More scientific errors from the Japan Times

In an article entitled “Food not checked for radiation poses risk in Fukushima: study“, we once again see a couple of errors in the Japan Times’ reporting.

Researchers followed nine people, who were the only ones out of 30,622 examinees from the city to have internal cesium-137 levels greater than 50 becquerels per kilogram in screenings between March 11, 2012, and March 10, 2013. That’s roughly equal to 0.1 to 0.2 millisieverts per year.

That’s pretty impressively reassuring figures – only 0.03% of the tested population coming up “hot”!

Cesium-137 levels among the nine participants ranged from 3,230 to 15,918 becquerels per body, which corresponds to between 0.07 to 0.53 millisieverts per year

I’m not quite sure why 50 becquerels per kilogram was 0.1 to 0.2 millisievers per year in the first quote, but now 0.07 in the second. Furthermore, the report says the figures were calculated as 0.14 to 0.97 mSv/year, although that figure combined both Cs-134 and Cs-137.

Finally, the introduction to the report itself says:

Accordingly, serious health threats have emerged in radiation-contaminated areas after nuclear accidents such as the Chernobyl accident [2], and similarly, cumulative radiation exposure is currently a serious public health concern in Fukushima [3].

Looking at the two references here, both are not, as the casual reader might suspect, related to contamination, but instead appear to focus on the mental health of people from the worry about radiation. They seem to be odd references to me.

  1. While I cannot find at quick glance a “becquerel to millisievert conversion chart”, I think the difference you highlight, Ken, is fairly simply explained:

    9 people had internal cesium-137 levels of 50Bq per kg or more. 50Bq per kg of body weight results in a ROUGH exposure range of 0.1 to 0.2 mSv per year.

    I take that to mean that the more kg one has, the more total Bq, the more mSv. In other words, a 40kg person at 50Bq/kg would be 2,000Bq total, with whatever the resulting mSv would be from that (and, doubtless, the mSv would also depend on what element we are dealing with, but in this case it is Cs-137), a 50kg person at 50Bq/kg would be 2,500Bq total, etc.

    Now, for the nine participants their PER BODY total Bq of Cs-137 was between 3,230Bq and 15,918Bq – according to the JT article anyway, reading the report the low end was 2,130 Bq/body and 50.7 Bq/kg, which gives me a 42kg individual. Double checking the Cs-134 numbers for that 69-year-old woman also gives me 42kg body weight.

    On the high end we have a 64-year-old male with 15,918Bq/body or 216.8Bq/kg, which gives me 73.4kg in body weight.

    Now, if I divide the low Bq/body (2130) by the high (15918) I get 13%. If I divide the low mSv/year (0.07) by the high (0.53) I again get 13%. So these things seem to correlate.

    However, the low per kg (50.7) divided by the high per kg (216.8) gives me 23%, which tells me that what sets the exposure in mSv per year is TOTAL Bq/kg. This explains why “50Bq/kg results in exposure of 0.1 to 0.2 mSv/year”. For the two individuals listed, given Bq/body divided by given mSv/year gives me a factor of 30427 and 30034 respectively. A quick inverse calculation:

    50Bg/kg x (body weight) = (Bq/body)

    then

    (Bq/body) ÷ 30230.5 (average of two factors above) = mSv/year

    Thus a 60kg individual would get 0.099 (0.1) mSv/year and a 120kg individual would get 0.198 (0.2) mSv/year. Average body weight for a Japanese man in his 30s is about 68kg, for a Japanese woman in her 30s 51kg, average of the two 59.5. Some fluctuation with age but “average weight of an adult Japanese = 60kg” seems like a good starting point if one was trying to set a low-end estimate of mSv/year exposure for 50Bq/kg, while 120kg is a not-too-unrealistic high-end figure.

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  2. The basic conversion factor is 0.000014 mSv/Bq for an equivalent “committed dose” over a person’s lifetime.

    At 15,918 Bq of Cs137, the committed dose is 0.223 mSv. Not 0.53 mSv/year.

    See:
    http://www.laradioactivite.com/en/site/pages/Dose_Factors.htm

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  3. Andrew in Ezo

    In Osaka this past weekend- the hotel put a Japan Times in the holder on my room door- ugh! Removing it, I was consoled to find an International New York Times folded inside- it was actually 2x thicker than the JT, which I promptly binned.

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