"Cesium-137 that has longer effects, ranging from 3 million to 14.7 million becquerels per square meter, was detected in Namie, Futaba, Minamisoma, Iitate and Katsurao, northwest of the Fukushima No. 1 Nuclear Power Plant, in Fukushima Prefecture. The levels far exceeded 550,000 bacquerels per square meter, the level the then-Soviet Union had used as a criterion for urging people to evacuate at the time of the 1986 Chernobyl nuclear disaster."The article reveals measurements only for Cs-137, but at least we have one isotope to calculate exposures with. Keep in mind that these exposures are coming from radioactive sources located in just a square centimeter of surface area.
|Half life: 30 years|
Don't underestimate the penetrating ability of electrons. I can't find Bragg peak info for beta at the energies Cs-137 gives off, but would estimate that 10-15% of any dose would penetrate the sides of shoes.
The above chart also illustrates why it is so dangerous for very young children to be in an environment with high radioactivity in soil. Their critical bodily structures (gonads, bone marrow, spleen) are physically much closer to the source of radiation than that of adults. During the crisis, the public was urged to stay indoors, and hopefully they heeded that advice.
So what was the most important data point officials relied on to determine the need for evacuation procedures? It obviously was not exposures to lower extremities, as the article further points out:
"Based on recommendations from the International Commission on Radiological Protection (ICRP), the Japanese government used 20 millisieverts per year of radiation in the atmosphere as the criterion to designate evacuation areas in the wake of the nuclear accident in Fukushima. Therefore, there are areas that have not been designated as evacuation zones although they have larger amounts of accumulated radiation."Sort of like waiting for E Coli deaths to pop up in 20 different restaurants before action is officially taken.