Japan Nuclear Emergency: How Much Radiation is Safe?

From Time Online, Posted by Eben Harrell Sunday, March 13, 2011 at 5:55 pm

Government officials have confirmed that radiation has leaked from the Fukushima power plant site in Northern Japan, where workers are scrambling to prevent a meltdown at two damaged reactors. The surrounding area has been evacuated. It's difficult to ascertain how much radiation has already leaked from the plant--or what the exposure will be if either or both of the reactors suffer a total meltdown. But one thing seems likely: Japan will soon have to ask one of the most vexing scientific questions related to public health: what is a "safe" level of radiation? When can residents return to their homes?

As my colleague Jeffrey Kluger wrote over the weekend, an overheating nuclear power plant can let off vapor packed full of high energy by-products. Scientists have a pretty firm grasp on what this radiation does to the human body in high doses: it causes agonizing and often fatal damage in a matter of days or hours. But controversy remains over the effects of low-dose radiation—the sort that does not make an individual ill but which may lead to cancer.

Most regulatory agencies around the world--and the IAEA--rely on a mathematical model to estimate radiation risk. Using this model— known as the linear-nonthreshold dose-response model (LNT model)—agencies extrapolate down in a linear fashion from high radiation dose levels known to be harmful.  This method results in extremely conservative safety levels—the U.S. Nuclear Regulatory Commission (NRC), for example, sets an acceptable level of radiation exposure from any one source at 100 millirem a year. In contrast, the average  level of natural background radiation in the United States is about 350 millirem a year. (A chest X-ray, for further comparison, gives the equivalent to 1 or 2 millirem to the whole body.)

But some scientists say the model is based on a false assumption. It assumes that there is no safe dose of radiation and that the risk of getting cancer or genetic damage increases in a linear fashion along with exposure. These scientists argue that there is a threshold below which radiation poses no hazard to health, as cells only slightly damaged by radioactivity can heal themselves without long-term ill effects.

Peter Zimmerman, a physicist and former chief scientist of the Foreign Relations Committee says that when his staff looked into radiation safety limits as part of legislation pertaining to nuclear terrorism, “it became pretty clear to me that the linear-nonthreshold needed to be revised.”

“Japan will have to decide what it considers an acceptable level of radiation exposure following an emergency” Zimmerman adds. “That's a conversation it needs to have with itself."

As an example of the controversy surrounding low-level radiation, the U.S. federal government can't even agree on a safe dose. The Environmental Protection Agency differs from the NRC  in that it advocates the more stringent standard for all radiation exposure from a single source or site at 15 millirem—rather than 100 millirem— a year. The NRC has a different set of guidelines following a severe nuclear emergency such as a meltdown or dirty bomb attack, however.  In these cases, the safety threshold increases to a 25 rem per 30 days—a huge increase and right on the threshold of when people begin to show physical signs of radiation poisoning after a single dose such as changed blood work (physical symptoms usually doesn't appear below 100 rem in a single dose). Areas receiving more radiation would be evacuated. When citizens would be allowed to return home following a clean-up operation would be decided on a “case by case” basis, according to an NRC spokesman speaking on background.

Source: Time Online