The recent death of a Russian ex-spy in Britain has been linked to the presence of a radioactive polonium-210 in his body. What is Polonium? How can it cause death?

Polonium is a chemical element with the symbol Po and atomic number 84. It is a silvery white metal. Over 25 isotopes of Polonium are known, with atomic masses ranging from 192 to 218. Polonium-210, the predominant isotope of polonium, is radioactive and emits alpha particles on decay. It has a rather short half-life of 138 days.

Polonium occurs naturally in very low concentrations in the earth crust. The element was discovered by Mr. and Mrs. Curie in 1898 and was later named after Mrs. Curie's homeland of Poland (Latin: Polonia). Polonium-210, a naturally occurring isotope of Polonium, is found in all environmental media, such as in soil, air and even in human body, at very small amounts. Polonium-210 is also present in the fine tobacco hairs, coming from the phosphorus fertilizer used in growing tobacco plant. Due to its scarcity, Polonium-210 for commercial use is produced artificially in a nuclear reactor.

Polonium-210 can be used to eliminate static electricity in machinery, remove dusts from photographic film and camera lens, and provide heat energy for thermoelectric power devices in space applications.

The alpha particles released from Polonium-210 have a very low penetrating power and can be stopped easily by the outer layer of skin or a sheet of paper. It is unlikely that the particles can affect human body through direct irradiation. However the ionization power of alpha particles is very high. If Polonium-210 is taken into our body by inhalation or ingestion, the released alpha particles can directly damage the internal organs in the body. Even a small amount of intake may impair our health. A high acute dose of alpha radiation may result in 'Radiation Poisoning'.

'Radiation Poisoning' is a form of damage to organic tissue due to excessive exposure to ionizing radiation. The term is generally used to refer to acute problems caused by a large dosage of radiation in a short period. The ionization process often results in chemical changes in living tissue, and damages the function and metabolism of the cells which can lead to injury in the organism. The latent period varies for different types of biological effects of radiation. The acute effect can appear from several hours to a few days, but the long-term effect may not appear after a few years. Different organs have different sensitivity to different types of ionizing radiation. The pathological symptoms are different.

The lethal dose for acute radiation exposure is generally about 4 Siverts (Sv). One Becquerel (Bq) of Polonium-210 causes a radiation dose of 0.51 micro Sivert (Sv) if ingested, and 2.5 Sv if inhaled. Since Polonium-210 has an activity of 1.66 x 10¹⁴ Bq per gram, a fatal dose of 4 Sv can be caused by ingesting about 5 x 10 ^-8 gram or inhaling about 1 x 10 ^-8 gram of Polonium-210.