Saturday 10 September 2011

Equation for internal exposure

In my post Radiation (4) a week or so ago, I quoted Professor Kunihiko Takeda who says that to work out how much internal exposure you get from food, you divide the becquerels by 100, to get the equivalent in mSv/year. I added that 'if you ate one kilogram of food with 500 bq of caesium in it (divide by 100) you would be exposed to 5mSv/yr'.

A reader challenged this and said the figure should be in μSv (microsieverts) not mSv (millisieverts), therefore harmless. I e-mailed Professor Takeda and got a reply confirming that the answer is in millisieverts and directing me to the details. For the record here is the full equation.

You take the bq/kg in the food. Multiply it by the amount you eat. Multiply that by the number of days. Then the equation is  x 2 / 100000. So to take an extreme example, say all the food you eat in a day (1.4 kg) has a measure of 500bq/kg and you eat that everyday for a year, you get the following:
500 bq x 1.4 kg x 365 days x 2 / 100000 = 5.11 mSv/year.

Let's take a more realistic example. In yesterday's paper, most fruit and veg were clear but figs from Minami Soma were showing some radiation. One kind had 77 bq/kg of Caesium 134 and 86 bq of Caesium 137. Add them together and that's 163 bq/kg. So, say you eat 100 grams just once the equation would be:
163 bq x 0.1 kg x 1 x 2 / 100000 = 0.000326 mSv or 0.326 μSv. Negligible.

But the point is that those of us who're living here have to add up external exposure, what we breathe in, and what we take in from food and water. If you think of it that way, the 'safe limit' of 500 bq/kg from food alone is not safe. Especially for children.

Professor Takeda is a professor at Chubu University, has been a member of the Atomic Energy Safety Committee, has received an award for his work on uranium enrichment. He is also a maverick with outspoken views on recycling and the environment. Since the disaster and in the absence of advice from the government that people can trust and understand he has taken on the role of fatherly advisor, especially to parents worrying about their children, helping us understand the figures and doling out lots of practical advice.
I don't know how authoratative he is. Comments welcomed. 



  1. "In yesterday's paper, most fruit and veg were clear". Which paper are you referring to? Thank you for your blog.

  2. Hi
    Thanks for your comment. I'm referring to our local paper, the Fukushima Minpo, which has been brilliant in this crisis giving us detailed information that's just not available in the national press. It's only in Japanese though. This is the link for a pdf of its 'Daily Life Information' page. At the bottom left are the radiation levels for every village and forecasts for weather and wind at the reactor. At the bottom right, levels for food. In this example, September 13th, the table is showing levels for grass, silage and straw for fodder. Sunday's paper had rice, mushrooms and beef.

  3. Anne,
    (sorry I replied to your original post without reading this one).
    The calculation as presented looks OK with the IRCP dose equivalent of 0,000000013 Sv/Bq for Cs-137, but I am not a specialist.
    Compared to your previous post, I did not understand that it was 1 kg of contaminated food at 500 Bq/kg EVERYDAY.

    Now, from my (external) point of view, this situation is not good but not "not safe" with a risk to your life, people are living in higher irradiated zones everyday without prejudice (natural radiation).
    You can do the calculation yourself: take the external exposure in your area (?) + plus the worst case scenario with food (everything is contaminated at 500 Bq/kg) so 5 mSv/y. I am assuming that water is fine.

    Now, for the children, it is difficult to be cold and rational, I have no comment.

    I understand this is a shitty situation, let's hope that decontamination efforts may bring the dose down.

  4. Thank you for sharing these insights, Anne.

    For reference, contamination values after Chernobyl observed in Germany and elsewhere.
    Estimates of body burden.

  5. Thank you for your comments.
    I'll be looking at this again soon.

  6. I doubt people will be eating food at that level every day.

    But in a bad area would eg 0.5 mSv/year ingested be unrealistic. That's half the normal yearly dose but internal which is a lot worse.

    The limit might be safe for a few meals per year or even at the limit constantly for almost a week. But eating stuff between 100-500 every day for years isn't 'safe'. Give the kids some cigarettes and give 100,000 kids chainsaws. I'm quite sure only 1% of them will kill someone which is 'safe'.