The Japanese government will look at altering the threshold for radiation exposure as it evaluates whether people can return to the exclusion zones around the damaged Fukushima Daiichi nuclear power plant.

The government ordered people to evacuate from a 20 kilometer (12 mile) radius around the plant, and has advised people to leave voluntarily if they are within a 30 kilometer radius. The United Nations has urged the government to expand the area of the evacuation zone, and the U.S. government advised Americans in Japan to stay more than 80 kilometers (50 miles) from the plant.

Questions have come up about altering the evacuation plan because the amount of ionizing radiation that populations are exposed to doesn't vary directly with distance; some places got more even though they were outside the zone, while others inside it got much less.

Chief Cabinet Secretary Yukio Edano said at a news conference on Thursday that the government is considering allowing people to return to their homes to gather possessions and expanding the evacuation zone. He added that no decisions have been made yet.

Another issue is how long people will have to stay outside the area. Edano said the question for the government is about long-term exposure versus short-term. Edano said that the long-term exposure was an issue when deciding whether people should be allowed to move back into the area. Edano also said the government is looking at the threshold levels of exposure.

While the amount of radiation outside the plant, measured in microsieverts and millisieverts, is small, dosages are cumulative. Radiation levels 1,100 meters west of the number 2 reactor, one of the more damaged ones, was measured at 58.5 microsieverts (0.0585 millisieverts) per hour on April 6. At that rate, someone living there would get a dose that could cause medical problems after about two years, assuming the radiation level stayed that high.

Levels have dropped in the last few days; at the same point exposure was 74.2 microsieverts per hour, and on March 29 it was 109.4 microsieverts per hour.

The current government standard is 1 millisievert per year when the plant is operating normally, and 5 millisieverts during a nuclear emergency. The annual limit for nuclear power plant workers is 50 millisieverts. A typical chest X-ray exposes someone to 0.1 millisieverts, and background radiation runs at under 5 millisieverts annually. It takes 1,000 millisieverts to cause immediate health problems, and 10 sieverts (10,000 millisieverts or 10,000,000 microsieverts) is lethal.

Edano said efforts are being made to figure out what exposure levels were before March 23, when radiation was being released from the stricken nuclear plant but there were fewer monitoring stations in place. No decisions have been made yet, he said.