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.