Challenges to Radiation Limits Getting Harder to Ignore

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In Japan, the only nation ever to have been subject to nuclear attack, radiation victims are outliving others of the same age.

Half a century of monitoring of atom bomb survivors has found - as expected - that, the people closest to ground zero have died in high numbers of cancers that began in a white-hot flash of nuclear radiation. But as one moves farther from the blast site, the death rate drops until it actually dips below the baseline.

The finding has raised a question: Could ionizing radiation in small doses actually be good for you? Could it be like so many other substances - iodine, for example - that are lethal in excess but essential to good health in …