1) Those interested in recently (?) measured radiation levels (at different distances from the Fukushima reactors in Japan) should see:

http://www.meti.go.jp/earthquake/nuc...nitor02_01.pdf
(dose levels measured 1 meter above the ground)

http://www.meti.go.jp/earthquake/nuc...nitor02_02.pdf
(dose levels measured 1 centimeter above the ground)

Note that the color code is explained near the lower left corner of each dispay. Radiation levels are expressed in micro-Sieverts per hour. [The 10 micro-sieverts, for example, is the same as 0.01 mSv, etc. And 10 micro-Sievert/hour is the same as 0.24 mSv/day, or 7.2 mSv/month.]

2) 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.

A dose of 10 Sv will most likely results in death, within a day or two.
5 Sv would 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 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

Ludwik Kowalski (see Wikipedia)
.