Here is an interesting chart I found via Small Dead Animals about radiation, Sieverts, exposure, and illness.
The only quibble I have is where they place the accumulated dose for the onset of acute radiation sickness[ARS]. Their chart lists a reading of 2 Sievert for ARS or 200 rem, CDC says ARS may start appearing with as little as 75 rem/0.75 Sievert exposure and other sources say 100 rem/1 Sievert. Reason for this variation? Time of exposure and energy of the particles can shorten the onset. The higher energy particles can cause the skin to act like it has a burn with redness and swelling, called a beta burn after the particle. This radiation can also affect internal organs as it penetrates into the body. People can still be cured, but there will be an increased chance of cancer. Past 400-500 rem/4-5 Sievert, your chance of survival is minimal even with extraordinary efforts.
In previous post I forgot to mention a very important bit of information, accumulated or cumulative exposure. As the terms imply, this is the exposure you pick up over time. Its like accumulating debt, the more you pick up the worse you are.
So lets look at that reading of 0.00000035 Sieverts per hour from the earlier post. And lets be conservative and go with the 0.75 Sievert threshold for ARS. To reach an accumulated reading of 0.75 Sieverts with such a small exposure, it will take 2,142,857.15 hours standing in that one spot to reach 0.75 Sieverts. Or 244.6 years. And this is not even a fatal dose, just the lowest limit you might start to show signs of ARS.
Want to calculate your own normal yearly exposure, NRC has a nifty calculator.
For further reading, UN report on Chernobyl from 2005, the 20th anniversary of that terrible accident.
Update - this article has now been linked at Stop the ACLU. I guess I am moving up. Thanks Rose.