Nulcear Radiation: Random Effects

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The other type of effects are called stochastic or random, and most significantly, they include cancer. A lot of what we know about stochastic effects in humans, Seward said, comes from long-term studies of the survivors of the atomic bombs dropped on Hiroshima and Nagasaki. These studies show that above doses of 100 millisieverts, the risk of getting cancer—but not the severity of the disease—rises in a straight line with exposure.

At 100 millisieverts, the increased risk of cancers is very small: around 0.5 percent, he noted. As he explained it, the average Japanese male, throughout his lifetime, has a 26 percent chance of developing a fatal cancer from any cause; the average female, 16 percent. With the addition of a 100-millisievert radiation exposure, that risk rises to about 26.5 and 16.5 percent, respectively. For individuals exposed as children, the numbers are slightly higher.

Below that 100-millisievert level, however, the picture becomes much less clear. “There is only limited evidence to show a dose-related effect,” Seward said. Safety standards established for these low-level exposures—for power plant workers, for instance, and radiation technicians—depend therefore on something called the linear no-threshold model, which, as Seward noted, is simply extrapolated from the impacts seen at higher doses. In essence, the model conservatively holds that any dose of radiation increases cancer risk: There is no bottom threshold.

The hitch is that scientists by no means agree on the validity of this theory. Some say low-dose exposures below some threshold are harmless. Some even claim they have a beneficial effect on DNA repair (a phenomenon known as hormesis). Others argue that low-dose risks may be higher than currently predicted.

“The key point,” Seward said, “is that the linear no-threshold model is best applied for setting safety standards.” It errs on what many scientists consider to be the safe side. It is unlikely to be an accurate predictor of the numbers of cancers resulting from exposures when very low doses are involved.

The challenge, Seward said, is that in a population where radiation exposures are very low, it may be difficult to detect a significant change in cancer rates and attribute that with certainty to the Fukushima releases.

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