An illustrative graphic, below, describing “the ionizing radiation dose a person can absorb from various sources” — from sleeping next to someone through spending ten minutes next to the Chernobyl reactor core after meltdown.
The chart’s creator, Randall Munroe, warns people that he is not a radiation expert, and “If you’re basing radiation safety procedures on an internet PNG image and things go wrong, you have no one to blame but yourself.”
You can click on the image below to view a larger version of the chart. Even better is to look at the original chart at this link: http://xkcd.com/radiation/
[Thanks to Maj Gordon Hunter, 8th Civil Support Team (WMD) for the lead. He asks how come "no one in the media has yet actually quantified what 'a large amount of radiation' is? 100 Alpha particles? A chunk of Cobalt 60 the size of one's head? The background rad being reported on the [Japanese] reactor is actually less than the background normally found in Colorado just by walking outside. Sometimes, knowing the math behind rad can make life hard on your TV (being shouted at, things flung into, etc).”]