Answer to Question #15350 Submitted to "Ask the Experts"

I am looking at renting a newly constructed home which has a geothermal heating system. This is in an area where radon levels are relatively high. I read that radon levels indoors are more elevated when using geothermal energy. I am concerned as I have a young child and I would value your input.

Thanks in advance.

In general, the use of geothermal energy should not have a significant effect on indoor radon levels. 

To give more detail we need to understand how geothermal energy works. Geothermal systems take heating and cooling for indoor spaces from the earth beneath the surface. The temperature of the earth at 3 m is constant at about 13 °C. Geothermal systems take advantage of this. They either pre-heat cold ambient air or cool hot air. They consist of an underground system that taps into the thermal energy under the ground surface and a heat pump powered by electricity that moves liquid through pipes to heat or cool the air. In other words, you are not making heat as you would with a furnace but moving the warmth (or cooling) that Mother Nature already provides. All that moves between the depths and your home is the water flowing through the sealed pipes, so there is no way for radon to enter your home from a geothermal system such as this. 

A recently published Canadian study that includes data from over 13,000 residences shows that homes heated by geothermal energy have an average indoor radon level slightly higher than homes heated by other sources such as natural gas and electricity. However, only 180 of the more than 13,000 homes in the study used geothermal energy so it was a pretty small sample, on top of which it is quite probable that the higher radon levels in these homes are due to other factors. For example, the same study showed that homes built after the year 2000 had higher radon levels than homes built during the 1900s, most likely due to better insulation and other energy-saving efforts. Since geothermal energy is relatively new, the houses employing that technology were probably built recently. Houses with basements had 50% higher radon levels than houses without basements (crawl spaces or slabs), another factor that could impact homes using geothermal energy. Homes with private well water also had indoor radon levels a bit higher than homes using municipal distribution systems, possibly from radon dissolved in the well water. Most important, though, is that in all cases, the average indoor radon concentration in the homes involved in the study were below the Canadian indoor radon standard of 200 Bqm^-3 (5.4 pCi l^-1—the US EPA guideline is 4 pCi l^-1). The bottom line is that homes using geothermal energy may have slightly higher average indoor radon concentrations, but it is likely due to factors other than the type of heating and cooling employed.

Regardless of these conditions, it would be wise to assess the actual radon concentrations in your prospective home to give you some comfort regarding your risk. It is not necessarily accurate to predict the indoor radon concentrations based on home characteristics or location. Homes constructed side by side may have different indoor radon concentrations, and radon concentrations will also change from day to day depending on temperature, precipitation, and other factors. If the measured concentration exceeds the appropriate limit or guideline, radon mitigation strategies can reduce the concentrations regardless of the type of heating and cooling employed.

The major factor in the risk from exposure to indoor radon is smoking. While it is important to measure and, to the extent practical, mitigate the radon in a home, refraining from smoking is the critical factor in reducing the risk of radon-induced lung cancer. 

We should also mention that there is a form of geothermal energy that is very different from the typical home systems—these are massive geothermal systems that can provide power or heat for entire cities (e.g., Reykjavik, Iceland), and these systems are associated with elevated levels of radiation, albeit not necessarily radon. These systems drill much deeper than 3 m to tap the energy from hot rocks or even magma chambers thousands of feet underground. Like your system, these geothermal systems pump water underground to be heated; unlike your system, they heat the water to temperatures of hundreds of degrees, bringing that heat to the surface where it can be used to spin turbines (to make electricity) or to heat water and homes. Because geological activity is frequently associated with radioactivity, these large-scale geothermal systems are often associated with elevated levels of radioactivity, but not with radon in homes.

Jan Johnson, PhD, Certified Health Physicist (emeritus)
Norman Dickenson, PhD, Certified Health Physicist
Jess Joyce, MS, Certified Health Physicist