Radon is a by-product of the radioactive decay of uranium, an element that occurs naturally in the Earth's rocks, sediments and soils. 
The Earth's crust typically contains between 1 and 3 parts per million (ppm) of uranium.  
Earth materials with over 3 ppm uranium are considered potential radon sources, and buildings  constructed on top of such materials may have elevated levels of indoor radon.  
In Ohio, most houses with indoor radon levels above 4 pCi/l are associated with two geologic materials:  a rock formation known as the "Ohio Shale," and a soil developed on sediment (called "till")  deposited by  glaciers during the last Ice Age. 

For more information on the geology of radon in Ohio, Click here



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