The testing of nuclear bombs in the atmosphere three decades ago left a radioactive imprint on the world. Today, scientists are using that imprint to determine the age of people's gallstones.

Their findings, reported this week in The New England Journal of Medicine, are helping doctors figure out how long gallstones are present in the body before they cause pain, information that may eventually suggest ways to keep small stones from becoming troublesome.

In the late 1950s and early 1960s, when the United States and Soviet Union were conducting nuclear tests in the atmosphere, the level of radioactive carbon-14 in the air was unusually high. Since atmospheric tests were banned by treaty in 1963, those levels have declined steadily.

This carbon, in the form of carbon dioxide, is absorbed by plants, which in turn are consumed and metabolized by people. By measuring the density of carbon-14 in gallstones -- which are deposits of cholesterol or calcium in the gallbladder or bile ducts -- Dr. Henry Y.I. Mok of California's Woodland Clinic and his colleagues can determine when the stones began to form.

Gallstones that began to form during nuclear testing have high levels of carbon-14 toward their centers, said Mok. The stones form from the inside out, like the rings of a tree -- which also have higher levels of carbon-14 in rings formed during the atmospheric test era.

"Nobody knows the potential long-term side effects" of this mildly radioactive form of carbon, said Mok, who worked with colleagues from the University of California at San Diego with a grant from the Veterans Administration. His team also studies the health effects of nuclear tests.

Gallstones are usually removed surgically. Sometimes medication is used, and more recently sound waves have been used to dissolve them.

Analyses of gallstones removed from 11 patients revealed that the stones were an average of 11.7 years old, that they grew at a rate of 2.6 millimeters a year and that they were at least 2 years old before symptoms appeared.

A similar technique, carbon dating, is used to determine the age of fossils. But while the fossil technique uses the rate of decay of radioactive carbon-14 to the stable carbon-12 over thousands of years to determine the age of a specimen, Mok's method looks at the artificially high levels of carbon-14 over a short period.

The light shed on gallstones through his research "is possibly the only benefit of nuclear weapons testing," Mok said. "I'm not sure if this use is great enough to justify testing."