Media playback is unsupported on your device Media caption Prof Malcolm Sperrin, a medical physicist, on the Japan quake health risks

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, and is being extended to five communities outside the zone to the north west of the plant, where radioactive contamination is most significant.

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 there 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 high levels of radiation - above one gray (the standard measure of the absorbed dose of 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 a gray weighted by the effectiveness of a particular type of radiation at causing damage to tissues, and is 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 on average to around 2mSv of radiation a year from the natural environment, although there is considerable variation in this dose between individuals.

In the UK, the legal limit 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 emergencies, the upper limit is set higher - 5mSv or more - but these figures are set conservatively, at levels far below those that would significantly raise health risks.

How is radiation sickness treated?

The first thing to do is to try to minimise further exposure to contamination by removing clothes and shoes, and gently washing the skin and hair with soap and water.

Drugs are available that increase white blood-cell production to counter any damage that may have occurred to the bone marrow, and to reduce the risk of further infections due to immune-system damage.

There are also specific drugs that can help to reduce the exposure of internal organs caused by radioactive material by speeding up its removal from the body.

How does radiation have an impact on health?

Radioactive materials decay spontaneously to produce ionising radiation, which has the capacity to cause significant damage to the body's internal chemistry, breaking the chemical bonds between the atoms and molecules that make up our tissues. Damage to the DNA of a cell is particularly important.

The body responds by trying to repair this damage, but at high doses it is too severe or widespread to make repair possible, leading to short-term acute health effects.

There is also a danger of mistakes in the natural DNA repair process, which can lead in the long-term to cancer.

Regions of the body that are most vulnerable to acute radiation damage include the cells lining the intestine and stomach, and the blood-cell producing cells in the bone marrow.

The extent of the damage caused is dependent on how long people are exposed to radiation, and at what level.

Radiation and cancer Most experts agree even low doses of ionising radiation can increase the risk of cancer - this risk becomes clear at doses above 100 millisieverts - but by a very small amount.

In general, the risk of cancer increases as the dose of radiation increases. Exposure to one sievert of radiation spread out over time is estimated to increase the lifetime risk of fatal cancer by around 5% on average.

The thyroid gland and bone marrow are particularly sensitive to ionising radiation, especially in children.

Leukaemia, a type of cancer that arises in the bone marrow, is the most sensitive radiation-induced cancer. Leukaemia may appear as early as a few years after radiation exposure.

Other cancers can also result from exposure to radiation, but may not develop for at least a decade. These include cancers of the lung, skin, thyroid, breast and stomach.

What are the most likely long-term health effects?

Cancer is the biggest long-term risk. Usually when the body's cells reach their "sell-by date" they commit suicide. Cancer results when cells lose this ability, and effectively become immortal, continuing to divide and divide in an uncontrolled fashion.

The body has various processes for ensuring that cells do not become cancerous, and for replacing damaged tissue.

But the damage caused by exposure to radiation can completely disrupt these control processes, raising the risk of cancer.

Failure to properly repair the damage caused by radiation can also result in changes - or mutations - to the body's genetic material (DNA), which are not only associated with cancer, but may also be potentially passed down to offspring, leading to deformities in future generations. These can include smaller head or brain size, poorly formed eyes, slow growth and severe learning difficulties.

Are children at greater risk?

Potentially yes. Because they are growing more rapidly, more cells are dividing, and so the potential for things to go wrong is greater.

Following the Chernobyl nuclear reactor accident in the Ukraine in 1986, the World Health Organization recorded a dramatic increase in thyroid cancer among children in the vicinity.

This was because the radioactive materials released during the accident contained high levels of radioactive iodine, a material that accumulates in the thyroid.

Children continued to eat and drink heavily contaminated foodstuffs, such as milk.

What risk does Fukushima pose currently?

The Japanese authorities have recorded a radiation level of up 400 millisieverts per hour at the nuclear plant itself.

Professor Richard Wakeford, an expert in radiation exposure at the University of Manchester, said exposure to a dose of 400 millisieverts was unlikely to cause radiation sickness - that would require a dose of around twice that level (one sievert/one gray).

However, it could start to depress the production of blood cells in the bone marrow, and was likely to result in a lifetime risk of fatal cancer of 2-4%. Typically, a Japanese person has a lifetime risk of fatal cancer of 20-25%.

A dose of 400 millisieverts is equivalent to the dose from 20 -100 CT scans.

Several emergency workers at the plant were exposed to doses in excess of the emergency dose limit of 250 millisieverts, but no-one has exceeded a dose of 1Sv (1000 mSv). A dose of 1Sv would raise the lifetime risk of fatal cancer by 4-8%, depending on the length of time the exposure lasted.

The lower legs of two workers came into contact with highly contaminated water, which led to a high localised dose to the skin (1-2 gray) that was sufficient to cause temporary skin reddening. The dose to the trunks of their bodies was much less, at 100-200 mSv.

However, even a dose of 100 millisieverts over a year is enough to raise the risk of cancer, and a dose of 250 millisieverts could produce an additional lifetime risk of around 1-2%.

The level of exposure for the general population, even those living close to the plant, was unlikely to be anywhere near as high.

However, levels equivalent to around 25 millisieverts a year have been recorded in the village of Iitate, to the north west of the plant. Professor Wakeford said estimating the impact on human health of such a level was difficult, but there was some evidence to suggest that it could raise the risk of cancer - albeit by a small amount.

The pattern of contamination around the plant is patchy, reflecting wind direction and rainfall at the time of any release. The risk to people living in less contaminated communities will be much lower than the areas to the north-west of the plant.

What radioactive materials have been released?

Experts are concerned about two types of radioactive material, created as by-products of the nuclear fission process, both of which are easily released and can contaminate the soil relatively easily, and get into the food chain. This is why foodstuffs sourced from some areas have been banned.

The radioactive form of iodine - iodine-131 - is easily absorbed by the thyroid, the gland which regulates growth and cell production.

This would raise the risk of thyroid cancer in those exposed as children.

To counter that risk, people - in particular children - can be given tablets containing a stable form of iodine which would prevent the body absorbing the radioactive version.

Radioactive iodine decays quite quickly and will disappear from the environment within weeks or months.

The Japanese already have a lot of iodine in their natural diet, so that should help too.

Another potential source of contamination is the radioactive form of the metal caesium (caesium-134 and caesium-137), which once released into the environment continues to pose a potential risk for many years.

Unlike iodine, caesium does not concentrate in one particular tissue, but gets into soft tissues, where it can cause cancer.

How can the Japanese authorities minimise the cost to human health?

Prof Wakeford said that provided the Japanese authorities acted quickly, most of the general population should be spared significant health problems.

He said in those circumstances the only people likely to be at risk of short-term serious health effects were emergency workers at the plant potentially exposed to high levels of radiation.

The top priority would be to evacuate people from the area and to make sure they did not eat food contaminated above the limit.

Distribution of iodine pills would also help to minimise the risk of thyroid cancer in the general population should high levels of radioactive iodine be present in the environment.

Is there evidence that food has been contaminated?

Yes. Japan's health ministry has urged some residents near the plant to stop drinking tap water after samples showed elevated levels of radioactive iodine - about three times the limit in Japan.

Radioactive iodine has also been found in water supplies in Tokyo at twice the levels deemed safe in Japan for babies and infants to drink, although still some way short of the safety level for adults.

Raised radiation levels have also been found in samples of milk and spinach, in some cases well outside the 20km exclusion zone, particularly to the north west of the plant.

Japan's chief cabinet secretary Yukio Edano, said the level of radioactivity found in the spinach would, if consumed for a year, equal the radiation received in a single CT scan. For the milk, the figure would be much less.

Professor Wakeford stressed that safe limits in Japan for radiation in food were kept very low and assumed long-term consumption, so people should not necessarily be unduly worried by reports that they had been breached.

For instance, an infant would have to drink over 100 litres of water containing iodine-131 at the limit to reach the dose of radiation received in a year from natural background sources.

However, Professor Wakeford said it would be advisable to stop young children from eating products contaminated over the limit. But he stressed that contmination levels have fallen sharply since mid-March.

What is the threat to Tokyo residents from contaminated tap water?"

Officials said the tap water showed 210 becquerels per litre of iodine-131 - more than twice the recommended limit of 100 becquerels per litre for infants. The recommended limit for adults is 300 becquerels per litre. (Becquerels is a measure of radioactivity whereas millisieverts is a measure of dosage to the body of the radiation emitted on radioactive decay).

Professor Richard Wakeford says that infants who drank 100 litres of water contaminated at the Japanese limit of 100 becquerels of iodine-131 per litre would receive a radiation dose of about 2 millisieverts of radiation.

The Japanese limit for adults is 300 becquerels of iodine-131 per litre because adults are less sensitive to the effects of radiation.

In comparison, the average person is exposed to 2-2.5 millisieverts from natural background radiation in a year, although variations about this average can be large.

In theory, drinking water contaminated at 210 becquerels of iodine-131 per litre for a year would cause a very small additional risk of cancer - but in practice nothing much more than you could expect to get from normal background levels of radiation.

Is it safe to bathe or wash food in contaminated tap water?

Yes, because only small amounts of water would be ingested into the body from these practices.

What about contaminated seawater?

Levels of radioactive iodine-131 in seawater near the nuclear plant have reached 3,355 times the legal safety limit, but this limit is set at a very low level for normal operations.

Japanese officials admitted the reading was a concern - but said there was no immediate threat to human health.

Professor Wakeford said it would be sensible to ensure that fish and other seafoods contaminated beyond the limit did not enter the food chain.

However, he stressed that radioactive iodine would be diluted by the ocean currents, and would, in any case, decay away almost completely within three months.

How does Fukushima compare to Chernobyl?

Although Japan's Nuclear Safety Commission has upgraded the severity rating of the crisis from five to seven, the highest level and the same as Chernobyl, officials say emissions of radioactive materials at Fukushima currently stand at about 10% of those from the 1986 disaster.

Professor Gerry Thomas, who has studied the aftermath of the Chernobyl disaster, said: "It is very unlikely that this will turn into anything that resembles Chernobyl.

"In Chernobyl you had a steam explosion which exposed the reactor core, which meant you had a lot of radiation shooting up into the atmosphere."

Prof Thomas said although the Chernobyl disaster had led to a rise in thyroid cancer cases, the only people affected were those living in the areas of Ukraine, Belarus and Russia that lie closest to the site of the Chernobyl Power Plant, and who were young at the time.

What if the situation deteriorates?

If there were to be a meltdown or a fire at the nuclear plant, and unfavourable winds, then experts say radioactive material could reach as far as Tokyo, 150 miles (241km) away.

However, even in that situation, the level of radiation is likely to be such that simple measures, such as staying indoors with windows closed, should significantly reduce the risk.

The situation is much better now than at the time of the major earthquake on 11 March because the short-lived radioactive materials have decayed away - the radioactivity of iodine-131, a major concern in releases from the site, reduces by a half every eight days, so in a month there is only one-sixteenth of the original iodine-131 left.