It has been singled out as the second-leading cause of lung cancer in the United States, right behind smoking. But to Shiro Umeda, sprightly at 74, radon is the best thing since aspirin.

Every month for the past 10 years, he has come to a radon bath here to soak it up and breathe it in. He’s convinced it has helped ease his back pain and improve his overall health.

Undaunted by warnings from the scientific community that the highly radioactive gas is a carcinogen, tens of thousands of health-seekers like Umeda are drawn each year to hot springs in Japan that claim radon can cure an array of ills.

“Not a doubt in my mind,” Umeda said after a recent session. “It makes me feel better.”

The popularity of radon is nothing new.

At the turn of the century, its curative powers were believed to be so strong that products containing radon or radium, its parent element, ranged from toothpaste and beauty creams to chocolate bars.

Research has since led most health experts to make an about-face.

Most, but not all.

While acknowledging that high doses are undoubtedly dangerous, Yutaka Okumura, a professor of radiology at Nagasaki University, a leading center of radiation research, said the issue may be less simple than some of the more dire cautions suggest.

Okumura cited a study he participated in that found cancer fatalities between 1976 and 1993 among more than 4,300 people living near one of Japan’s most famous radon springs, Misasa, were significantly lower than rates elsewhere. Radon levels in the test area were roughly 70 becquerels per cubic meter, or about three times higher than those in the control areas.

More here.