Radioactive substances have been leaking from Japan’s tsunami-damaged Fukushima Daiichi nuclear plant for nearly two months, and the plant is likely to continue emitting radiation into the atmosphere, and possibly into the ocean and ground water, for many months to come.
People living within a 20 km (12 mile) radius of the plant have been evacuated, while those in five towns downwind from the plants have also been told to prepare to leave their homes.
Following are some questions and answers about the health risks from continued radiation exposure from the plant, 240 km (150 miles) from Tokyo, as operator Tokyo Electric Power Co struggles to bring it under control:
HOW MUCH RADIATION HAS BEEN MEASURED OUTSIDE THE EVACUATION
The highest radiation reading outside the evacuation zone on Thursday measured 0.046 millisieverts an hour in Namie, Fukushima, which is one of the towns whose residents are preparing for evacuation.
Most areas outside the evacuation zone logged readings around 0.001 millisieverts an hour or less.
Japan’s National Institute of Radiological Sciences calculated that in the month from March 14 to April 11, following the earthquake and tsunami, a person in Tokyo who spent eight hours a day outdoors and drank tap water would have been exposed to about 0.12 millisieverts.
In absolute terms, these quantities are extremely small. People in Japan are on average exposed to 1.5 millisieverts of natural background radiation a year.