Polonium 210 is so toxic that it is difficult to explain just how dangerous it is – a fatal dose is almost a million times less than is needed for cyanide, for example.
Its danger comes from its high radioactivity. It decays by giving out alpha radiation and once Po210 is absorbed and is circulating in the body, it silently and fatally damages each individual cell. Half of any amount of Po210 decays in this way every 138 days.
It is impossible to physically see what would be a fatal amount of solid polonium, making it impossible for anyone to detect that they have been given the poison. Obviously, this also makes it remarkably easy to give to someone in a drink or food.
Strangely though, it is completely safe to handle, because alpha radiation is stopped by even a sheet of paper or by human skin. It can be carried around in a test tube for years without harming anyone, until it gets into someone's blood stream.
I believe it did get into Yasser Arafat's bloodstream and here is why.
The Swiss scientists produced a truly excellent report, which has researched every issue and explored every possibility. Their conclusion about the cause of Arafat's death is based, quite correctly, entirely on their scientific results, since they have no scientific information about his illness.
They do comment that his symptoms fit with the known pattern of Po210 absorption, which means it is likely to initially cause damage to the intestine and lead to related symptoms.
Forensic science as presented to a court, however, always has to consider scientific results in context. In the case of Arafat, the context of his death is what changes their scientific assessment of up to 83 percent probability into an evidential certainty.
To start with, a key question is: How likely are we to find these levels – a lethal level according to the Swiss – of polonium in someone who did not die from polonium poisoning?
We have some scientific data on this. There are many bone samples of people who died naturally which have been analysed over the years, and they average out at between 25-50 milibecquerels (mBq) per gram of calcium.
Arafat's ribs were around 900 mBq. That is 18 to 36 times more than the average, even at the time of exhumation. And remember, that took place over eight years later when the Po210 had been reduced by 21 half-lives.
So at the time of his death in 2004, he had over two million times that level circulating in his blood and being deposited in his bones.
And the pattern of the results in the clothing stains done in 2012 and the bones from the exhumation show that the polonium must have been circulating in him before death. It is only found in actual stains from his urine, his blood and his sweat, and is highest in those bones which have the greatest blood supply.
Cause of death
The results in 2012 also show that the Po210 had been manufactured in a nuclear reactor. That means it could not have come from natural sources like the decay of radon. The tiny background level of Po210 decay of radon produces in humans has been well established by work in the nuclear industry.
Just consider how many people have died naturally in the whole world since 1950. Only five are known to have died from Po210. We can work out how likely it is that anyone at all would die of Po210 poisoning. And it is one chance in many billions.
So what are the odds that any person would just happen to have a fatal level in his tissues if he did not die of it?
The fact that Arafat has a level which would have been way into the accepted lethal range when he died in 2004, now 22 half-lives ago, is beyond dispute from the Swiss results in their report. And he shouldn't have had any reactor-produced Po210 in him anyway.
Toxicologists in general and the Swiss scientists in particular, can never state just from the science that someone definitely has died of, for example, cyanide or strychnine. That is because the person might have then jumped off a bridge, or died under a train, so death due to a poison always depends also on the absence of any other cause being present.
I am sure the Swiss also considered that although he must have had high levels of Po210 within the lethal range, he could have become ill by coincidence from some other unrelated cause. However, very exhaustive tests were performed by the French Percy Hospital during his final days, without result. Arafat had no other disease, no cancer, and no heart disease.
Questions for judge and jury
In my opinion, this evidential assessment would convince any investigator or member of a jury.
We can summarise just a few of the arguments by anticipating how the courts would assess the evidence. A court would have to ask four important questions to assess the significance of the results:
1. Are we satisfied that the results are scientifically correct, and represent levels circulating in life?
We are because the Swiss lab is excellent and their analyses are comprehensive; and because there is the conclusive pattern of results on clothing and bone, so the Po210 results could not have been manufactured by anyone after death.
2. How likely are we to find these levels of Po210 in any other person in the world, chosen at random, who has died in the last 50 years?
It works out at less than one chance in several billions, even taking the conservative view that some Po210 accidents may have gone unreported.
3. How likely is it that these levels are not directly associated with an illness/death occurring in 2004?
A conservative estimate might be 1 chance in 50 or so, even if we do not take into account what the levels must have been in 2004 – which would be over 2 million times higher. However, he had no other obvious cause of death.
4. Is there evidence that the levels present in 2004 would directly cause serious harm?
There is. The Swiss scientists also calculated how much should remain in his bones in 2013 of a dose in the accepted lethal range that would have been given in 2004. It fits very well with the figures they actually obtained from his bones after exhumation.
Taking the measured scientific figures obtained by the Swiss, together with the context, we can be absolutely certain that Arafat ingested a large dose of Po210, which was made in a nuclear reactor, and it was circulating in him during his terminal illness.
From death statistics, the chance of this happening accidentally must be less than one in several billions for any individual living on this planet, and maybe even smaller for Arafat since his food and drink supply was apparently controlled.
We are less certain, scientifically, that the calculated levels of Po210 caused his illness and death simply because the lethal dose is less well established; but as forensic scientists, we are obliged to take into account the context, including the fact that he had no other obvious cause of death.
And of course neither he, nor any of us, would have any reactor-made Po210 in our blood anyway. That fact alone would satisfy most juries that something really sinister was going on in 2004.
So, based on my decades of experience and the evidence before me, I have no evidential doubt that a lethal dose of Po210 was administered to or ingested by Arafat in 2004, and that it caused his death.
David Barclay is a veteran forensic investigator who specialises in the assessment of physical evidence in murder cases. He is also Professor Emeritus at Robert Gordon University, Aberdeen.