Polonium-210, The Perfect Poison

Figure 17.27 in the 5th edition of Intermediate Physics for Medicine and Biology shows the decay series arising from the radioactive isotope radon-222, which itself is produced by the decay of the long-lived isotope uranium-238. The last step in this long chain of reactions is the alpha decay of polonium-210 to the stable isotope lead-206. The half-life of this decay is 138 days. This is not the only isotope of polonium in radon’s decay series. A heavier isotope polonium-214 has a half-life of 160 microseconds, and polonium-218 has a half-life of 3 minutes.

Polonium was discovered by Marie and Pierre Curie in 1898 when analyzing pitchblende, a uranium containing ore. It was named after Marie’s homeland, Poland. Now ²¹⁰Po is produced by bombarding bismuth-209 with neutrons, forming bismuth-210, which undergoes beta decay to ²¹⁰Po.

²¹⁰Po is infamous for being a deadly poison. For a given mass, ²¹⁰Po is 250,000 times more toxic than hydrogen cyanide. Its toxicity comes from the 5.3-MeV alpha particle it emits. Because alpha particles are easily stopped by clothing and even skin, ²¹⁰Po is dangerous primarily when breathed or ingested, so that the alpha particles are emitted inside the body. A nearly pure alpha emitter, ²¹⁰Po rarely emits a gamma ray, making it difficult to detect this poison unless one measures the alpha particles directly. A lethal dose comes from ingesting about a microgram.

²¹⁰Po was used in the 2006 assassination of Alexander Litvinenko, a former Russian spy who was apparently given some polonium-laced tea by Russian agents (the investigation into this complicated murder continues--see here and here--and the details are still debated). Death by ²¹⁰Po is slow; the 44-year old Litvinenko needed 22 days for the radiation to eventually take his life.

Polonium was also suspected to play a role in the 2004 death of Palestinian leader Yasser Arafat. Just this month, a French investigation has concluded that there is not enough evidence for pressing charges. The issue is complicated because ²¹⁰Po is found in cigarette smoke, and Arafat was a heavy smoker. The National Council on Radiation Protection and Measurements reports that the annual effective dose equivalent to a smoker from radiation in tobacco is about 13 mSv, which is over four times the average annual dose of 3 mSv we are all exposed to (see Section 16.12 in IPMB), but is still a tiny dose.

The Environmental Protection Agency has published a report titled “Occurrence of ²¹⁰Po and Biological Effects of Low-Level Exposure: The Need for Research.” As with all studies of low-level radiation exposure, the results are difficult to assess, and depend on our assumptions about radiation risks at small doses. But Alexander Litvinenko’s death proves that at high doses ²¹⁰Po is very dangerous indeed; it’s perhaps the perfect poison.