I've been trying to get to grips again with radiation levels in this town. The last time I did this was two years ago. Levels which had been as high as 8 μSv/hour on 15 March 2011 fell to 2 in the summer of that year. Now they're down to about 0.3. That's micro sieverts (μSv) per hour. There are 1,000 micro sieverts in 1 milli sievert (mSv). 100 mSv (in a single dose, over the long term, over a lifetime?) cause cancer.
Koriyama has natural airborne radiation equivalent to 0.35 mSv/year. In normal times the ICRP dose limits are 1 mSv/year for members of the public and 20 mSv/year for radiation workers (for 5 years). After the accident at Fukushima Daiichi, areas over 20 mSv/year were evacuated. But there was confusion over what were safe levels for the rest of us. ICRP emergency standards were for a band of 1 to 20 mSv/year. There was outrage when the Japanese government set the acceptable exposure level for children at 20 mSv/year (the same as for evacuation) so they eventually brought it down to 1 mSv/year. Koriyama City is committed to decontaminating all areas to 0.23 μSv/hour which equates to 1 mSv/year.
Koriyama has natural airborne radiation equivalent to 0.35 mSv/year. In normal times the ICRP dose limits are 1 mSv/year for members of the public and 20 mSv/year for radiation workers (for 5 years). After the accident at Fukushima Daiichi, areas over 20 mSv/year were evacuated. But there was confusion over what were safe levels for the rest of us. ICRP emergency standards were for a band of 1 to 20 mSv/year. There was outrage when the Japanese government set the acceptable exposure level for children at 20 mSv/year (the same as for evacuation) so they eventually brought it down to 1 mSv/year. Koriyama City is committed to decontaminating all areas to 0.23 μSv/hour which equates to 1 mSv/year.
Two years ago there was only one monitoring post in Koriyama. Now, we have monitoring posts outside every school and nursery and in every park. And the City is offering free use of monitoring equipment. So about a month ago I pitched up at the ground floor of city office and borrowed a survey meter and a dosimeter. The survey meter (pictured below) measures levels of airborne radiation and I was able to borrow it for a week (14 to 20 June). The dosimeter measures accumulated exposure and I wore it on my belt for a month. Today let's look at the survey meter.
 |
| Outside city office - a standard 0.235 μSv/hour |
The meter resets every 30 seconds and the first thing you notice is that levels are constantly changing. So it's important to realise that the monitoring posts set up outside schools and parks only show levels at that position and at that moment in time. Two meters away and the reading could be quite different.
I found that my meter showed consistently higher levels than the monitoring posts (for example it was 0.365 μSv/hr on the post at Hayama no Yakata Park but showed 0.445 on my meter). When I mentioned this to the staff as I handed the meter back I was told that lots of people had made the same comment and that levels do vary according to the equipment used. The highest variance was at the post at the prefectural office (godo chosha) which is the official reading you see in the papers for Koriyama which when I was there showed 0.19 but the reading on my meter was 0.336. The monitoring post there was different from the others. Perhaps it takes an average over a longer period. I'd like to know the reason for the difference.
Generally speaking, levels were about 0.075 μSv/hr in my apartment and 0.09 in my office. There was not much variation between the rooms and whether the window was open or closed. I was told these are low because they are both concrete buildings and on the 7th and 8th floors. In the car, levels were about 0.2. Outside, levels varied from 0.2 on the east side of the city to 0.3 around the station area and 0.5 in the west. The highest I recorded was 0.9 in a park in the west after rain.
A lot of decontamination work is being done but actual levels were often higher than those shown on signs put up a few days or a week or so ago. Decontamination is needed but the winds blow, the rain falls and the levels change all the time. One thing I can say: don't go near any bags of soil, don't go near when they're doing decontamination work. Levels are high.
The whole thing was stressful and I was glad to hand the meter back.
BTW levels for Aizu were low at 0.1 μSv/hr, Inawashiro and Ura Bandai were 0.07, the same as indoor measurements in Koriyama.
Anne
PS Good discussion of the variation in monitoring posts here http://blog.safecast.org/
PPS Place your bets on the name for the Royal Baby. Favourite is 'George' followed by 'James'.
Interesting post, thanks.
ReplyDeleteYou said that 0.23 microsieverts/hour equates to 1 millisievert/year but I don't think that's right. According to my calculations, 0.23 uSv/hr x 24 x 7 x 52 is about 2 millisieverts/year.
I know that some of the readings might have been concerning when compared to places like London, for example, but compared to Cornwall, which has an average annual dose of 7.8 mSv/year (or 0.89 uSv/hr), even the highest readings you had are much lower than that.
http://www.hpa.org.uk/Topics/Radiation/UnderstandingRadiation/UnderstandingRadiationTopics/DoseComparisonsForIonisingRadiation/
I'm sure you're well aware of this anyway and it doesn't stop it from being stressful. Thanks for your posts, I always read with interest.
The calculations used here in Japan after the accident are based on being 8 hours outside and 16 hours in a wood construction house. That's how you get 0.23. As I've said in an earlier post that's completely unrealistic - people have different lifestyles, on different days - so a better method is to have everyone wear a dosimeter and work out which people are at risk. Focusing on people rather than places. As you know, I'm convinced there is no risk in Koriyama. But it's still stressful.
ReplyDeleteThanks for your interest. Anne
Thank you for the up date on radiation levels, but I must say I lost my £5 bet at the local bookies, I just cant understand why "tahj" was not given to the new baby.
ReplyDeleteHi, Anne -- Sorry, I haven't had time to visit your blog in quite a while. I continue to be amazed at the amount of work you're putting in to keep this up, and the quality of the writing and photos. Well done.
ReplyDeleteOne issue, though. At the beginning, you say "100 millisieverts (...) [can?] cause cancer." Actually, we don't know that. The RERF says that the "association remains unclear" between dose and cancer risk below "around 100 to 200 mSv":
http://www.rerf.or.jp/rerfrad_e.pdf
If (and that's a big "if") the linear no-threshold hypothesis is correct, 100 mSv all at once increases your cancer risk by 1% in absolute terms, which is 5% relative because the background risk is about 20%. But the risk from 100 mSv over a period of many years is "speculated by scientists to be smaller".
That is no longer speculation. It has been shown, by direct observation of the cellular repair processes induced by radiation damage in living cells, that those processes work better at low-rate doses but can be overwhelmed by high doses,
so the response is clearly not linear:
http://newscenter.lbl.gov/news-releases/2011/12/20/low-dose-radiation/
I would also like to give some details of how the monitors you're using work and what their limitations are.
The Horiba PA-1000 is a scintillation counter, meaning it contains a material that emits light when struck by radiation. Electronics then counts the flashes of light.
It records a running average of the rate of flashes over a minute, so it is a short-term reading.
By contrast, the "glass badge" is a long-term average. It works by clever addition of impurites (silver) to the glass, which works in a way somewhat analogous to photographic film. Radiation causes stable "trapping sites" to occur in the glass. Later exposure to ultraviolet light causes fluorescence which can be measured to determine the overall total radiation exposure. Unlike film, the glass badge can be reset and reused by heating to 400 degrees C.
http://www.intechopen.com/download/pdf/22740
But rather than talking about the differences between small numbers, maybe it would be more useful to try to keep the whole thing in perspective. I like this site, because about 60% of the way through, it compares radation risks to other common activities. Unfortunately, they use the older mR (milliRem) unit. To conver milliRem to microSieverts, multiply by 10. So 10 mR = 100 uSv.
http://www.umich.edu/~radinfo/introduction/risk.htm
So roughly 1/4 uSv/hr means 400 hours (16 days) to reach 100 uSv, which under LNT is a one-in-a-million chance of dying of cancer. But living in NYC for 2 days is also a one-in-a-million risk, due to air quality. So you're eight times safer where you are, post-accident, than being in NY. :)
Assuming Tokyo and NY aren't so much different, say within a factor of two on air pollution, this also suggests that the people of Tokyo would be safer moving to Koriyama and soaking up 1/4 uSv/hr rather than staying where they are and breathing Tokyo air. Of course, if they all did that, Koriyama wouldn't have such nice air anymore... :)
Thank you so much for your comments. It's really good to hear from someone who knows their subject. I thought the '100 mSv' causes cancer' was one of the few generally agreed facts so it's interesting to hear of new research which proves otherwise. I like your final comment!
ReplyDeleteIf you don't mind, I'll draw people's attention to your comment in my next post (23 August)
Thanks for your support.
Anne