The radiation levels from the Fukushima Daiichi nuclear power plant have fallen back from a great spike on Tuesday, Japanese authorities say. Japan government has advised residents living within 30km or 18 miles to leave the area.
Now let's take a look at the effects of nuclear radiation to humans. 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.
Based from World Nuclear Association, here is the radiation dose effect:
2 millisieverts per year (mSv/yr)
Typical background radiation experienced by everyone (average 1.5 mSv in Australia, 3 mSv in North America)
9 mSv/yr
Exposure by airline crew flying New York-Tokyo polar route
20 mSv/yr
Current limit (averaged) for nuclear industry employees
50 mSv/yr
Former routine limit for nuclear industry employees. It is also the dose rate which arises from natural background levels in several places in Iran, India and Europe
100 mSv/yr
Lowest level at which any increase in cancer is clearly evident.
350 mSv/lifetime
Criterion for relocating people after Chernobyl accident
400 mSv/hr
The level recorded at the Japanese nuclear site, 15 March
1,000 mSv single dose
Causes (temporary) radiation sickness such as nausea and decreased white blood cell count, but not death. Above this, severity of illness increases with dose
5,000 mSv single dose
Would kill about half those receiving it within a month
image credit: all voices
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