Be wary of granite that glows

Last August, Consumer Reports magazine — ever alert to stylish trends among its affluent readers — pinpointed the growing popularity of natural stone countertops, demand for which had increased 5 percent annually for the previous five years. But for Rice University physics professor W.J. Llope, the article's title, "Countertops: The Hottest Rocks," carried an unsettling irony.

Some granite countertops, he says, contain high levels of uranium, which, by generating gamma radiation and radon gas, can endanger human health.

"Most stones, in terms of radioactivity, are relatively quiet," Llope said. "But there are a couple I have found that are insanely hot."

Using a Rice University spectrometer, Llope has examined 55 stones, representing about 25 varieties of granite purchased from Houston-area dealers. Some, he said, could expose homeowners to 100 millirems of radiation — the annual exposure limit set by the Department of Energy for visitors to nuclear labs — in just a few months.

Llope, who said he plans to publish his findings in a peer-reviewed journal, declined to name the most hazardous varieties of granite he has thus far examined. But he said dangerously radioactive varieties include striated granites from Brazil and Namibia.