Polonium Poisoning

By Carey Sublette

Last changed 14 December 2006

Introduction

The death of Aleksandr Litvinenko from polonium poisoning is the first such case in history; it was also the first case of someone dying from the acute effects of alpha radiation from any source. Polonium-210 has a short half-life, 138.38 days. This means that the material used to poison Litvinenko had to have been produced in the relatively recent past - it could not, for example, have been obtained from an old disused radioisotope source. Such sources using other radioisotopes, like cesium-137 and cobalt-60, have long been a concern for their potential use in terrorism and other criminal actions, and have been the cause of serious accidental exposures in the past, most notably in the Goiânia, Brazil disaster in September 1987 that significantly exposed 244 people and killed 2. Although mathematically it would be possible to have a toxic amount still left from an initial enormous quantity after as long as a few years, it does not make a plausible theory of the case that Litvinenko's attackers acquired their material so long ago.

If someone is poisoned by a minimal lethal dose of a radionuclide they do not become ill right away. Instead they steadily accumulate radiation exposure as long as the material remains in their body. Radiation has relatively small ratios between an exposure that first shows any apparent illness, an exposure that has a significant chance of causing death, and an exposure that is almost certainly lethal. There is thus a latent period while exposure accumulates and finally reaches a level where illness becomes evident. After this point severity of exposure rapidly increases (see table below).

Acute (that is, intense short term) radiation exposures are measure in units called grays (abbreviated Gy). An older unit that is still often used is the rad, which is exactly equal to 0.01 Gy. It is thus convenient to refer to radiation exposures in terms of "centigrays" (cGy) since they match the older rads.

Polonium, when absorbed orally, tends to distribute widely through the tissues of the body, approximating whole body radiation exposure. Polonium is excreted fairly quickly, its effective half-life in the body is about 30 days (mostly due to excretion, but also due to radioactive decay). This results in a maximum total exposure equal to about 43 times the first day exposure. Most of this is accumulated in the first 30 days, but significant amounts occur of the next several weeks following.

Because of its short half-life and rapid decay, a small quantity of polonium-210 is intensely active. Its "specific activity" is 4490 curies/g (166 TBq/g). Thus 4 millicuries (a minimal lethal dose for 80 kg person) is 0.89 micrograms, an almost invisible speck of matter.

In the case of absorbing a minimal lethal dose no initial illness would result. The decay of the polonium would deliver an accumulating exposure of 16 cGy a day initially. No visible effect would be expected for about a week, with mild symptoms developing over the next week. Serious illness would not be expected for a month or more. This delay is partly due to the time is takes to accumulate dangerous internal exposures, and partly due to the time delay of 1-2 weeks after dangerous exposure before serious illness becomes evident. The exposure rises eventually to 700 cGy after a few months. The actual toxic effect of this exposure would be lower than what the table below indicates, because radiation received over a period of time is somewhat less harmful than a dose received all at once. Nonetheless death would be expected from the poisoning within two or three months.

This scenario contrasts strikingly with the reported symptoms of Litvinenko, who fell ill the day he was poisoned, was seriously ill on day 11 later, in critical condition on day 20, and was dead on day 23. The fact that he became ill so quickly indicates he was poisoned with many times the lethal dose of Po-210.

In the hour or two that elapses during gastrointestinal uptake of the poison, the GI system (which is about 3% of body mass) receives disproportionate radiation exposure, amounting to perhaps a few days worth of whole body exposure. If Litvinenko were poisoned with, say, 40 millicuries (ten lethal doses) then severe exposure on the order of 400 cGy would occur to the GI system. Even polonium that was not absorbed, but passed through the GI tract and was excreted would expose the radiation-sensitive GI lining. This would account for his falling ill apparently within hours of exposure. If poisoned with such a large amount, he would receive whole-body daily exposures of 160 cGy. The resulting gastrointestinal syndrome, which is due to the destruction of the GI lining with exposures of about 700 cGy, would account for the severe illness that led to his death in scarcely more than three weeks. By the time of his death his total exposure might have reached a few thousand cGy, levels that would injur his cardiovascular system directly and might be related to Litvinenko's heart attack shortly before his death.

On the other hand, Dmitry Kovtun who met with Litvinenko on 1 November, the presumed day of the poisoning, was reported to have become seriously ill from polonium poisoning on 7 December. This is 36 days after Litvineko becoming ill, and is fully consistent with someone who has absorbed a single lethal dose, or a near-lethal one. If Kovtun was present at the poisoning, perhaps being the poisoner himself, he could have ingested a small fraction of the very large amount administered to Litvinenko. Andrei Lugovoy, who was also present at the 1 November meeting was reported ill two days after Kovtun, on 9 December, but apparently was not as seriously ill as Kovtun.

(Most generally, radioactive nuclei are referred to as "radionuclides". The term "radioisotope" is used to distinguish radioactive nuclei of a particular element, such as cobalt, from its non-radioactive cousins).