Wednesday, March 23, 2011

Radiation risks

Or, how I stopped worrying and learnt to love the Fukushima Daiichi nuclear power plant.

Over the past rather abnormal week or so I've read a lot more about radiation and nuclear power than I ever thought I wanted to know. Mostly, I learnt that there are about half a dozen different and incompatible ways of measuring radiation, though the Sievert seems the most directly applicable, being a measure of the biological impact. Even this is sometimes presented misleadingly with people talking about Sv when they really mean a rate, Sv/h, and vice-versa. There's a typical example in this horribly confused Yomiuri article which talks about the likely fatal effects of 3,000mSv/h, when they presumably mean 3,000mSv total (over some short time). Another here in the Japan Times reports a reading of "0.000142 millisievert" in the Tokyo area, which obviously misses "per hour". It's possible that the Japanese media is particularly bad at this: their style is often to spew out context-free numbers, rather than actually educate (eg crime stats of foreigners, which are never actually directly compared to those of natives, because they are generally lower on a like-for-like comparison), and adding in mistakes makes it even harder to make sense of.

Xkcd (which I copied here), while not claiming to be a reliable resource, provides a convenient summary of a wide range of exposure factors and outcomes. David Spiegelhalter also has a good article here about the risks of Fukushima. He links to this article which suggests a 0.04% risk of cancer (29,000/70,000,000) from a CT scan, say 5mSv, or round it to 4 for convenience. That works out (under the linear assumption, which is disputed but let's gloss over that for now) to 1 cancer per 10Sv, whereas this perhaps somewhat rosy-hued article pegs the risk as 4% for 1Sv, or only 0.4 cancers per 10Sv. I'll stick with the worse figure and invent the new(?) unit of the microcancer (µC), analogous to micromort, and use 10µSv = 1µC. (The purpose of this post, as some of you may have guessed, is really just to show off that I've found that Option-m = µ on Macs.)

If we assume a typical cancer incidence of 25%, then a typical day of life averages about 9 µC (0.25 per 80 years). Typical background exposure is about 3mSv per year or 10 µSv per day, which would imply via my conversion factor that this background radiation is responsible for about 10% of all cancers. So far, these hand-waving approximations all seem to hang together surprisingly plausibly.

So, what about the effects of Fukushima? According to this (governmental) web site, 5 µSv/h is the the ambient radiation rate at which official measures have to be taken in Japan. That value is at the very top of the graph, with the observed data (for the prefecture I live in) nearly flat-lining at the bottom around 0.1µSv/h, though it's clearly been creeping up in the last couple of days. There were a couple of bigger bumps last week too which have dropped off the graph now, but nothing as high as 0.1µSv/h. There is certainly some leakage from the plant going on, that is reaching us. Other prefectures are a bit closer to the threshold (the parent page is here) and Fukushima, for which there is a pdf of lots of stations rather than a single graph, has much higher readings in some places.

5µSv/h is 120µSv/day or 43mSv/year, roughly the limit for workers at nuclear power plants. According to my conversion factor, this should translate to 0.4% additional cancer chance per year, which would add up substantially over a lifetime. Now, nuclear power station workers don't have such significantly elevated cancer levels, but they also probably don't actually get that much radiation either continuously throughout their entire careers - it's the limit, not a target. So I'm not ready to throw away the calculations yet.

100µSv/h sustained continuously with no attempt at mitigation would amount to about 70mSv in a month, and this would certainly mount up to a pretty substantial dose (and cancer risk) over time. Some of the Fukushima readings are up at that level around the edge of the 30km "stay indoors" region, and I don't think I'd want to stay there indefinitely if the level does not drop. However, even in this case, it would not amount to an immediately dangerous emergency requiring a panicked evacuation. It may be reasonable to stay there for a week or two, especially if it's possible to stay mainly indoors, and hope that the situation will be brought under control in that time. Assuming it is mostly iodine, the half-life is only 8 days so it won't hang around indefinitely.

It's also worth noting that cancer rates vary pretty widely from place to place and lifestyle to lifestyle anyway. Japan has famously high stomach cancer rates, for example, probably due substantially to the diet. Of course it has low rates for other types and a long life expectancy overall, but the point is that these risks vary substantially due to various lifestyle factors, that people show no signs of wanting to change. Some people even choose to smoke, probably because they are nervous about a nuclear catastrophe :-)

This page suggests about a 15% chance of lung cancer from smoking, so if we assume a 20 per day habit for 40 years, each cigarette is about half a µC, and the day's tally is 10µC or equivalent to 100µSv of radiation, a rate of 4µSv/h continuously through the day. So it seems that plenty of people find that sort of risk completely acceptable.

From the public health perspective, of course, these figures seem much more alarming. Even a single microcancer per person adds up to tens of cases over the Greater Tokyo area. And I think it's entirely right that there should be regulations to limit the extent to which TEPCO and similar can just dump pollutants on everyone. Beyond the health impacts, it is also (at least potentially) a major environmental problem for the immediate area, if substantial contamination does occur (or already has). Farming in the area may be completely wiped out. So I don't necessarily disagree with the Steve Bloom types who are going on about what a catastrophe it is. However, I'm approaching this analysis primarily from the selfishly personal level, as the British Embassy requested that I "consider leaving the area". I have done so (considered, that is, not actually left!), and it seems to me that the risks are entirely acceptable to me at the current level. By this I don't just mean the actual radiation which is currently negligible here, but the threat of an increase should things turn pear-shaped. I've got several orders of magnitude to play with before I need to feel worried. I can still resent TEPCO for imposing the risks on me, but reasonably decide that I should stay here rather than run away (taking a small additional radiation dose en route) for some uncertain, but potentially rather long, period of time.

If we assume (quite reasonably) that pedestrians suffer about 10 times as many serious injuries on the roads as deaths from traffic accidents, and (as a wild guess) that a typical pedestrian crosses a road every quarter-mile, then it seems from the micromort animations (1 micromort = 17 miles of walking) linked above that a single µSv of radiation exposure is quite literally not worth crossing the road to avoid. I'm equating a cancer with a KSI statistic here, which may be debatable but doesn't seem entirely ridiculous. Coincidentally, the peak radiation rate in Tokyo a few days ago, that had the media in a quite a tizzy, was a little less than 1µSv/h (and this level didn't last long). Hope no-one ran home in a panic.


Dallas said...

I was looking at the fall out numbers at this link and trying to get a estimate of health risk.

The non-linear cancer risk makes it very difficult to figure out. Comparing with Chernobyl was the only way I could a reasonable baseline. Chernobyl was about 7 orders of magnitude greater(EBq versus MBq).

Then over 80% of the Beta/gamma measured was I-131 which in a month or so will fade into background.

I guess this is why we under pay research scientists to figure it all out :)

PS, Hold off on spinach until next season, unless you happen to be older than 55.

Steve said...

James, maybe you can explain this basic point to me, or refer me to the best site to explain, because, as a non-scientist, I still find it unclear.

When they are measuring radiation at any significant distance from the reactor (say, more than a few km away, and definitely in Tokyo) the radiation is coming from particles or gases in the air, isn't it?

But a mere radiation reading doesn't tell you what type of gas or particle is causing the radiation, does it? As I understand it, some radioactive gases are not readily absorbed and do not represent as big a danger from ingestion. But others - I-131 and Cs-137 of course, do.

So, how does a simple radiation measurement (of the type you mention around Tokyo) help a person know whether the stuff in the air or on the ground causing the reading is the worst stuff, or the not as bad stuff?

I hope this does not sound too dumb.

Anonymous said...

According to the xkcd graphic then there are some sites 50km NW of flushing where 3.6mSv/day has been measured. On your scale that is 1 cancer unit every 7.6 years.

EliRabett said...

The biological effect of each type of radiation and source is incorporated into sieverts as a bugger factor

Jesús said...

Does anyone know within what time evacuated people might be able to get back home in Fukushima? Weeks? Months? Years? Decades? Thanks.

How about food coming from Fukushima, James? Do you think it may pose a risk? Or you trust the Japanese controls in this respect? Thanks.

Steve Bloom said...

Thanks for the plug! It gives me an RP Jr. sort of feeling. :)

But did I say it was already a catastrophe (relative to the southern Tokyo 'burbs) as opposed to there being risk of such? Being prepared for an out-and-out catastrophe (meltdown with major release) remains prudent, although the oddds on that improve with each passing day.

As it stands, the greater portion of the anticipatable fatalities from the incident are very likely to have already occurred as a consequence of attention diverted from rescue efforts. Let's hope that remains the case.

James Annan said...

Jésus, it will depend on the nature of the contamination. Iodine has an 8 day half life, but caesium will last for decades.

It will surely wipe out agriculture in the area for some time, though Japan has a strong culture of food labelling scandals and I bet the stuff gets into the food chain on a regular basis unless the authorities ban all production (rather than relying on testing).

Personally, as a 40+ adult with a (likely) finite duration here, I think the safety limits are so low that I'm not in the least bit worried. In comparison, about 1500 kids were given salmonella poisoning with a single batch of dodgy salad not so long ago. None of them died in that instance, but plenty of people do die of it each year.

James Annan said...

Steve, I think they measure iodine and caesium because these are the dominant risks (eg total load x harmful effects per atom). Xenon can be used as an early marker of a leak (eg for detecting secret underground bomb tests) and also was the first thing to get to the USA, but in (even more) trivial quantities.

I hope someone is testing for other things too, but being Japan it is not impossible that they are not mentioned in the legal schedule, and therefore no-one is looking for them. At the moment the ongoing leakage seems to be a bit of a mystery, my guess it is means something fairly important is more broken than they are letting on.

bluegrue said...

The PDF on measurements in the Fukushima province shows that most of the measurements were carried out using ionization chamber type survey meters and - to a smaller extent - NaI scintillator detectors. Either of them can in principle be used to record the energy of the ionizing radiation and hence fingerprint the emitting nucleids in a laboratory setting, however, I seriously doubt that survey meters have that capability.

The exposure limits for professionals vary from country to country. Whereas for American workers of nuclear plants it seems to be 50mSv per year in Europe it is 20mSv per year; Germany imposes an additional limit of 400mSv over the entire lifetime. As for actual exposure, the German Federal Authority for Radiation Protection has the following to offer: in the calendar year 2007, 99% of the supervised workers had an annual dose below 3 mSv; the average effective dose of all measurably exposed workers was 0.8 mSv, a value that reached only 4% of the permitted annual limit of 20 mSv. This data may be useful for your µ-cancer estimates.

Jesús said...

Thanks for your answer, James.

At least in sea water, they are measuring other isotopes too:

"Testing for a range of radionuclides showed amounts below regulatory limits for cobalt-58, iodine-132 and cesium-136. Detections were far above limits, however, for cesium-137, cesium-134 and iodine-131"