What do the Fukushima exposures really mean? A rough estimate is that for a 50-year-old male working at the Fukushima nuclear facility, his lifetime risk of cancer might increase from 42% to 42.2%. The magnitude of this increased risk is comparable to the added risk of living in Denver (where background radiation is higher because of the altitude and radionuclides in the Rocky Mountains) versus New York City for 10 to 15 years, or smoking one pack of cigarettes a day for one to two years. The Japanese public will, of course, get far less radiation.
Why are people still frightened given the relatively small risks of radiation from Fukushima? Mostly because governments and scientists have done a poor job of explaining radiation to the public. Until now, most reports compare doses of radiation or contamination with radiation of foodstuffs, without discussing the risk associated with these exposures. The implication is that if the estimated dose is below the dose or concentration used for comparison, there is no cause for concern. People are typically unconvinced by this argument.
Instead, radiation exposure and dose should be expressed using estimates about the risk of cancer in later life. This can be calculated in a few different ways. For example, a person's lifetime risk of cancer regardless of cause; or, after an accident like Fukushima, the total and/or excess numbers of cancers anticipated in an exposed population over its lifetime. Because these risk estimates are uncertain, a range of estimates should be given to reflect what we know and don't know about the chance that radiation exposure might lead to more cancers in later life.