How harmful is nuclear radiation? It depends on the dose received.
Tsunami-related Fukushina accident will probably renew debates about nuclear electricity. Such debates should be based on what is known about negative effects of nuclear radiation. Numerical data below should be useful in that context.
The effect of penetrating radiation on a person depends on the dose received. The common unit of dose is Sievert (Sv). Smaller doses are expressed in milliseverts (mSv) or microseveret. The old unit of dose, rem, is also used widely (1Sv=100 rem)
A dose of 10 Sv will most likely results in death, within a day or two.
5 Sv would is kill about 50% of exposed people.
2 Sv can also be fatal, especially without prompt treatment.
0.25 Sv = 250 mSv is the limit for emergency workers in life-saving operations.
0.10 Sv = 100 mSv dose is clearly linked to later cancer risks.
0.05 Sv = 50 mSv is the yearly limit for for radiation workers.
0.004 Sv= 4 mSv typical yearly dose due to natural radiation (cosmic rays, etc).
0.003 Sv= 3 mSV typical dose from mammogram
The one day dose, due to Fukushima accident, at a distance of 30 miles from the damaged reactors, was reported (on 3/16 and 3/17) as 0.0036 mSv. I do not have data on doses, probably very large, received by those who worked near or inside reactors. But I have no doubt that each of them was carrying an individual dosimeter. No deaths due to radiation have been reported in Japan, as far as I know. Many lives, however, were lost in Chernobyl, by those who worked to minimize damage.
Montclair State University, USA