Radioactive Food and Soil Reports – What do they mean?

[ add-on afterword:  CAUTION: The below is likely to contain errors.  It has been up for over a year, yet no one has pointed out my mistake…  In any case: I suggest you contact experts to have all its errors pointed out to you (and to me, please!  Unless I was right… in which case this is really disturbing…   / Calculation adjustment made 3/25/2011  7pm SMT.  Please do not quote from this post- It should be proofed by an expert first.  Thank You.]


I’m trying to make sense of the radiation unit ‘Becquerel’, in regards to radioactive food… How bad is it what they are reporting?

Two reported recent measurement, both for Cesium-137:

  • 890 Bq/kg of leafy green, grown in Tokyo, 240 km from Fukachima.
  • 163,000 Bq/kg of soil, detected in Iitate Village, about 40 kilometers northwest of the Fukachima- Daiichi plant.


– [CNN [via Twitter-streamed link]:  9:51 p.m. ET Thursday, 10:51 a.m. Friday in Tokyo] “Japan’s health ministry says radiation above the legal limit has been detected in a vegetable grown in Tokyo, NHK reportsRadioactive cesium was found Thursday in a leafy vegetable taken from a field in Edogawa ward. The vegetable, called Komatsuna, or Japanese mustard spinach, contained 890 becquerels per kilogram, exceeding the legal limit of 500. This is the first time that radioactive cesium exceeding the legal limit has been found in a Tokyo vegetable.”

Legal limit: 500 Bq/kg     In a Tokyo Leafy Green:  890 Bq/kg

My Radiation Units page doesn’t cover now also DOES cover “The Becquerel” yet, and, At the time Wikipedia didn’t give me a clue on how to calculate a dose equivalent for humans from such readings…  A Google search lead to this excellent website: In their FQA they state:  “… Many people are searching for How do I convert activity to dose or dose-rate?  We think that conversion is the wrong term. Conversion usually means, what number do I multiply Ci and Bq by to obtain R, rad, rem and Gy or Sv?  What you should be asking is how do I calculate dose-rate or dose for a given activity of an isotope? It is indeed a complicated calculation, not a simple conversion.  There is no number that you multiply Ci and Bq by to get R or rad and Sv or Gy.  Different isotopes emit different energy gammas and some emit more than one gamma.  At the same rate of emission or decay, an isotope that emits more and higher energy gammas will give a higher rem/hr or Sv/hr. Ci and Bq are based on dpm/sec (dps).  Consider the same emission rate (dps) for Cs-137 and Co-60.  Co-60 will give you a higher dose-rate and dose because it emits two gammas above 1000 keV where Cs-137 emits only 1 gamma at 661.8 keV. Obviously Co-60 is emitting more electromagnetic energy per decay than Cs-137 which will give one a higher dose-rate in R/hr or Sv/hr.  Also, the dose and dose-rate varies with the distance from the source. You cannot convert from Ci and Bq to R, rem or rad and Sv or Gy. You must do a complicated calculation on an isotope by isotope and a distance by distance basis. The formula can be found here.  If you can eliminate the word “convert” from your question and replace it with “calculate”, Rad Pro Calculator can give you your answer, here. Rad Pro Calculator also calculates dose-rate in rad/hr and Gy/hr from beta emitter activity here.”

The gamma calculator it is:

So what I need to do the calculation is:  The isotope + the distance:  Cesium-137, and since we’re talking food, I suppose we can set the distance at one millimeter (0.1 centimeter (0.1 cm)), as it passes through the body when ingested.

–> Run through the calculator above, for 500 Bq/Kg, with gamma @ 0.1 cm, for Cs-137, that gives me a reading of 4,356.3 µSv/hr.

For 890 Bq/Kg, that would translate to 7754.2 µSv/hr in gamma radiation ‘equivalent dose’. Cesium’s half-life is 30 years, so the radioactivity won’t drop quickly enough to take into consideration if it’s in your food.   See more below at “So, what does it mean?”, after I’ll look at the soil sample, and combining it with exposure from Cesium-137’s Beta rays…


[NHK-WORLD NEWS:  Wednesday, March 23, 2011 19:02 +0900 (JST) Extremely high radiation found in soil.  … 40 kilometers away from the troubled nuclear power plant in Fukushima Prefecture.  The disaster task force in Fukushima composed of the central and local governments surveyed radioactive substances in soil about 5 centimeters below the surface at 6 locations around the plant from last Friday through Tuesday.  The results announced on Wednesday show that163,000 becquerels of radioactive cesium-137 per kilogram of soil has been detected in Iitate Village, about 40 kilometers northwest of the plant.  Gakushuin University Professor Yasuyuki Muramatsu, an expert on radiation in the environment, says that normal levels of radioactive cesium-137 in soil are around 100 becquerels at most. The professor says he was surprised at the extremely high reading, which is 1,630 times higher than normal levels.  He warns that since radioactive cesium remains in the environment for about 30 years it could affect agricultural products for a long time. He is calling on the government to collect detailed data and come up with ways to deal with the situation.  [from / my emphasis – mvb]

So, we’ve got Cesium-137 in soil, what radiation dose do I get from walking on that?  Let’s take gamma distance @ 1 cm  –> Run through the calculator: for 163,000 Bq/Kg, with gamma @ 1 cm, for Cs-137, that gives me a reading of: 14,200 µSv/hr.  Wowza.  This is 40 km from the plant?  Holy sm…  (And, gotta say, on a sidenote: it doesn’t “stay in the environment for 30 years”, no, it’s half-life is 30 years, meaning it remain noticeably in the environment as a radioactive contaminant for 100 to 600 years. (see endnotes on that)

And I haven’t even touched on the Beta rays.  (there’s alfa, beta and gamma rays.  To learn more about these, see )


Okay, that was just for gamma rays, pretending for a moment Cesium-137 didn’t also emit Beta rays.  From :“Beta particle radiation is generally a slight external exposure hazard, although prolonged exposure to large amounts can cause skin burns and it is also a major hazard when interacting with the lens of the eye.”

The Rad Pro Calculator also has a Beta calculator:

Now the problem is, I don’t know how much of ‘the activity’ (in Bq/Kg) comes from gamma rays and how much comes from beta rays.  So just like the above calculation is for “all from gamma rays”, these calculations are from “all beta”.  The truth is probably somewhere in between.

So I ran the data run through Rad Pro Calculator (Beta) and then converted to µSv/hr for easy comparison to the Radiation Exposure Effects page.

– The leafy green of 890 Bq/Kg —>  if all from GAMMA: 7754.2 µSv/hr

–> if all from BETA:  17.48 mGy/hr ≈ 17,480 µSv/hr

I have no clue where in between these two values reality would lie. For calculation-ease, I’ll pick 10,000 µSv/hr

– The burning soil of 163 kBq/Kg –> if all from GAMMA: 14,200 µSv/hr

–> if all from BETA:  32.84 mGy/hr ≈  32,840 µSv/hr

I have no clue where in between these two values reality would lie.  For calculation-ease, I’ll pick 20,000 µSv/hr



— For the highly contaminated leafy Green found growing in/near Tokyo…  we have a guestimate of the equivalent dose rate (10,000 µSv/hr); and it really is a guess, because it is not reported anywhere in the media what distance is used for calculations from activity to equivalent dose.

think it means this: (I’m making these digestion/elimination numbers up just for a rough estimate of the effects):  Say you eat 100 grams (tenth of a Kg), and let’s say you eliminate 90% after 1 day, another 9% the next day and 0.9% over the rest of the weekand have 0.1% of these hundred grams (so 0.1 g) left circulating around in your body for, (let’s just say:) another week.  What extra ‘dose’ of radiation would that give me, over these 2 weeks since I ate that damned leaf of delicious Japanese mustard spinach?

890/kg = 78/100g (gamma + beta) =>  1,000 µSv/hr  x 24 hrs =  24,000 µSv

(90% eliminated, 10g left):  ==> 100  µSv/hr x 24 hrs =     …………………. 2,400 µSv

(99 % eliminated, 1g left):  ==>  10 µSv/hr x 24 hrs x 5 days = …………..  1,200 µSv

99.9 % eliminated, 0.1g left  1 µSv/hr x 24 hrs x 7 = ……………………………..  168 µSv

Total: ………………………………………………………………………………………… 27,768  µSv/2wks

On a year-basis, eating just once a portion of 100 grams of such contaminated food would (if these calculations and assumptions about distance and eliminations are reasonably correct) amount to approx. “at least 25,000 µSv/yr” EXTRA, on top of the normal background radiation of about 1,500 to 4,000 µSv/yr.

My conclusion from that is that eating just 100 grams of this highly contaminated leefy green ONCE would not necessarily cause health problems, at least not that year or the few following it, BUT 1) have a similar dose just 10 times that year and cancer risksincrease notably (only affecting some, not all), and various other radiation effects (before full-on radiation sickness) may occur too.  2) Long-term health effects, from chromosome damage to reproductive cells (potentially leading to birth defects), chronic fatigue, headaches, mental problems, various cancers later in life, etc may not be easily traced (statistically) to those radiation exposures from such a relatively small amount of badly contaminated food, but it is known that ANY amount of ionizing radiation can do damage to cells.  So, yeah: don’t eat it, they’re not kidding, it isn’t safe.  And this is 240 km from the plant…

— As for the walking on the contaminated soil 40 km from Fukachima Daiichi…

Receiving an equivalent dose of 20,000 µSv/hr means your cancer risks starts to increase notably after an afternoon of walking there (at least for the exposed feet soles).  But hang around for months and you’re bound to get radiation sickness.  This is, of course, if you were to be in close contact with this kind of contaminated soil day in day out.   But If you were to just take a nap on a lawn with such sol, within 5 hours you would receive the maximum yearly dose allowed for civilians.  Even if you didn’t lay on the ground, it’s likely that your exposure will reach the 230,000 µSv/hr within the year, the criterium used for relocation at Chernobyl. What this tells me is what the Japanese government has yet to admit:  an entire region is already or will soon be de facto uninhabitable.

This is beginning to look a whole lot more like Chernobyl.



  • Websites visited in the process of writing this, see further below.
  • For conversion help, see ‘Radiation Units’, and further below.
  • This article helped me get started:  Written by Jeffrey Kluger [Senior Time writer, licensed attorney, and intermittently taught science journalism at New York University -mvb]:  “There are four kinds of isotopes that are likeliest to be emitted by the crippled Fukushima Daiichi plant […] :

iodine-131cesium-137strontium-90 and plutonium-239.

Iodine-131 … can lead to cancer — specifically thyroid cancer …

Strontium and cesium are the next up the danger scale. While iodine tends to concentrate its damage to the thyroid, those two are not nearly so selective. “Strontium is chemicaly similar to calcium,” says Dr. Ira Helfand, a board member for Physicians for Social Responsibility. “So it gets incorporated into bones and teeth and can stay there, irradiating the body, for a long time.” Strontium is most commonly linked to leukemia.

Cesium works in other ways, behaving more like potassium when it’s inside the body — which means it circulates everywhere and can contaminate anything. Cesium doesn’t linger as long as strontium does — it gets excreted in urine over the course of months or years — but that’s more than long enough to cause cancer of the liver, kidneys, pancreas and more. …   Strontium has a half-life of 29 yearscesium’s is 30.

A radioactive isotope is generally considered dangerous for 10 to 20 times its half life, …

Most worrisome of all is plutonium-239 — for a number of reasons. First of all, the vast majority of a fuel rod is made of plutonium, which means there’s just more of it in play. What’s more, says Helfand, “It’s extraordinarily toxic.” Plutonium exposure usually comes from inhalation rather than ingestion, so it’s mostly associated with lung cancer. What’s more, plutonium’s half life is 24,000 years, which means anything released in Fukushima today could be around at dangerous levels for up to half a millon years.
“Wherever it is that a power plant is leaking radiation, you want to be somewhere else — preferably a very distant somewhere else.”   [my emphasis and deleting.  Read FULL ARTICLE HERE(=]

  • Also checked for this post:

  • Used in conversion from milliGray to microSievert:

1 mGy = 1000 µGy = 1,000,000 nGy  ≈ 1,000 µSv

Thus  1 mGy ≈  1,000 µSv/hr

17.48 mGy/hr ≈ 17,480 µSv/hr

32.84 mGy/hr ≈ 32,840 µSv/hr

—— end of Post —–

PS:  Please somebody proof this and let me know if there’s errors.  I write this stuff late at night before I dose off, to get show up at work just 7 hours later… Yawn.

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