Pre- and Post-market Measurements of Gamma Radiation and Radon Emanation from a Large Sample of Decorative Granites

Reports of radiation from granite counter tops generated public interest in the potential exposures to external gamma radiation and internal exposure from radon decay products created by building materials. The gamma radiation, radionuclide content and surface dose rates were measured in 322 slabs of 254 named stone types (area ~50 ft per slab) and 14 smaller samples (area ~1 ft). Average surface gamma dose rates ranged from 1 to 24 μrem h with hot spots up to 160 μrem h. Radon emanation, measured for 60 slabs of 24 named stone types, ranged from 1 to 400 pCi ft h. Emanation fractions ranged from roughly 5% to 50%. Individual sampling points on the slabs showed emanation rates as high as 1300 pCi ft h. Model calculations suggest that some home occupants might receive doses that warrant remedial actions when substantial areas of some types of granite are installed in small, minimally ventilated living spaces. Based on these results, pre-market screening protocols have been developed and are being used to remove potentially problematic slabs from inventory. Post-market screening measurements are being tested in a pilot study of 35 homes. The maximum result radon emanation measured from granites in 11 houses was 90 pCi ft h. INTRODUCTION Media reports of radiation from granite counter tops recently generated public interest in this issue. While any building material derived from rock has the potential to create radiation exposures, some natural decorative stone like granite can have elevated concentrations of naturally-occurring radioactive materials (NORM). The U family, and to a lesser extent the Th family, create two pathways for exposure. In addition to external radiation dose from gamma rays, these families have a member which is a radioactive gas that can escape from the rock matrix into the environment. These gases, often referred to as radon and thoron respectively, decay into chemically active decay products that can deliver internal radiation doses when inhaled. Customers buying decorative granite, and industry that supplies them, want an answer to a deceptively simple question, “Is it safe?” There are a number of underlying issues that help formulate an answer to that question. These finer points may not be of interest to everyone but can help support the answer if the questioner asks for details. For example, while public health officials might measure the “safety” using the average impact of the radiation exposures for the general population, individuals are usually more concerned about their 1 This work was supported, in part, by a grant from the CSB/SJU faculty development program.