistry in 1911, which was also a recognition of her discovery of another element, radium.10
In its natural state at room temperature, polonium is a solid metal with a silver color. Polonium-210 is one of 25 known radioactive isotopes of polonium and it has a half-life of 138 days (meaning half its radioactivity dies away in this time), decaying into a new, stable metal - lead.
Polonium is very rare, as Marie Curie found when she discovered it in a source of uranium known as pitchblende. As little as about 100 micrograms (0.0001 grams) of polonium occurs in one ton of uranium ore.
Curie's pioneering work, however, produced enough polonium to fatally expose her daughter Irène in a lab accident in 1946 - she went on to suffer leukemia, dying in 1956 at the age of 58.11
Modern production of the substance is usually done by creating another radioactive element in a nuclear reactor, which itself then decays to polonium-210.
Polonium-210 is abbreviated to Po-210, (210)Po, or 210Po.
Beneficial uses of polonium include:
- Insulating instruments in Russian lunar landing craft - polonium decay generates significant heat relative to its light weight
- Application in commercial devices to remove static electricity, in which polonium is usually electroplated onto other metals
- Inspecting oil wells (using the same principles as when using polonium to trigger nuclear weapons)
- To measure the thickness of industrial coatings (through alpha radiation attenuation).
How dangerous is polonium-210?
Hydrogen cyanide is well known to be highly poisonous, but is far less toxic than polonium-210. Matched milligram for milligram, polonium-210 is 250 billion times more toxic.8,11
This makes polonium-210 one of the most toxic substances known to man.6
Toxicologists estimate that one gram of polonium could be enough to kill 50 million people, on top of another 50 million who would become ill; in the case of Litvinenko, less than one millionth of that amount would have been enough to cause his death (less than a microgram).8
How could an assassin use polonium-210 for poisoning?
Clearly, polonium-210 is extremely dangerous - but it is also extremely difficult to obtain. Its commercial application onto devices that remove static has largely been replaced by non-radioactive technology and is done in such a way that it is infeasible to separate the polonium for use as a poison.9,12
Even buying pure polonium from legitimate sources would not be feasible since it is sold in such tiny quantities - 15,000 orders would have to be placed to enable enough for poisoning.9,12
If a person was to acquire some polonium, say from a nuclear industrial process, it is not particularly dangerous to carry around as its high-energy radiation can be blocked by a relatively thin barrier for a radioactive material. Polonium-210 is unable to penetrate the epidermis (the skin), and is blocked by just a few sheets of thin paper. Additionally, alpha particles usually lose all their energy after traveling through a few centimeters of air.11,13
The rapid application and damaging effects of polonium-210, added to its relatively safe transportation and minimal risk of detection make it an attractive potential poison.
The perpetrator(s) of Alexander Litvinenko's murder by polonium poisoning would have needed access to "a reactor capable of producing and irradiating materials, and a radiochemical laboratory," according to quotes in New Scientist from Professor Nick Priest, one of few UK experts to have worked with polonium-210.3
Speaking at the British inquiry into Litvinenko's death, Norman Dombey, emeritus professor of theoretical physics at the University of Sussex, said the polonium used was produced at a closed nuclear facility in the city of Sarov, 450 miles south-east of Moscow.22
This Soviet-era plant (known as Avangard) was the only place in the world with a polonium production line, and the last remaining source of commercial polonium. The US and UK stopped making polonium in the 1970s as did all other countries.
Dombey also told the inquiry that the dose of polonium used to kill Litvinenko was exceptionally large (26.5 micrograms).22 He determined that polonium was likely used in this case because it is unlikely to be detected, given that it emits alpha radiation rather than the gamma radiation emitted by most radioactive materials.
This means that the poison is unlikely to be detected. Indeed, it was only hours before Litvinenko's death that polonium-210 was discovered as the poison that was killing him.
In all likelihood, Dombey says, the polonium-210 came from Avangard, after having started out as irradiated bismuth in the Mayak nuclear reactor in the Urals. 22 To convert the metallic polonium-210 into liquid form, Dombey says that a "state institution" would have had to be involved, before the two Russians charged with Litvinenko's murder would have had to smuggle it to London.22
Polonium has a half-life of just 138 days, meaning that it loses half of its radioactivity in that time. The 138 days for polonium-210 to lose half of its radioactivity means that the specimen used to kill Litvinenko would need to have been made relatively recently.9,11
Finally, when polonium-210 has reached its target, the poison must be introduced into the body to do its fatal damage internally.8,11
The high-energy alpha particles are harmless externally - they are blocked by a few sheets of paper, or the top layer of the skin, the epidermis. But deadly tissue damage can be done via:8,13
- Ingestion, or
- Entry through skin abrasions or wounds.
Alexander Litvinenko is thought to have ingested the polonium via a cup of tea, which would have hidden the heat generated by the radioisotope.
How does polonium poison humans?
Polonium-210 does not itself have toxic chemical properties - the danger comes purely from the radiation it emits. But the alpha particles have a very short range across biological tissue. This is a tiny fraction of a millimeter (40-50 microns or 0.04-0.05 mm), and the particles are stopped by the top layer of the skin.6
Before polonium-210 becomes dangerous to the human body, it must therefore be introduced internally. This can be through broken skin or through inhalation of ingestion by mouth, as in the Litvinenko case.
Before the 2006 assassination, polonium-210 was considered to be a highly unlikely murder method. However, since the event, there has been an increase in expert literature on this kind of poisoning.14
After oral ingestion, polonium-210 is initially concentrated in red blood cells, followed by the liver, kidneys, bone marrow, gastrointestinal tract, and gonads (testicles or ovaries).6
By mouth, polonium-210 is absorbed more readily than some other alpha-emitting radionuclides.8 Once absorbed into the blood, polonium is then distributed through the body mainly in soft tissues and roughly as follows:6,8
- 30% to the liver
- 10% to red bone marrow
- 10% to the kidney
- 5% to the spleen
- The remainder to the wider body, including the skin, particularly the hair follicles.
Environmental levels of polonium are extremely low and harmless to human health, and very low levels of the substance are found naturally in the body (tobacco plants concentrate the radioisotope though - so smokers have higher levels).1
In sufficient amounts, polonium can be lethal within days or weeks. Polonium poisoning typically presents as unexplained vomiting, followed by bone marrow failure and hair loss. Different organs and tissues greatly vary in their sensitivity to the alpha radiation damage, with the blood-making (hematopoietic) tissue in the bone marrow, and the lining of the gut especially vulnerable.8
The bone marrow failure seen in polonium poisoning is accompanied by damage to other organs where polonium-210 concentrations are high, meaning that transfusions to rescue the bone marrow may not be enough to save someone.15
Why is alpha radiation so toxic?
As polonium is distributed around the body, it leaves a trail of reactive radicals because it steals electrons from any molecule in its path.9
Damage to DNA from the alpha particle radiation can cause apoptosis ''" cell suicide - and even low-level DNA damage can cause genetic changes that affect cellular replication.9
How much polonium is needed to cause death?
Scientists have a number of ways to quantify amounts of radiation exposure.
Polonium is extremely dangerous when inhaled, ingested, or introduced through broken skin. It is relatively safe to transport though, and its radiation will not penetrate the top layer of the skin.
The radiation given off (emitted) by a quantity of polonium is measured in Becquerels (Bq), a unit that describes the rate at which a radioactive material decays in terms of the number of disintegrations of its nucleus per second; a gigabecquerel (GBq) is 100,000,000 disintegrations per second.16
The amount of radiation absorbed by the tissues, meanwhile, is measured in gray units (Gy); the absorption of one joule of radiation energy per one kilogram of matter.16
Scientists estimate that the equivalent of between 0.1-0.3 GBq or more of polonium absorbed into the blood of an adult male would likely prove fatal within one month.15
Assuming 10% of ingested polonium would be absorbed into the blood, 1-3 GBq or more would need to be swallowed for the poison to be deadly15; just a few micrograms of polonium would be enough to kill someone.3
On the next page, we look at the diagnosis, symptoms and treatment of polonium poisoning.