Granite is a natural source of radiation, like most natural stones. However, some granites have been reported to have higher radioactivity thereby raising some concerns about their safety.
Some granites contain around 10 to 20 parts per million of uranium. By contrast, more mafic rocks such as tonalite, gabbro or diorite have 1 to 5 ppm uranium, and limestones and sedimentary rocks usually have equally low amounts. Many large granite plutons are the sources for palaeochannel-hosted or roll front uranium ore deposits, where the uranium washes into the sediments from the granite uplands and associated, often highly radioactive, pegmatites. Granite could be considered a potential natural radiological hazard as, for instance, villages located over granite may be susceptible to higher doses of radiation than other communities. Cellars and basements sunk into soils over granite can become a trap for radon gas, which is heavier than air and is formed by the decay of uranium. Radon can also be introduced into houses by wells drilled into granite. Radon gas poses significant health concerns, and is the #2 cause of lung cancer in the US behind smoking.
However, in the majority of cases, although granite is a significant source of natural radiation as compared to other rocks it is not thought an acute health threat or significant risk factor. Various resources from national geological survey organizations are accessible online to assist in assessing the risk factors in granite country and design rules relating, in particular, to preventing accumulation of radon gas in enclosed basements and dwellings.
"A study of Granite Countertops by National Health and Engineering Inc of USA , undertaken in November, 2008 however, did not find a single granite that poses any health risk. Quantities of radon and radiation emitted by stones included in the analysis all fell well below average outdoor background levels that are commonly found in the United States. Scientists conducted more than 400 tests of 115 different varieties of granite countertops, including stones cited in media reports as being potentially problematic. The stones tested include types of granite that comprise approximately 80 percent of the annual U.S. market share for granite countertops, based on the most recent market data available.