Putting Fukushima Into Perspective
Rhetoric has the power to influence how people judge nuclear risk.
Journalists, public officials and experts may not realize the influence they have over the public's psyche, but their rhetorical power recalls that of the Latin-literate clergy who interpreted the Bible for the common folk in medieval times. Consider how some have explained the radiation dose of 1 millisievert.
In 2011, the New York Times equated this dosage (accurately) to that of the outermost contamination zone around the 1986 Chernobyl nuclear disaster. But one could just as well liken 1 millisievert to one-third of what physicist Richard Muller has called a "Denver dose," or the amount of extra natural background radiation that residents of Denver, Colorado, receive annually as compared to the average U.S. resident. The first comparison generates fear, the second comfort—no trifling matter when the mental impact of such crises can be so damaging.
We can limit the role of such rhetoric by demanding more precise language that quantifies risk on an absolute scale, effectively assigning a risk "temperature." Then average Joes and Janes could compare the riskiness of reported events to other episodes in common experience.
Units such as the "micromort" and "microlife" were created for this purpose. Introduced by Stanford Professor Ronald Howard, the micromort—a one-in-a-million chance of dying—allows one easily to compare the risks of hang-gliding (eight micromorts per jump) and going under general anesthesia (10 micromorts) to that of giving birth (120 micromorts in the United Kingdom). On a typical day, the average person endures about one micromort of risk from non-natural causes.
The micromort's close kin, the recently introduced "microlife"—brainchild of Cambridge statistician David Spiegelhalter, from whom most of these numbers originate—represents a 30-minute period in one's expected lifespan. This unit is especially helpful for discussing longer-term effects. A young adult has roughly a million microlives to expend before expiring, or 48 microlives per day. But factors such as lifestyle choices and, yes, radiation exposure affect how quickly one uses up one's allotment.
Now, instead of comparing 1 millisievert of radiation exposure to Chernobyl or Denver, we can state the risk as a once-off loss of approximately 18 microlives (calculated by conservatively extrapolating from far higher doses for which there is good data). That compares to the effects on a young adult of smoking (20 cigarettes daily costs about 10 microlives a day) or exercising (daily 20-minute sessions add about 2 microlives per day). Living in Fukushima city now, with its elevated radiation levels, costs residents roughly one additional microlife every five days, compared with before the 2011 meltdown. (In Tokyo today, radiation levels are back to pre-meltdown levels.)
Of course, quantifying risk does not remove the underlying uncertainties—I may be overestimating the risk of living in Fukushima city, for example, due to complexities such as extrapolation and radioactive decay. Nor can such risk numbers determine how individuals will or should act.
While the quantification of risk and its uncertainties is scientific, perception of acceptable risk is subject to personal judgment that will vary with life experience and factors such as whether the risk is voluntary or not. However, providing a risk temperature can help express risk in less emotive terms and allow people to draw on personal experience in understanding extraordinary circumstances.
It will take time for the public to develop a good intuition for risk temperature, but it would be good to start now. Then, when the next crisis comes, average citizens may better know how heated the situation really is—and whether it's something to sweat about.
Mr. Roberts, a former academic physicist, was science advisor to the U.S. ambassador to Japan from 2011-12.