Arafat may have been poisoned, but what is polonium?

The 138-day half-life of 210Po is short, so the element is very radioactive. While it has a melting point of 254°C, it is so radioactive that if you made 1g piece of 210Po it would create so much heat it would melt itself. The liquid would appear to glow blue due to the interaction of the alpha particles with the surrounding air.

Polonium’s effects
The toxicity of radioactive materials is usually measured in terms of the radiation emitted and/or absorbed. However, to compare with more conventional toxins, the median lethal dose (LD50) for 210Po that is usually quoted is about 1μg, or one millionth of a gram.

That is one ten thousandth the dose of VX — the most potent nerve gas.

Contamination is treated in the same way as other heavy metal poisoning, with chelating agents that bind to the metal and make it more likely to be excreted. However, once a victim shows the symptoms of 210Po poisoning, the effects are likely to be fatal.

The type of radiation is also a factor in how dangerous a material is. The radiation released by 210Po is called an alpha particle.

The alpha particle is a helium nucleus (two protons and two neutrons). This relatively large particle will not travel far through air and is stopped by a piece of paper. However, it pulls electrons out of other elements (ionising them). In turn, the ionised elements are highly reactive and able to undergo reactions that would not normally occur in a human body.

So unlike the image of radiation damaging DNA and causing cancer, alpha particles act more like a normal poison, but damaging many different biological systems rather than targeting one type of molecule.

The effects of polonium poisoning are effectively those of acute radiation poisoning. These occur within one day of exposure to a large dose of ionising radiation. The effects are all based on damage occurring to the body’s fast-growing cells:

  • bone marrow – a drop in number of blood cells causing tiredness
  • gastrointestial cells – causing vomiting and nausea
  • follicular cells – causing hair loss.

Detecting polonium
Due to polonium’s high radioactivity, it is usual to detect it by making use of the way it decays. The speed (energy) of alpha particles produced by radiation is specific to the isotope that emits them and leaves a kind of signature, with which the original isotope can be identified.

A sample of fluid suspected of containing polonium is dried onto a surface and the energy of the emitted particles is measured. The number of particles with a particular energy is directly proportional to the amount of that isotope in the sample.

The short half-life makes 210Po very hard to study. It is particularly hard to look for the remains of 210Po contamination once a significant amount of time has passed.

Measurement of ratios of stable breakdown products of the various isotopes of polonium could give some insight into if poisoning has occurred, but this depends on the initial composition of the polonium sample and it is susceptible to contamination.

Martin Boland is Research Fellow at University of Melbourne. This story is published courtesy of The Conversation (under Creative Commons-Attribution/No derivatives).