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How can Fukushima crisis be rated as severe as Chernobyl?

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It's more than a quibble over numbers, he says. Instead, it's about uncertainty and confusion among a public already stressed by the effects to themselves and their loved ones of the quake and tsunami. "To the extent that this tool is adding to that, it's making things worse," he says.

What the scale measures

The scale was developed in Chernobyl's aftermath.

"People felt there needs to be some kind of way to explain the safety significance of events associated with radiological accidents," including events at nuclear plants, says Olli Heinonen, a senior fellow at Harvard University's Belfer Center for Science and International Affairs in Cambridge, Mass.

Each increase in the scale represents a 10-fold increase in the amount of radioactive iodine-131 released. IAEA members agreed on that benchmark because iodine-131 is among the most readily released isotopes during a reactor accident. But it also is among the fastest to decay, losing about half of its remaining radioactivity every eight days.

Actual or potential human exposures to radioactive materials released are explicitly considered in the index's first five levels, many of which have the potential to contaminate only people working at a nuclear facility.

The index doesn't specify exposures for the higher levels because the index's architects figured it would be difficult to measure that as an event unfolded. Much depends on how a government responds with evacuations, for instance, and on factors such as weather patterns, which might tend to steer plumes of radioactive material toward or away from populated areas.

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