It is fair to say that politics and money usually trump science in the short term, under enough social duress. Records now show that the Japanese government set radiation exposure levels for evacuation zones around Fukushima based not on health safety but on a desire to limit the magnitude and cost of population flight. A 5 millisievert threshold was thrown out in favor of 20 millisieverts, because officials feared that otherwise too many people would be unable to return home and the cost of compensating them would be too high.
Five years after the 1986 Chernobyl accident in Ukraine, a dose of 5 millisieverts was set as the tripwire for relocating residents for safety reasons. But at Fukushima, areas falling in the range of 5 millisieverts would represent a loss of 13 percent of the territory of the prefecture, which was deemed untenable in a country with so many people living on so little land. Five mSv is still more than three times the average annual dose from natural background radiation in Japan. The threshold level of 20 mSv/year is in stark contrast to the statutory legal limit imposed by a 1972 industrial safety regulation for the nuclear industry. For workers at a nuclear power plant, the maximum limit of exposure prescribed by law is 20 mSv/year and a cumulative dose of 100 mSv in five years. The law forbids the entry of ordinary citizens into controlled areas of the plant with a radiation dose of 1.3 mSv/quarter and further prohibits workers from eating, drinking or sleeping in those areas. It also prohibits pregnant women from being exposed to radiation in a controlled area of more than 2 mSv/year.