<P> Potassium - 40 is a radioactive isotope of weak emission, and a constituent of alkali feldspar, which in turn is a common component of granitic rocks, more abundant in alkali feldspar granite and syenites . Naturally, a geiger counter should register this low effect . </P> <P> Some granites contain around 10 to 20 parts per million (ppm) of uranium . By contrast, more mafic rocks, such as tonalite, gabbro and diorite, have 1 to 5 ppm uranium, and limestones and sedimentary rocks usually have equally low amounts . Many large granite plutons are 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 . Cellars and basements built into soils over granite can become a trap for radon gas, which is formed by the decay of uranium . Radon gas poses significant health concerns and is the number two cause of lung cancer in the US behind smoking . </P> <P> Thorium occurs in all granites as well . Conway granite has been noted for its relatively high thorium concentration of 56 ± 6 ppm . </P> <P> There is some concern that some granite sold as countertops or building material may be hazardous to health . Dan Steck of St. Johns University has stated that approximately 5% of all granite is of concern, with the caveat that only a tiny percentage of the tens of thousands of granite slab types have been tested . 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 . </P>

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