Two weeks after the Quake, tsunami and start of the Fukuchima nuclear crisis, I figured out what to make of the various radiation units and I have a basic idea now of how to gauge reported radiation levels for their potential health effects. The key thing I touched up on last post, though, is that exposure to radiation from radionuclides cannot be linearly compared to radiation from very distant sources in terms of dose rates.
The dose received from a steady flow of radiation, such as from normal background radiation, or from 1-deal limited exposures like medical radiological and security screenings is easy to calculate received doses from. With radionuclides that float around in air, water and soil, or that end up in cells (where they may accumulate in much higher concentrations in certain parts of specific fauna or flora), the received dose rate changes dramatically with the distance from the radionuclide. On contact the dose rate is very high, yet diminishing exponentially with distance, which makes gauging the doses received from inhalation/ingestion extremly complex, as not only is the flow rate different at different distances, the tissue itself also functions as a shielding factor, with different tissues shielding differently.
So when soil samples are reported with dose rates in µSv/hr, such as in this MEXT table from yesterday –continued below table…
… To pick the fist sample, 30 km NW of Fukachima Daiichi NNP, taken 3/23 at 11:10am, you can see that the 2 radionuclides Iodine-131 and Cesium-137 tested for 200,000 Bq/Kg and 45,000 Bq/Kg respectively, and that there was a dose rate measured of 103 µSv/h, but nowhere do they indicate at which distance you would receive such an dose rate (assuming it is flowing from the radionuclides in the soil), or the relative contributions of Io-131 and Cs-137 to the dose rate, nor how for each isotope that breaks up in gamma and beta rays. 103 µSv/hr LOOKS like it could be compared to natural background radiation (which was about 0.1 µSv/hr for much of Coastal Japan prior to March 11). It looks like “1000 times normal”. But IF, say, you took a mud bath in such soil, your exposure could easily amount to 200,000 µSv/hr, which after 10 hours would get you well in the radiation sickness ballpark.
With the limited data released I can’t figure out what distance they’re using, but from toying with values in the Rad Pro Calculator, it seems to be somewhere in 10 cm to 1 meter range (3-4 inches to a yard (sorta) – range), so it’s a general environment reading, it seems, one you would get if you were walking around and took measurements holding the Geiger Counter just below the waist, perhaps. If you were barefoot, your feet would get significantly higher doses, though. This important nuance I haven’t seen explained in the press coverage I’ve come across.