What kind of radiation does granite emit?

Is it harmful?

Radioactive uranium, thorium, and radium occur naturally within the stone's veins. The radiation levels emitted are usually negligible and pose little to no risk. However, “once in a blue moon,” I encounter a countertop that registers above 100 CPM (counts per minute). This is too high for my comfort level and should probably be for yours.

Why? Let's put this into context. According to the EPA, an average person living at sea level (NYC) acquires about 300 milliRems per year (mrem/yr) of naturally occurring ionic radiation.  EPA.gov article on radiation exposure. (Celestial radiation exposure increases with altitude).

There is no direct equation for converting CPM (counts per minute) to mSv (milliSieverts) or mrem (milliRem) because beta CPM is an electron count, and (micro and milli) sieverts account for bodily damage. The EPA suggests an intervention level at 300 CPM with the equivalent ≈ of 10 µSv/hr (microSieverts per hour) ≈ , 0.01 mSv/hr (milliSieverts per hour) ≈ , 1.0 mrem/hr (milliRems per hour).

Therefore, 100 CPM ≈ 0.33 mrem/hr. So, if someone spent 2 hours every day, 365 days per year, at this particular granite countertop, their yearly exposure from this countertop alone would be ≈ 241 mrem/yr. If you add the typical total exposure at sea level estimated by the EPA, this person’s yearly exposure ≈ would be 541 mrem.

Another stat adds some context: the Federal Government restricts exposure to 500 mrem for anyone under 18 years old who works with or near radiation, and 5,000 mrem for an adult. The EPA recommends that a typical citizen’s exposure to background radiation should not exceed 100 mrem per year. In light of this, the background exposure of 241 mrem per year exceeds EPA guidelines by 141 mrem/year, but if you were employed as the home’s chef, you are far below the 5,000 mrem per year.

 THERE'S MORE TO GRANITE THAN WHAT THE SALESPERSON TELLS YOU.

Radon gas levels vary from stone to stone, and the age of the granite will directly affect its emission levels. Radon gas is a natural residue of the radioactive decay of radium. An old granite stone will emit some radon gas as it ages. As most know, radon gas is the second leading cause of lung cancer, second only to smoking cigarettes. In some homes, radon gas is emitted from the ground beneath the foundation and can leak into the basement. This is more of an issue than any radon gas emission from a granite countertop.

Fortunately, radon gas's ill effects from a granite countertop can be mitigated by increasing the airflow in the surrounding room, as with other volatile compounds. Proper ventilation of any kitchen area is highly recommended.

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