As kitchen granite countertops became more common, consumers raised questions about radon exposure from different types of granite. Radon is a natural radioactive gas that can accumulate in buildings as a carcinogenic pollutant. The most common sources are seepage from groundwater and the pore space atmosphere in soils naturally high in radioactive elements. The usual remedy is proper household ventilation. Radon in most granite is not a health hazard, unless the home has an existing radon hazard.
Origin of Radon
There are more than 1,600 types of granite worldwide. Unlike volcano magma, which reaches the surface as liquid and cools quickly, granite magma cools slowly and under pressure deep inside the earth's crust. Slow cooling allows minerals to crystallize in beautifully colored patterns of mica, quartz, feldspar and horneblende. Mica in granite varies from the clear mineral muscovite to the dark mineral biotite. The radioactive element uranium most often occurs in granite as a trace mineral inside biotite. Over time, uranium decays to radium, which then decays to radon gas.
Radon Escapes from Granite
When radium decays, it emits an alpha particle of neutrons and electrons. The remaining part of the radium atom is then radon, which is propelled in the opposite direction of the alpha particle emission in the same way that a fired bullet causes a rifle recoil. Radon can only escape from granite if the radium was close to the surface and the recoil propels it outward from the surface.
No Safe Level of Exposure
There is no safe level of exposure to radon. Geiger counter scans of commercial countertops can measure significantly different levels of radiation even from place to place on a typical length of kitchen countertop. For that reason, the surface of granite countertops should be sealed in accordance with the manufacturer's instructions at least once every year, and homes with potential radon hazards should be completely ventilated for half a day more than once each month.
Tobacco Smoke
Special consideration should be given to evaluating the accumulation of radon over several risk factors, such as building materials, tobacco use, soil and water. When radon is inhaled into the lungs, radiation damages DNA in lung cells and creates a risk of lung cancer. Radon can enter secondhand tobacco smoke in the ambient atmosphere of a home and thereby increase the inherent risks of lung cancer for tobacco users and others exposed to secondhand smoke.
Background Radiation
Everyone is exposed to 360 milli-rems of background radiation annually from cosmic rays, soil, dental x-rays, water and food. Lung cancer risks from radiation are statistically linear: If the exposure doubles, the risk also doubles. The 1 milli-rem exposure per 1,000 miles of commercial air travel or the minor amount of radon escaping from countertops are not usually dangerous. However, a few granites do contain unusual amounts of uranium, so countertop selection should be based on more than just a pretty face.
