Measured analysis of Fukushima:
The Panic Over Fukushima - WSJ.com
Denver has particularly high natural radioactivity. It comes primarily from radioactive radon gas, emitted from tiny concentrations of uranium found in local granite. If you live there, you get, on average, an extra dose of .3 rem of radiation per year (on top of the .62 rem that the average American absorbs annually from various sources). A rem is the unit of measure used to gauge radiation damage to human tissue.
The International Commission on Radiological Protection recommends evacuation of a locality whenever the excess radiation dose exceeds .1 rem per year. But that's one-third of what I call the "Denver dose." Applied strictly, the ICRP standard would seem to require the immediate evacuation of Denver.
It is worth noting that, despite its high radiation levels, Denver generally has a lower cancer rate than the rest of the United States. Some scientists interpret this as evidence that low levels of radiation induce cancer resistance; I think it is more likely that lifestyle differences account for the disparity.
Now consider the most famous victim of the March 2011 tsunami in Japan: the Fukushima Daiichi nuclear power plant. Two workers at the reactor were killed by the tsunami, which is believed to have been 50 feet high at the site.
But over the following weeks and months, the fear grew that the ultimate victims of this damaged nuke would number in the thousands or tens of thousands. The "hot spots" in Japan that frightened many people showed radiation at the level of .1 rem, a number quite small compared with the average excess dose that people happily live with in Denver.
What explains the disparity? Why this enormous difference in what is considered an acceptable level of exposure to radiation?
In hindsight, it is hard to resist the conclusion that the policies enacted in the wake of the disaster in Japan—particularly the long-term evacuation of large areas and the virtual termination of the Japanese nuclear power industry—were expressions of panic. I would go further and suggest that these well-intended measures did far more harm than good, not least in limiting the prospects of a source of energy that is safe, abundant and (as compared with its rivals) relatively benign for the environmental health of our planet.
If you are exposed to a dose of 100 rem or more, you will get sick right away from radiation illness. You know what that's like from people who have had radiation therapy: nausea, loss of hair, a general feeling of weakness. In the Fukushima accident, nobody got a dose this big; workers were restricted in their hours of exposure to try to make sure that none received a dose greater than 25 rem (although some exceeded this level). At a larger dose—250 to 350 rem—the symptoms become life-threatening. Essential enzymes are damaged, and your chance of dying (if untreated) is 50%.
Nevertheless, even a small number of rem can trigger an eventual cancer. A dose of 25 rem causes no radiation illness, but it gives you a 1% chance of getting cancer—in addition to the 20% chance you already have from "natural" causes. For larger doses, the danger is proportional to the dose, so a 50-rem dose gives you a 2% chance of getting cancer; 75 rem ups that to 3%. The cancer effects of these doses, from 25 to 75 rem, are well established by studies of the excess cancers caused by the atomic bombs at Hiroshima and Nagasaki in 1945. (A recent study of butterflies near Fukushima confirms the well-known fact that radiation leads to mutations in insects and other simple life-forms. Research on those exposed to the atomic bombs shows, however, no similar mutations in higher species such as humans.)