Friday 6 April 2012

Food 1

Hi again,
As I said earlier, we have new stricter food standards as of April 1st. The level of Caesium allowed in the new category of 'general foods' is 100 becquerels/kilogram. It was 500, so it's a big change. But three days into the new regime, bamboo shoots from Chiba (outskirts of Tokyo, Narita airport) were showing 120bq/kg and shiitake mushrooms grown outside in Miyagi  (north of here) registered 350 bq/kg.  Japanese are great foragers, and there are lots of lovely buds and shoots to be had at this time of year (make wonderful tempura) but mushrooms and berries really soaked up the radioactive nasties at Chernobyl so it's probably not a good idea to forage in the woods. You can buy cultivated ones anyway - though that's not as satisfying as getting 'summat for nowt'.

Talking of Chernobyl, I remember last summer bemoaning the fact that we didn't have facilites like they have in the Ukraine where people can go along and test their own food. Well, now we do. Koriyama, which covers a wide area, has installed 40 machines in village halls. I went to the one opposite my office in the Big-i  building next to Koriyama station and here's a picture of me with my newly tested rice from Hirata village. Yes, fellow rice planters and harvesters of the Odaira Club, you'll be pleased to know that the rice from Sato-san's farm is as near as dammit free from contamination.

It works like this. You take along a bag of rice (5-6 cups) and they put it in the machine and you go back half an hour later for the results. I slipped out on a Thursday afternoon and it wasn't at all busy but the woman on duty said it would get busy from now on as spring produce appears in gardens and small farms. (You can take in food from shops as well.) To test vegetables you have to chop them up small and you need 1 kilo of stuff, as closely packed as possible. I got a slip of paper to say that no radioactive Iodine 131, Caesium 134 or Caesium 137 had been detected in the sample. But with the rider that the machine can't detect below 12.1 bq. According to the lady at the centre, it's not zero as there will always be some background radiation. A germanium machine gives more accurate results but those machines are very expensive (10 to 20 million yen, 75-150,000 GBP). The machine she was using (that little thing at my feet) costs 3.6 million yen (28,000 GBP).

By  international standards 100 bq/kg is low. The EU was 1,250 but tightened it to 500 after the accident in Fukushima. In Canada it's 1,000 bq/kg. Had some positive feedback from a reader in Canada. Take a look at the end of the post 1st April - All Change

He (she?) says, "The new limits on Cs-137 levels in food seem quite conservative to me, perhaps overly so. Here is a previous discussion following an article in the Vancouver Sun, which at least had the journalistic integrity to label the source as an "anti-nuclear group" in the headline:

Mine is the third comment. I calculated the dose from eating 400g a day of fish contaminated at twice the new limit to be 0.4 mSv, about 1/6 of average annual background radiation. So I think the new limit mainly serves to hurt farmers and fishermen without really improving protection."

I'll have more to say next time about how farmers are coping with the new regime.
Cherry blossom at its peak in Tokyo but still cold here.
Goodnight all


  1. Hi, Anne --

    It's "he". :)

    Many years ago, I used to work for a company which made germanium detectors like the ones you're talking about. The less expensive one probably uses NaI (Sodium Iodide), which also allows isotope identification but with much less precision.

    It takes half an hour for your test. For really exacting measurements of the specific mix of isotopes in a sample, scientists using germanium detectors leave the sample in a much more heavily shielded container than the one in your photo for a full day or more. The shielding itself is made out of special "low background" lead which has very low levels of radioactive contaminants, usually salvaged from pre-WW II ships and buildings:

    By reducing the interference from natural radioactivity and cosmic rays, and counting for tens of hours, it is possible to identify specific isotopes in really tiny quantities (picograms, a millionth of a millionth of a gram).

    So, to put things in a bit of perspective, an average 70-kg person contains 8100 Bq of natural radioactivity:

    Scroll down about 60% of the page to "Human Body", and you'll see 4400 Bq of K-40 and 3700 Bq of C-14, ignoring all the minor isotopes. If we divide 8100 by 70, we get 116 Bq/kg. So you'll no doubt be pleased to know that the new food limit effectively bans cannibalism in Japan, because people are just too radioactive to eat!

    Of course, that's cheating a bit, because Cs-137 does not occur naturally and any amount ingested is additive to the natural internal dose. Still, it gives a sense of scale for how small 100 Bq/kg really is.

    Cs-137 is used as a test source to calibrate Geiger counters and other types of portable radiation detectors. For $125 USD, I can buy a little pellet with up to 10 microCuries of Cs-137:

    Scroll down to the "Radioactive sources" section. These are amounts which can be shipped anywhere by regular post and kept with no license from the NRC; in other words, it's not regarded as excessively hazardous by the US Government. But if we convert to SI units 10 uCi is... 370 thousand Bq! Eating it would be equivalent to consuming 3.7 metric tons of material contaminated at 100 Bq/kg, and would expose me to a 5 mSv dose (twice annual background in the US).

    Here's how I get that dose conversion. The NRC specifies annual allowed occupation exposures for all radionuclides. Here's the page for Cs-137:

    The oral ingestion ALI is 100 microCuries (recall this is an occupation exposure limit for radiation workers, not the general population). ALI is defined as the amount of ingested material per year which results in 50 mSv of annual dose:

    So we can scale down to 10 uCi at 5 mSv, or 1 uCi at 0.5 mSv. 1 uCi is still 37000 Bq, or 370 kg of food contaminated at the maximum allowed value. So a full kilogram a day of worst-case contaminated food would result in only 1 percent of the annual exposure in the "Red Zone" of the ground contamination map you published a while back.

    Sorry to be so long-winded here, but there are so many people actively trying to scare you that it's worthwhile getting a deeper understanding of what the numbers mean.

    What matters is the cumulative average, not any particular meal. So go ahead, enjoy the wild mushrooms once in a while -- just don't eat tens of kg of them. :)

  2. Hi Anne, I fully appreciate your concern for the farmers, and what has happened to them breaks my heart. But we do have to think about the children of Japan, and it is safer to stay on the conservative side. These poor children will be accumulating radioactive materials in their bodies for their entire lives, and no one really knows what the effect will be. It's all guesswork. I am married to a Japanese and have two young children, so this issue is something that affects me profoundly.

  3. Although the evidence seems to suggest that effects will be minimal (see comments by Diogenes NJ above and after the post 'Still Waiting', also my post 'Expert Advice') you are right when you say it's all guesswork. There is no data yet on long term low dose radiation and we are all part of a giant experiment. I sympathise with you for it must be very hard when you have children to think of.