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Images of helicopters dropping sea-water on the Fukushima nuclear power station, and of some worried residents leaving Tokyo, present a startling and unsettling picture. Watching the news and reading the papers it would be easy to be left with the impression that the health risks from the nuclear leak are rapidly escalating and spreading far beyond the exclusion zone.

The may also add to the sense of unease:

"Due to the evolving situation at the Fukushima nuclear facility and potential disruptions to the supply of goods, transport, communications, power and other infrastructure, British nationals currently in Tokyo and to the north of Tokyo should consider leaving the area."

But if you read on, the travel advisory from the FCO is more reassuring:

"The most recent advice from the UK's Chief Scientific Adviser (Sir John Beddington) remains that for those outside the exclusion zone set up by the Japanese authorities there is no real human health issue that people should be concerned about. This advice is kept under constant review."

The exclusion zone is 20 kilometres (12 miles) around the plant. Those living between 20-30 kilometres away (12-18 miles) are being advised to stay indoors.

Sir John Beddington spoke by phone two days ago to British Embassy staff in Tokyo and a is, I think, worth setting out in detail. He said:

"....do we have any concerns now in terms of human health? Well the answer is yes we do, but only in the immediate vicinity of the reactors. So the 20 kilometre exclusion zone the Japanese have actually imposed is sensible and proportionate. If they extended out a little bit more to 30 kms, that is well within the sort of parameters that we would think are extremely safe."

Of course, everyone is concerned about what might happen if the situation at Fukushima gets worse. Sir John went on to talk about the worse case scenario - a meltdown at the plant. He did not think this was likely, but what would be the result if it did?

"In this reasonable worst case you get an explosion. You get some radioactive material going up to about 500m up into the air. Now, that's really serious, but it's serious again for the local area....The problems are within 30 km of the reactor. And to give you a flavour for that, when Chernobyl had a massive fire at the graphite core, material was going up not just 500m but to 30,000 feet (9,144m) . It was lasting not for the odd hour or so but lasted months, and that was putting nuclear radioactive material up into the upper atmosphere for a very long period of time. But even in the case of Chernobyl, the exclusion zone that they had was about 30km. And in that exclusion zone, outside that, there is no evidence whatsoever to indicate people had problems from the radiation. The problems with Chernobyl were people were continuing to drink the water, continuing to eat vegetables and so on and that was where the problems came from. That's not going to be the case here. So what I would really re-emphasise is that this is very problematic for the area and the immediate vicinity and one has to have concerns for the people working there. Beyond that 20-30km, it's really not an issue for health".

Sir John is not alone in his assessment of the health risks. It is similar to comments I have heard by many other scientists.

On , Professor Laurence Williams, former UK chief inspector of nuclear installations, stressed that Fukushima was not another Chernobyl and made it clear if he was living in Tokyo he would not leave:

"....I wouldn't be concerned. This is not a Chernobyl. We are not going to see high levels of radioactivity being put up high into the atmosphere and distributed on the winds. If we do get to the worse situation where the fuel in those reactors slumps because it doesn't have any structural integrity because of over-heating, then there will be a release of caesium and iodine (although the iodine is decaying all the time) and there will be strontium and ruthenium and other things that will come out....my guess is it is a low-level release and very localised it will not be like Chernobyl. People living in Tokyo are 150 miles (241km) away, so I would not be worried."

So even in the worst-case scenario, the risks to human health beyond the exclusion zone appear to be low. The main immediate threat to health is to the emergency workers trying to stabilise the plant. Radiation levels at the plant have fluctuated and we don't know exactly what materials are leaking out, nor in what quantity, nor over what time period.

Finally a note on potassium iodide tablets. Radioactive iodine from a nuclear leak can pollute the air and contaminate the food chain. Potassium iodide tablets can block radioactive iodine from being taken into the thyroid. The Japanese authorities have distributed these to people in the vicinity of Fukushima as a precautionary measure. But sales of iodide tablets have also soared in the US, thousands of miles away. It is not proportionate or sensible, but an indication of the level of unease that a nuclear accident can cause.

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