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Polonium-210 poisoning

There's fascinating science behind the media hype. Steven Kennish and Stuart Currie explain

In November 2006 Alexander Litvinenko, a 43 year old former officer of the KGB (the former Soviet Union's committee for state security) and a political exile in the United Kingdom was poisoned by a radioactive substance known as polonium-210. The story gripped the nation and made news headlines worldwide.^w1-w3

Litvinenko fell ill on November shortly after meeting some contacts in central London, at locations including a sushi bar and hotel restaurant. In hospital he tested positive for ²¹⁰Po. His condition progressively deteriorated, and he died of multiple organ failure on 23 November.

In many respects the political intrigue has overshadowed the science behind the case. The general public can be forgiven for skipping the technical details, but if we dig a little deeper these are just as fascinating.

Polonium (Po) is a rare radioactive element that occurs naturally in the earth's crust (see box 1). It has position 84 in the periodic table. It was discovered by Marie Curie in 1898 and was named after her native country of Poland.^w4 It exists as more than 25 isotopes, which range in atomic mass from 192 to 218. As a consequence of recent events, ²¹⁰Po has become the most famous isotope of the element.

Polonium-210 exists in minute quantities in the human body and in soil and air. It has been used in devices to eliminate static charges and has also been alloyed with beryllium to act as a trigger for nuclear weapons.^w5

The isotope can be artificially produced in nuclear reactors by bombarding bismuth-210 with neutrons. Polonium-210 has a half life of 138 days and decays to stable lead-206 by emitting α particles (box 2).^w6

Irene Joliot-Curie, the daughter of Marie Curie, was the first known person to be thought to have died from exposure to polonium. She came into contact with it when a capsule exploded in her laboratory. She later died of leukaemia, most likely induced by inhalation of polonium, although we cannot verify this, a decade later in 1956 at the age of 59.^w7

FABIAN BIMMER/AP/PA

Fig 1 The α particle (helium nucleus)

Fig 2 Decay of ²¹⁰Po

X rays, γ rays, α particles, and &bgr; particles are all ionising radiation (box 3). Ionisation results in damage to molecules, primarily DNA and ultimately cells, tissues, and organs.

Fig 3 Decomposition of water molecules

The greater the absorption of ionising radiation in the body the greater the biological effects will be. X rays are used in diagnostic imaging because many will not be absorbed. They are detected once they have passed through the patient. Some x rays are absorbed creating the shadow, or image. Lower energy x rays are filtered out of the beam before leaving the x ray machine to reduce the amount of radiation absorbed. Biological effects are, therefore, minimised by relatively low rates of absorption.

X rays also have indirectly ionising effects because they knock electrons out of atoms. These electrons then go on to interact with other atoms, primarily water in the body (indirect ionisation) before coming to rest.

Alpha particles are directly ionising because they are charged particles. Their biologically damaging effects are 20 times greater than electrons released by x rays because their energy is completely deposited in a much shorter range. They are heavier and cannot travel far into tissue. Their complete absorption maximises biological damage. Although a sheet of paper—and human skin—is thick enough to stop the penetration of α particles, once they are inside the body the ionising effect and, therefore, biological damage is severe.

The complete absorption of highly radioactive ²¹⁰Po once ingested seems to be so damaging in the short term that mechanisms of cell repair are overwhelmed and mass cell death occurs, leading to organ failure (fig 4).

Fig 4 Mechanisms of cell damage

Deterministic effects occur because of cell damage once a threshold dose of radiation has been achieved. This is usually a dose beyond which cell death occurs at a greater rate than cell repair. Above and beyond the threshold the severity of those effects increases. It is the deterministic effects of the massive doses of radiation that killed Litvinenko.

Stochastic effects are based upon probability of occurrence. Increasing dose increases the probability of a stochastic effect rather than its severity. Malignant change is a good example of a stochastic effect and is more probable because of cell transformation with high radiation exposure. Very low doses of radiation can lead to malignant change in cells, but this is much more likely with higher doses. Stochastic effects can take years to manifest.

Both these effects are why we have such strict measures in hospitals and other facilities where ionising radiation is applied, to keep radiation exposure to patients and staff as low as possible.

Thankfully, acute radiation poisoning is a rare occurrence, however, this means that treatments are not well established and experience of caring for such patients is limited.

The difficulty with an isolated and unsuspected case of radiation poisoning is identification. The initial symptoms of vomiting are fairly non-specific, and delay in diagnosis and, therefore, early treatment are almost inevitable.

With external contamination, occurring after a wider environmental release of radioactive waste, removing the offending radioactive particles is important and can be achieved in the main by simply washing the patient with mild soap and water. Internal contamination is more complex and a number of chelating agents for specific radioactive isotopes have been found (box 4). As for decorporation of ²¹⁰Po from the human body, experience is limited.^w8 D-penicillinamine is a recommended chelating agent.^w10

The most radiosensitive cell lines in the body are those that are most susceptible to the damaging effects of exposure to radiation. They generally have the highest turnover, or rate of cell division. This is why tumour cells are often more sensitive to radiotherapy than other cells.

Normally lymphocytes are the most sensitive cell line and rate of fall of lymphocyte count is a sensitive marker of the amount of radiation exposure received.^w11 The other haematopoietic cells are also affected. Falling platelet counts predispose to haemorrhage, and platelet transfusions are advised. Packed cell transfusions are also recommended for anaemia. Clinical guidelines produced from a study in the United States recommend cytokine therapy and haematopoietic colony stimulating factors to replenish depleted cell lines.^w12 Antibiotic and antifungal drugs are advised for prophylaxis in newly immunocompromised patients.

Damage to the high turnover cells of the small bowel can lead to villous shedding and large fluid volume losses due to malabsorption. Resuscitation with intravenous fluid is vital.

Unfortunately for patients exposed to high levels of radiation supportive treatment is not enough to prevent overwhelming cell death and consequent failures of organ systems.

The case seems to centre around three key points related to the science:

(1) The inability of ²¹⁰Po to penetrate skin has restricted its toxic effects to the patient alone, who presumably ingested the poison. Its high levels of local radioactive deposition have led to the death of Litvinenko without causing other casualties, at least in the short term.

(2) The long half life of 138 days seemingly left a radioactive trail, which has been picked up by the people investigating the case. It has taken them to a number of locations in the UK and abroad.

(3) Polonium-210 is incredibly difficult to acquire in any quantity without substantial resources from the nuclear industry.

On 22 May 2007, the director of public prosecutions in the UK recommended that Andrei Lugovoi, a former KGB officer, should be charged with Litvinenko's murder.

The Russian authorities have cited a constitutional bar to extradition of its citizens in its refusal to the UK's request for Lugovoi to be handed over to face trial. Understanding the science has helped the investigation. But it cannot help to navigate the legal and diplomatic impasse that now ensues.

Competing interests: None declared.

References w1-w12 are on studentbmj.com.

       

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