Saturday July 26, 2008 (foodconsumer.org) -- Rice University physics professor W.J. Llope found some granite countertops generate gamma radiation and radon gas at a level that is considered dangerous by the U.S. government, Houston Chronicle reported.

The newspaper cited Dr. Llope as saying that some granite countertops contain high levels of uranium, which releases the radiation and radon gas that can cause cancer.

Llope has tested 55 stones of 25 varieties of granite stones purchased from local dealers and found some homeowners would be exposed to 100 millirems of radiation in just a few months, exceeding the annual exposure limit set by the Department of Energy for visitors to nuclear labs.

Llope did not publish the names of the granite countertops that he found are most dangerous, but he was cited as saying that the highly radioactive varieties include striated granites from Brazil and Namibia.

In a document published on his website, Llope said there is no safe threshold for radiation and the general guideline is that each rem of radiation would cause cancer in 4 people in a population of 10,000.

Some granite countertops he tested released one rem of radiation in just 250 hours or 10 days.

Llope recommended that homeowners who have granite countertops in their homes should keep their windows open at least for half a day once in every few weeks to dilute the indoor radon concentration.