Dr. Kitto's latest granite study is published

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Dr. Kitto's latest study was published in June, just now making it into print. Published in the Journal of Radioanalytical Nuclear Chemistry, June 2009. Among the conclusions is that:

A radiation index and measurements with a radiation meter established that the annual effective dose rates due to a 1 hour per day exposure to gamma rays emitted by the granite samples were often low, but can occure as high as 1 mSv per year.

When used as an indoor accent meterial, such as a kitchen countertop, granite can increase human exposure to gamma ray radiation, and radon released from the materials can increase inhalation exposures.

Given the health implications and the paucity of measurement data, goals of the present study were to determine the gamma ray activities of decorative stone countertops and to estimate an associated dose to teh occupant.

A total of 55 granite slabs, ceramic tile samples, and 17 engineered stone samles were studied. A core was extracted from each slab with NO attempt to core "hot spots". Gamma ray specrometric measurements were taken using a HPGe detector.

15% of the samples contained more than 100 Pq/kg or Uranium 238, 5% of the samples contained over 1,000 Bq/kg U 238 with the engineered stones having the lowest levels of U 238.

Thorium 232 was found in most of the samples with 15% of the samples containing more than 100 Bq/kg of Thorium 232.

Most of the samples contained 60 to 30 times more Potassium 40 ( K 40) than U 238 or Thorium 232 with most of the granites containing more than 1,000 Bq/kg of K 40. Engineered stones were mostly 10 times lower than the granites. K 40 emits only a single energy gamma ray, so it is far less dangerous than the U 238 or T 232.

About 22 of the granite samples had average activity concentraions of 31, 61, and 1210 Bq/kg for U 238, T 232, and K 40 respectively. Those averages were similar to reported findings in other granite countertop studies around the world. But nine granite samples had much greater concentrations, with 1140, 290, and 1060 Bq/kg.

Radiation indexes were calculated for each sample, with a 6 being equal to about 1 mSv/year and a 2 being equal to .3 mSv/year. Engineered stone samples were the lowest, with less than 0.1, many of the granite samples were less than 2 but six of the granite samples were over 2 and three of the granites were over 6 which is greater than the doses allowed for the general public.

Using a Palm Rad geiger counter, the engineered stones had an average count of 37 counts per minute (cpm), the ceramic tiles had an average count of 69 cpm, and the granites had between 140 and 1500 cpm due to their greater U 238 and T 232 content.

For a granite with 350 cpm, an hours contact per day would give you approximately 10% of your allowed radiation dose per year. Contrast this to the MIA's use of four hours per day exposure, which would put your dose at 40% of what is allowed.

The final conclusion was that a small percentage of granites could expose occupants to a significant absorbed dose of radiation. It was also determined that hand held geiger counters are not suitable for stand alone determination of radiation levels due to the varibility of isotopes found in granite.

It has taken several years to get the data out, but it was worth the wait.