Since the Great East Japan Earthquake and the following Fukushima disaster, there have been quite some maps of nuclear radiation sensor data out there (for those interested, in my sketchpad, I noted down: the just posted and beautiful 311 Scale map, the NYTimes version, a radiation data-specific 3D globe and some others).
In today’s world, we cannot avoid reading about millisievert numbers and alternative denominations of “dose equivalent radiation” in the news, together with comparative statements as “7.5 million times the legal limit" (Does it matter? What about 8.5 million? 7.5 trillion anyone?) and other relatively complex comparisons and limits that combine exposures by hours, with those of days or years equivalent.
This is a chart of the ionizing radiation dose a person can absorb from various sources. The unit for absorbed dose is “sievert” (Sv), and measures the effect a dose of radiation will have on the cells of the body. One sievert (all at once) will make you sick, and too many more will kill you, but we safely absorb small amounts of natural radiation daily. Note: The same number of sieverts absorbed in a shorter time will generally cause more damage, but your cumulative long-term dose plays a big role in things like cancer risk.