Ionactive - further thoughts on our Polonium 210 (Po-210) Litvinenko activity and dose assessment

On the 27 December 2022 Ionactive released a blog article from our archives, specifically – ‘2022 Alexander Litvinenko - "Polonium-210 Poisoning" - revisiting an Ionactive 2006 blog article’. If you have not already read this then please follow the above link first and then come back to this later post (it will make more sense).

Our 24 November 2006 blog post was written with very little official information – all we knew, through the media, was that radioactive Po-210 (Polonium 210) had been identified as a probable cause of the death of Litvinenko (who died on the 23 November 2006). Investigations were at an early stage and it would take months to establish the facts. This eventually lead to the official ‘The Litvinenko Inquiry’ released in January 2016 (this link will open in a new window).

In our original post we set out to provide 1) useful information regarding radioactive Po-210 for media / public consumption, and 2) provide an initial estimate of the likely Po-210 radioactivity (activity) which could lead to death (from deterministic radiation effects – essentially radiation sickness). For this blog post (2) is considered a little further given the extensive research, investigation and analysis that took place after the death of Litvinenko.

Terminology - feel free to visit the Ionactive Radiation Protection Glossary for a concise explanation any terms used in this post (this will open in new window for easy reference).

Ionactive initial Po-210 activity estimation to cause death by deterministic radiation effects (Nov 2006)

It is important to note that we made this assessment out of pure curiosity, an exercise in looking at some fundamental health physics. The assessment is not included completely here, if you wish to consider our thoughts at the time then please visit our preceding blog post dated 27 December 2022. Our overall summary at the time was as follows.

Radioactivity (activity) of Po-210 in the systemic system

462 MBq of Po-210 by ingestion. We reduced this down to 115.5 MBq (equivalent to 0.7 micro g) on our assumption that alpha particles would be more effective at inducing a deterministic radiation effect than x-ray / gamma rays. We choose a factor 4 which was a complete guess! What we did NOT do is introduce a factor “20” - we note that some other commentators at the time used this (incorrectly). The reason for this is that the factor 20, based on radiobiological effectiveness (RBE), is related to assessment of potential fatal radiation induced cancer – and this relates to stochastic radiation effects. We knew in 2006, and certainly by the 2016 enquiry, that the clinical observations and death of Litvinenko were deterministic in nature (sickness, vomiting, diarrhea, nausea and depletion of white blood cell count etc).

Intake vs activity in the systemic system

However, an important issue that we missed in 2006 (and could not have estimated reliably without additional knowledge), was that our calculation looked at the theoretical activity required to cause death – i.e. the activity of Po-210 in the systemic system (blood) which then is delivered to the target organs. In this respect, our systemic system calculation at the time is quite close to the official findings (more of this in a moment).

What needs to be added to our 2006 assessment, and clearly was missing, is that there is a difference between intake (i.e. through the month via ingestion) and what is actually is delivered to the target organs. Think of it as ‘efficiency’ – at 10% efficiency (this being deliberately chosen, see below) the ‘intake’ needs to be 100% of the systematic activity (calculated) in order to yield a systemic uptake at the required Po-210 activity. Taking our estimates noted above and rounding, that is 0.11-0.46 GBq (110 MBq – 462 MBq), then at 10% efficiency we would need 1.1 GBq – 4.6 GBq delivered by ingestion (mostly likely via the mouth).

What do the official scientific papers say?

What we are expressing is the % uptake in the blood. At this point we can turn to a number of supporting papers released after the event. Some of these support evidence submitted to the official enquiry - two of them being ‘Litvinenko: Estimated radiation doses and expected health effects following intake of Polonium-210, 26 April 2007’ and ‘The evidence for the use of Polonium-210 to poison Mr Litvinenko, 5 February 2014’. These were issued by the Health Protection Agency (HPA) in 2007, now known as the United Kingdom Health Security Agency (UKHSA).

At the time of writing this blog post, the above referenced documents appear to be illusive on the web(!). However, we can turn to other papers, by many of the same authors, that were produced around the same time (i.e. data / findings used as evidence appeared elsewhere in numerous scientific papers). One such paper is ‘The polonium-210 poisoning of Mr Alexander Litvinenko’ by John Harrison et al (2017). This link takes you to a freely available copy. Here are a few interesting findings from this paper – most are rather pleasing in that they support the Ionactive assessment of November 2006.

RBE – for alpha particles (deterministic effects)

Ref: section – 2.3. ‘Alpha particle RBE values (RBEα) for acute effects are generally ill-defined but a value of two (range of 1 – 3) has been estimated by Scott (1993,2004) for the bone marrow syndrome, and is used here’ (from above linked paper).

So (not surprisingly) this report does NOT use a value of “20” (neither did Ionactive). It suggests a range of 1-3 with a value of “2”, Ionactive chose “4” – so near enough!

Estimates of intake of Po-210

It is important to remember that much of the above referenced paper is based on real measurements (i.e. radioanalysis of urine and tissue samples).

Ref: section – 3.1 ‘The best estimate of absorption to blood following intake on 1st November 2006 was 0.44 GBq

Wow ! This was based on real analysis and they state 0.44 GBq. This compares favourably with the Ionactive November 2006 estimate of 0.46 GBq ! Ionactive then lowered this to 0.11 GBq using a RBE of “4” for alpha particles (a guess at the time). Since the above referenced paper states an RBE of 1-3, then both activity figures are comparable (depending on what value of RBE you choose).

Ref: section 3.1 ‘Assuming intake by ingestion and intestinal absorption of 10% of the ingested amount, absorption to blood of 0.44 GBq corresponds to ingestion of 4.4 GBq. The value of 10% absorption was used previously (Harrison et al 2007), based on human and animal data for absorption following administration of simple soluble forms of polonium (ICRP, 1979; Leggett and Eckerman, 2001)’.

This is the data that Ionactive did NOT consider in 2006 – the actual intake by ingestion (i.e. the minimum activity of Po-210 in the tea consumed by Litvinenko). The maths is really simple here – 10% absorption – so the intake was estimated to be 4.4 GBq (the Ionactive estimation would have been 4.6 GBq if we had used the 10% absorption factor and a lower RBE). Despite this, and considering what we knew at the time, the figures are all remarkably close and of the same order of magnitude (an order or magnitude is the best we could have hoped for anyway).

The above referenced paper concedes their own range in ingestion values (i.e. 2.2 GBq at 20% and 8.8 GBq at 5% absorption)

It is noted in section 4 of the above paper that absorbed doses to specific tissues were in the order of 6-12 Gy. The paper analyses is organ specific so absorbed doses are attributed to various organs. In comparison, the Ionactive November 2006 analysis worked the other way around (given there was no analytical data available) - we started by assuming a whole body dose of at least 10 Gy (given the reported illness displayed by Litvinenko) and then working back to calculate likely intake (or more precisely for our analysis, the activity of Po-210 in the systemic system).

Any there we will lay this subject to rest. It has been rewarding to look back at a 2006 blog post and see if it has stood the test of time. In our opinion it has – in fact, it is remarkable how close our guesstimate was to the actual values published in the above paper and those official UK papers supporting the findings of The Litvinenko Inquiry issued January 2016.

Mark Ramsay, Ionactive Consulting Limited, 28 December 2022.

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Physics is, hopefully, simple. Physicists are not

– Edward Teller -