Effect Measure

[This is Part II of our explanation of some of the science behind the Polonium-210 poisoning case of Alexander Litvinenko]

In Part I. we sketched the physical background to understand radioisotopes like Polonium-210, the agent in the Litvinenko poisoning, but have yet to explain its connection to the biology. That's today's order of business.

Remember that the chemistry is located in the orbital electrons of each atom. Sharing electrons or pairing charges is how atoms bond with each other -- literally and figuratively. So if you mess up the electron structure you can mess up the chemistry. In particular, if you knock an electron out of its orbit (called ionizing the atom) lots of things can happen to the chemistry. If you do this to a whole lot of atoms and they are ones that are biologically important (like DNA) then you can get some untoward biological effects -- like turning a cell malignant or killing it. Sometimes this happens indirectly, when an abundant molecule like water has some atoms ionized or split off into highly reactive forms called free radicals and they then do some chemical mischief of their own. To make a long story (too) short, if a radioactive particle or gamma ray has enough energy to disturb the electron structure, it can do damage. This kind of radiation is called ionizing radiation, for obvious reasons. We are subjected to natural background radiation all the time and we have numerous ways to repair the damage that causes. It's when the damage accrues faster than it can be repaired that we start to have problems.

In the first post we discussed three kinds of radioactive emissions, beta particles (electrons ejected from an unstable nucleus), gamma rays (electromagnetic radiation of varying energies) and alpha particles. Polonium-210, the poison that apparently killed Litvinenko is a weak gamma emitter and a champion alpha emitter. But first let's talk about beta radiation because it sets the context for the other two.

Beta particles (being energetic electrons) are pretty light and have a single negative charge force field around them. As they scoot through your tissues they strike glancing blows at odd intervals to your body's atoms. Meaning no disrespect to any of you, you are mostly empty space as far as beta particle is concerned. Atomic nuclei are little pinpoints separated by vast distances, so a very energetic beta will knock off electrons (ionize atoms) at irregular intervals along a long path before the collisions eventually slow it down to a slow aimless drift with no remaining force. Some of the knocked off electrons will themselves have enough velocity to cause ionizations nearby, too, so beta particles can do some damage before they are stopped by repeated collisions. Here's an analogy. Think of a huge field with a big parking lot in the middle of it. The cars are not closely packed together. They are your body's chemical molecules. A beta particle is like a fast motor scooter shooting through the lot, carooming off one or another car. It can cause some damage to a bunch of cars and maybe even push one car into another (I'm stretching here; maybe not a scooter, say the beta particle is a Morris Mini). The beta particle, being small and fast, can get a little ways into the lot from the edge but eventually it is stopped by the repeated collisions.

Gamma rays are electromagnetic radiation and they interact with atoms completely differently. Since they aren't particles but oscillating fields, they interact probabilistically with matter and most of them pass straight through without interacting at all. If x-rays didn't do that we could expose the film on the other side of the x-ray machine from you. X-rays are more likely to interact with dense matter containing calcium than with air, hence your bones and fluid leaves shadows. Returning to gamma rays, when they interact with an atom, which is infrequently and not often while passing through your body, they can also knock an electron off which then can do damage of its own, like a beta particle. But the gamma rays of Polonium-210 are pretty weak and interact weakly not doing much damage. Imagine a car thief wandering through the parking lot looking for a particular make and model. If he finds what he is looking for, somewhere in the lot, he hot wires it, jumps in and drives off. Unfortunately since he is surrounded by other cars, he does damage that way. That damage is not confined to the edges of the lot, however, but can be anywhere, even deep within the lot, wherever the chance encounter with the right car occurred. So gamma radiation can damage any part of your body, and if there is massive amounts of it it can kill you.

Now the alpha particle. It is 7400 times as heavy as a beta particle and has twice the electric charge. It is an ionizing behemoth. Last post I compared the alpha particle to the beta particle as a cannonball versus a bee-bee. As long as I doing the parking lot, it's a Mac truck versus a scooter or Mini. In the distance it travels, however, it does an immense amount of very dense damage, far too much for any biological repair mechanism to keep up with. It is a nuclear wrecking ball. An alpha particle interacts with so many other things because of its size and its forcefield it only travels a few centimeters in air before slowing down completely and in water -- i.e., in you, since you are mainly water -- it only travels 5 or 10 microns. That's hardly the width of a single cell and if external to your body it never gets past the dead layer of skin cells on your surface. It's as if the parking lot was surrounded by the hulks of old car bodies. The Mac truck would crash into them and be stopped. What this means is that no matter how big a hunk of Polonium-210 you have, you can put it in your front pants pocket and it won't hurt you. So what's the big deal?

An alpha particle isn't a danger to you outside your body (unlike a gamma source), but if it gets into your body, look out. And Polonium-210 emits 5000 times as many energetic alphas as Radium, on a mass basis. It is extremely dangerous. A lethal dose is estimated to be only 12 micrograms (12 millionths of a gram; a gram is 1/30th of an ounce). An estimate in Wikipedia gives 3370 lethal doses in a cube of Polonium-210 the size of the period at the end of this sentence.

It's radioactive half-life is 138 days, i.e., it takes that long for half of the Polonium to decay. Meanwhile, however, your body is eliminating it in your feces and urine. It dissolves readily in acid solutions and presumably can be distributed throughout the body, including to radiosensitive tissues like the gastrointestinal tract and bone marrow. The biological half life is 30 to 50 days, more than long enough for this stuff to kill you. The UK authorities are trying to stay upbeat:

The Health Protection Agency is providing expert advice on the public health issues surrounding the death of Mr Alexander Litvinenko. Following the results of further assessments we are updating our advice.

Some small quantities of radioactive material have been found in a small number of areas at the Itsu sushi restaurant at 167 Piccadilly, London , and in some areas of the Millennium Hotel, Grosvenor Square , London , and at Mr Litvinenko's home in Muswell Hill.

We are therefore asking anyone who was in the Itsu restaurant, or who was in The Pine Bar or the restaurant of the Millennium Hotel on 1 November to contact NHS Direct on 0845 4647 where they will be given advice on what to do.

The substance found is Polonium-210. The Chief Medical Officer, Professor Sir Liam Donaldson, is issuing advice to GPs and hospitals on the risks and clinical implications of exposure to Polonium-210.

We want to reassure the public that the risk of having been exposed to this substance remains low. It can only represent a radiation hazard if it is taken into the body ? by breathing it in, by taking it into the mouth, or if it gets into a wound. It is not a radiological hazard as long as it remains outside the body. Most traces of it can be eliminated through handwashing, or washing machine and dishwasher cycles.

The Agency is also investigating the clinical areas of the two hospitals where Mr Litvinenko was treated.

The police investigation continues. We will provide further public information as appropriate.(Update Statement on Public Health Issues)

This is not just an assassination. It is a potential public health disaster. Polonium-210 volatilizes (evaporates) relatively easily, so if there is a source out there it could spell trouble.