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Concern remains over the potential effect on human health from radiation leaks at the stricken Fukushima Daiichi nuclear plant.
A 20km (12 mile) evacuation zone affecting about 70,000 people has been imposed around the plant.
Residents living within 30km (18 miles) have been advised to leave the area, or to stay indoors, and try to make their homes airtight.
Experts believe that swift action of this sort should have minimised the risk to human health, but are worries about the level of radiation to which emergency workers have been exposed, and about possible contamination of food and water supplies.
What are the immediate health effects of exposure to radiation?
Exposure to moderate levels of radiation - above one gray (the standard measure of absorbed radiation) - can result in radiation sickness, which produces a range of symptoms.
Nausea and vomiting often begin within hours of exposure, followed by diarrhoea, headaches and fever.
After the first round of symptoms, there may be a brief period with no apparent illness, but this may be followed within weeks by new, more serious symptoms.
At higher levels of radiation, all of these symptoms may be immediately apparent, along with widespread - and potentially fatal - damage to internal organs.
Exposure to a radiation dose of four gray will typically kill about half of all healthy adults.
For comparison, radiation therapy for cancer typically involves several doses of between one and seven gray at a time - but these doses are highly controlled, and usually specifically targeted at small areas of the body.
A sievert is essentially equivalent to a gray, but tends to be used to measure lower levels of radiation, and for assessing long-term risk, rather than the short-term acute impact of exposure. There are 1,000 millisieverts (mSv) in a sievert.
People are exposed to around 2mSv of radiation a year from the natural environment.
In the UK, the legal limt for radiation exposure from sources such as nuclear plants for members of the public is 1mSv a year, based on recommendations from the International Commission on Radiological Protection.
For accidents, the upper limit is set at 5mSv - but these figures are set conservatively, at levels far below those that would damage health.
BBC News - Q&A: Health effects of radiation exposure
Concern remains over the potential effect on human health from radiation leaks at the stricken Fukushima Daiichi nuclear plant.
A 20km (12 mile) evacuation zone affecting about 70,000 people has been imposed around the plant.
Residents living within 30km (18 miles) have been advised to leave the area, or to stay indoors, and try to make their homes airtight.
Experts believe that swift action of this sort should have minimised the risk to human health, but are worries about the level of radiation to which emergency workers have been exposed, and about possible contamination of food and water supplies.
What are the immediate health effects of exposure to radiation?
Exposure to moderate levels of radiation - above one gray (the standard measure of absorbed radiation) - can result in radiation sickness, which produces a range of symptoms.
Nausea and vomiting often begin within hours of exposure, followed by diarrhoea, headaches and fever.
After the first round of symptoms, there may be a brief period with no apparent illness, but this may be followed within weeks by new, more serious symptoms.
At higher levels of radiation, all of these symptoms may be immediately apparent, along with widespread - and potentially fatal - damage to internal organs.
Exposure to a radiation dose of four gray will typically kill about half of all healthy adults.
For comparison, radiation therapy for cancer typically involves several doses of between one and seven gray at a time - but these doses are highly controlled, and usually specifically targeted at small areas of the body.
A sievert is essentially equivalent to a gray, but tends to be used to measure lower levels of radiation, and for assessing long-term risk, rather than the short-term acute impact of exposure. There are 1,000 millisieverts (mSv) in a sievert.
People are exposed to around 2mSv of radiation a year from the natural environment.
In the UK, the legal limt for radiation exposure from sources such as nuclear plants for members of the public is 1mSv a year, based on recommendations from the International Commission on Radiological Protection.
For accidents, the upper limit is set at 5mSv - but these figures are set conservatively, at levels far below those that would damage health.
BBC News - Q&A: Health effects of radiation exposure
