Answer to Question #13502 Submitted to "Ask the Experts"

We are considering purchasing an expensive home that is made ENTIRELY of granite. This home, built in 1924, was purchased in 2017 by the current home owners, who were then required to install a radon abatement system. Our concerns surround the issue that the tremendous granite walls above ground might be emitting dangerous radon levels, despite the abatement system in the basement of this large home. Additionally, the home has a large amount of pinkish marble tiles installed on the first floor approximately 20 years ago. The home is quite an investment and our hopes are that our 16-year-old son would one day inherit the house. We are trying to be prudent in reducing any detrimental exposures, within reason, for our family. The listing photos show the enormous amount of granite that was used in building this home. Our question is: Will we still be getting radon exposure from the walls above ground, despite the radon abatement/mitigation system in the basement? Would we need air exchange units on every floor to reduce our risk?

First, I've got to say that it looks like a beautiful home! And I've been to Asheville a few times—it's in a beautiful part of the country. But let's get away from the aesthetics and on to more interesting stuff like radon.

There are actually two questions here:

• Why are radon levels high?
• Does all the marble and granite pose a risk?

Let's take them one at a time.

Why are radon levels high?

It's hard to give a precise answer since I'm not sure what the radon levels were that required mitigation. I know that there's a difference between the radon concentration at which, say, a real estate agent or radon testing company will recommend mitigation, the level at which a regulatory agency will suggest you install a mitigation system, and the level at which radon poses a significant health risk. Not knowing what the levels were makes it hard to know if the mitigation was installed to try to enhance the resale value, to get below the magic number of 4 pCi L^-1 (150 Bq m^-3),¹ or because radon posed a risk. Nevertheless, I can still make a guess as to where it's coming from and why it might be elevated.

About 10 years ago I travelled to Hot Springs, North Carolina, for the wedding of a friend's daughter, flying into Asheville. It turns out that many hot springs contain elevated levels of radioactivity because what frequently warms them up is the decay of uranium and other elements (including radon) that come from the decay of the uranium. In areas with higher-than-normal levels of uranium in the rock, the rock tends to be warmer than normal and it heats up groundwater circulating through it, creating hot springs.

In addition, I took a look at a geologic map of the Asheville area and it looks as though there are a number of rock types in the area that are often associated with elevated levels of uranium, which leads to elevated levels of radon emanating from the soils. So, I feel reasonably comfortable in saying that the radon in the basement—regardless of the level it had been—is more likely to be due to the local geology.

Does all the marble and granite pose a risk?

With regards to the materials from which the house is built, it's true that granite tends to have elevated levels of uranium and other radioactive elements compared to many other rocks. But in looking at the photos of the interior, it looks as though all of the granite is on the outside of the house (with the possible exception of countertops). This means that any radon emitted by the granite would be emitted into the atmosphere, not into the interior of your home. In fact, the same general airtight construction that helps to concentrate radon in the basement will also help to exclude radon emitted by the exterior granite from the interior of the house. As far as granite countertops are concerned, they just don't emit enough radon to pose a problem and any radon they do emit will tend to end up at floor level or in the basement as radon is heavier than air.

With the marble—this just isn't much to worry about at all since marble tends to have less radioactivity than most rocks, and much less than most granites. I should also mention that neither granite nor marble will emit dangerously high levels of radiation, even if you live there for decades.

The only really important question you need to answer is "How much radiation exposure are we getting from our home?" Since the building materials (marble and granite) aren't emitting much radiation, it comes down to "What are the radon levels in our home?" And since radon, as I noted earlier, sinks to the lowest level, this comes down to "What are the radon levels in the basement?" As long as radon concentrations are low (i.e., as long as your regulators and real estate agent aren't looking uncomfortable) then you and your family are not at risk.

What you should ask for is a copy of the radon testing reports to see what the levels were before the mitigation system was installed. Also ask how long the radon test was conducted—radon concentrations will change significantly based on the weather (rain, dry, warming up, cooling down) and a single one-week or two-week test might not be indicative of the typical radon concentrations. But regardless of what the original tests show, see what the current radon concentrations are and as long they're less than 4 pCi L^-1 then everybody ought to be happy. And then, consider testing it every now and again (maybe every few years) to make sure they're staying low. As long as they are, you're okay.
 
P. Andrew Karam, PhD, CHP
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¹ The radon concentration units are given here on pCi L^-1 (called traditional units) because that is the unit used by the Environmental Protection Agency. However, the Health Physics Society has adopted the SI (International System) of units and those are given in parentheses.