How Much Radiation Is Too Much?

And how do we know?

An official scans a woman with a geiger counter

Ongoing problems at Japanese nuclear reactors have sparked runs on potassium iodide tablets in California, as residents fret that a radioactive plume will drift across the ocean. These fears are unfounded, according to most experts, who say that even if the plume reaches the United States, it won’t carry enough radiation to harm us. How do we know how much radiation is too much?

By studying Hiroshima and Nagasaki survivors. The joint Japan-U.S. “Life Span Study” includes about 120,000 residents of Hiroshima and Nagasaki, and  more than 86,000 survivors  who were within 2.5 kilometers of either explosion. Since 1950, researchers have tracked rates of cancer and other diseases among these subjects. From this data, they’ve been able to estimate the increased health risks associated with various degrees of exposure. Scientists have also conducted research on subjects regularly exposed to extra radiation from medical tests (such as CT scans) or through their jobs. Although the Department of Energy  funds research on how low-dose radiation  affects animals and humans at a cellular level, most health-risk studies still refer primarily to population-based research.

Based mostly on the atom bomb-survivor studies, scientists generally agree that every additional sievert of exposure (or about 160 times the typical annual dose) is associated with around a 4 percent or 5 percent increase in lifetime cancer risk. (This increase varies, however, with age and other personal factors.)

There is no universally accepted “safe” level of radiation, in large part because measuring the health effects of very small doses—like what we might expect from airport backscatters—is extremely difficult. A typical American is exposed to about  6.2 millisieverts, or 620 millirems, of radiation per year—roughly one-half of which comes from natural sources, like radon gas and cosmic radiation. * (Click to  calculate your annual exposure.) For its part, the National Council on Radiation Protection and Measurements  recommends keeping excess radiation from non-natural and nonmedical sources below 1 millisievert per year and defines a “negligible individual dose” as below 0.01 millisieverts at a time.

The United Nations has said that monitoring devices in California “may be able to detect extremely low levels of radiation” on Friday. Although the U.N. refrained from making any specific dose estimates, it’s probably safe to say that it expects the radiation to be lower than the “negligible individual dose.”

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Explainer thanks David A. Schauer of the National Council on Radiation Protection and Measurements.

Correction, March 18, 2011: An earlier version of this article used an out-of-date estimate of an American’s typical annual radiation dose. In 2009, that number was revised from 3.6 millisieverts to 6.2 millisieverts, an increase based primarily on additional medical-radiation exposures. (Return to the corrected sentence.)