Another thread pointed to a chart showing current exposure rates in Japan. Few people seem to understand what the numbers represent. Below is a short explanation.
The exposure rates are reported here:
[link to translate.google.com
Explanations of the units of radiation as well as annual exposure limits are contained in 10 CFR 20. (CFR = Code of Federal Regulations)
10 CFR 20 may be read here:
[link to www.nrc.gov
The chart being linked to in this thread is reporting radiation levels in nano-Grays per hour (nGy/h).
What we are concerned with is the amount of Beta-Gamma radiation people are being exposed to. Using a little math, the numbers presented may be manipulated into a more meaningful unit.
Where beta-gamma radiation is concerned:
1 Gray (Gy) = 1 Sievert (Sv)
1 Sievert (SV) = 100 Rem
therefore, 1 Gy = 100 Rem
Per 10 CFR 20, the annual exposure limit to the whole body (that is your core; everything above the knees and elbows) is 5 Rem per year.
The highest dose rate I see reported currently on the chart is 3817 nGy/h.
Alright then, time for a little math.
1 nano-Gray (nGy) = 10 to the minus 9 Grays (10E-9 Gy)
(3817 nGy/h)(10E-9 Gy/nGy)(100 Rem/1 Gy)(1000 mRem/1 Rem) = 0.3817 mRem/h
So, the highest reported beta-gamma radiation thus far is 0.3817 millirem per hour.
As an aside, people are typically exposed to 2 - 3 millirem during one cross-country flight across the U.S.
Back to math.
(0.3817 mRem/h)(24 h/day)(365 day/yr)(1 Rem/1000 mRem) = 3.82 Rem/yr
Recall that the Federal exposure limit to the whole body is set at 5 Rem/yr. So, while the radiation exposure is much higher than normal, it is still only ~76% of the federal annual limit. The federal limit of 5 Rem/yr is a conservative number as well. Studies performed on survivors from the atomic bomb attacks during WW2 showed that actual limits are closer to 50 Rem/yr.