Since everyone has radiation on their minds right now thanks to the situation in Japan, and since XKCD was nice enough to post this handy chart, I thought it might be useful to put up a link to my old “basics of radiation” guide from teaching radiation labs to undergrads many years ago.
(PDF Link: Handy Facts About Radiation)
A big thing this doc tries to do is explain the difference between things which will and won’t kill you. XKCD’s diagram is good at showing you the relative dangers of different levels of “primary radiation” — things like gamma rays and neutrons hitting you directly. What it doesn’t talk about is the relative dangers of radioactive substances, which can mostly kill you by getting inside your body and continuing to emit small doses of radiation.
For reference, so far the Fukushima-Daiichi plant has mostly emitted steam with radioactive noble gases in it, which is great from a safety perspective — those decay quickly and bond chemically with nothing, so they’re probably the safest radioactive leak you could ever get. XKCD’s diagram is appropriate for those. If the core were to be breached and spray some of the heavier materials inside there, like Cesium or Uranium, the results would be much nastier.
Also, a footnote: This doc works in units of rems, while a lot of the recent news reports have used Sieverts. 1 Sv = 100 rems.