For more than two weeks now, number after number describing horrifying sounding doses of radiation have emerged from Fukushima. We hear continuously about elevated levels measured in various places and of significant increases in the measured radiation, relative to the usual background levels.
Yesterday it was reported that radioactive iodine in the water near the plant had spiked to 3,555 times the legal limit. On Sunday, a report emerged – later to be withdrawn – that radiation reads had spiked at 10 million times the normal level. To the general public, not used to the standard units of sieverts and becquerels, those figures can be extremely disturbing.
So it's worth paying closer attention to exactly what they mean. The sievert is a measure of the "dose" and provides an estimate of the possible biological damage which a specific level of radiation can cause; the becquerel simply refers to the amount of radiation present. To put that into some context, a healthy human has a radioactivity of tens of thousands of becquerels a second.
The figures we've heard on the damage that workers have been exposed to have probably seemed the most frightening. Some of them have been reportedly exposed to around 200 milliSieverts, or mSv, in a short period. The annual limit in the UK is just 20 mSv. These are scary numbers, but when taken in the context of the expected increase of a fatal cancer they are a little less so. A 200 mSv exposure would mean an increased risk of a fatal cancer of 1 per cent, something like the same lifetime risk of a Californian dying in a road accident.
The levels of increased radiation outside the exclusion zone in Fukushima, and indeed outside of the plant, are much smaller. Indeed, while there were reports of a radiation "spike" in the air in Tokyo on 16 March, by 27 March they were back at (or in some cases below) the annual average for the background.
Radiation is certainly dangerous with major exposure: there is no doubt about that. But for most of the numbers that are being quoted for radiation levels in Tokyo and other places, the levels are about the same as one might get from eating a (normal) banana.
The author is a professor of nuclear physics at Surrey University
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