More On Japan's Cesium Contaminated Milk (Or "All Bq's Are Not Equal")

I've already posted on the issue, and the cancer risk is low.

But this recent The Japan Times Online article makes a couple of fundamental errors.


First, you can't compare one becquel (Bq) of potassium-40 (K-40) with one Bq of cesium-137 (Cs-137) for radiation safety purposes.

Why not?

Because a Bq is just the rate of radioactive decay....one atom disintegrating in one second. 

That's important, but it's also important to understand what radiations (and at what energies) are emitted per disintegration.  Not all atoms disintegrate with the same emissions.

K-40 emits a 1.31 MeV (max) beta 90% of the time and a 1.5 MeV gamma 10% of the time.

Cs-137 emits a 0.5 MeV (max) beta 95% of the time followed by a 0.662 keV gamma, and a 1.17 MeV beta (max) 5% of the time

So the absorbed dose by the body is different for each disintegration of each type of atom.  We can compare the allowable limit on intake (ALI - the amount required to yield a dose of 5cSv) for ingestion for both radionuclides to estimate the difference.

For K-40, it is 11 MBq.

For Cs-137, it is 3.7 MBq.

This means that 1 Bq of Cs-137 is about 3 times as radiotoxic as 1 Bq of K-40.

The article also mentions the ICRP risk estimate (100 mSv increase cancer risk about 0.5%).    However, this risk is averaged over a broad population (including adults, teens, babies).  The risk estimate would be higher just for babies because they are more radiosensitive.  ICRP Publication 103 (2007) states that the risk might be as much as 3 times higher.