I don’t have any clearer picture than any of you on what is happening in the Japanese nuclear power stations at the moment. I’ve only got the official statements to go on, the same as anyone else. But one thing I can talk about a bit is the nature of radioactive risk and to try to untangle the plethora of different measures of radiation and their units.
Radiation can cause damage to cells in your body, and this in turn can cause nasty effects such as cancer or genetic issues with any children you have in the future. The biology of this I’m not so sure about, but in terms of the physics, each radioactive particle carries energy and that energy can cause damage when it hits you. It’s rather like a cricket ball carrying kinetic energy that will damage any car that gets in the way. Now, you can’t escape radiation. We are continually bombarded by cosmic rays (and, if you live for example in a granite region – radiation from rocks as well) – and so lots of these minature cricket balls are hitting you every second. Each one of these has the potential to cause damage to a cell that leads to something nasty.
This means that the more that hit you, the greater the risk. The risk is cumulative. A dose-equivalent (I’ll mention what that means in a moment) of 1 milliSievert for 10 days gives the same level of risk as 10 milliSieverts for one day. It also means there is no such thing as a ‘safe’ level of radiation. Any radiation at all gives a chance of an adverse effect on you. It’s like playing a backwards lottery. Imagine a zillion tickets, of which a zillion minus one are winning tickets. Just one is a losing ticket. Each time you are exposed to a radioactive particle you enter the lottery, staking your life. Chances are that you’ll win, and live, – but, play the lottery enough times, and your chances of at some point buying that losing ticket become greater overall.
So what about those units? An easy measure of radioactivity is ‘Activity’. This is a measure of the number of radioactive decays (when an atom changes its status, emitting an alpha, beta or gamma particle – or sometimes more than one) every second. It’s measurable with a Geiger counter. The Systeme Internationale units are the becquerel (after Henri Becquerel, the discoverer of radioactivity), but often the old curie unit is used.
Activity doesn’t correlate well with risk to you. A further measure, that is relatively easy to do in a lab, is exposure (measured in roentgens). This is a measure of the amount of ionization of air that a radioactive material causes – ionization being when a molecule has electrons stripped from it by the particle.
Then we bring in Dose. The dose is a measure of the energy absorbed (e.g. by you). This is measured in grays. It’s a physics-based measure, not a biological-based measure, so still isn’t the best measure of your risk factor. A better measure is the dose equivalent. The unit is the sievert, and it is a reasonable measure of the chance of causing cell damage that leads to cancer etc.
Remember, it’s a cumulative risk, so you want to add up the total dose equivalent you have had over your life. To give you some idea, a typical individual gets about 2 milliSieverts per year. I was hearing on the radio today that they are talking about 200 mSv per hour at places within the Fukushima complex – that’s basically a lifetime’s worth in an hour. You would not want to be there for a particularly long time. But, even so, if you were not exposed for too long a period of time, it would still not be a terribly large risk.
Radiation safety decisions really come down to ‘risk’. I’ll close by saying that we are pretty bad at estimating risk generally speaking, and when presented with a figure we might scream in fright, but be prepared to take much greater risks everyday, for example when driving to work or crossing the road. (E.g. do a quick estimate on your chance of dying in a car crash this year – given that about 400 New Zealanders die a year out of a population of 4 million – the calculation is an easy one. Would you be prepared to enter a backwards lottery with those chances? Most people do.)