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Radon is an invisible, radioactive gas created during the natural breakdown of uranium in rocks and soils. It is found in nearly all soils. Radon typically moves up through the ground and into homes and buildings through cracks and other holes in the foundation, although there are other radon sources.

Homes trap radon inside, where it can accumulate.

Any home can have a radon problem, not just those built on soil and rock types with high geologic potential for radon release. According to the U.S. Environmental Protection Agency (EPA), nearly 1 out of every 15 homes in the United States is estimated to have elevated levels of radon.

Health Effects of Radon Exposure

Breathing air that contains radon can cause lung cancer. In fact, the United States Surgeon General has warned that radon is the second leading cause of lung cancer in the United States today, and is the leading cause of lung cancer in nonsmokers. If you smoke and your home has high radon levels, your risk of lung cancer is especially high.


According to the U.S. Environmental Protection Agency (EPA), radon is estimated to cause about 21,000 lung cancer deaths per year. Detailed information about the health effect of prolonged radon exposure can be found on the EPA's Radon Health Risks Webpage.


Image of Radon Warning


Potential Sources of Radon

There are three main ways that radon can accumulate in your home:

Infiltration through the Foundation and Building Envelope

Infiltration of radon from the soil beneath a home through cracks and other openings in a building's foundation is recognized as the most important source of residential radon. Infiltration points in a building envelope (foundation, floor, walls, and roof), can include cracks in concrete floors and walls, construction joints, gaps around service pipes, and other penetrations through foundations and walls.

Although newer houses, (those built after 1995 in Montgomery County) are built to resist radon entry through the use of certain construction techniques, the only way to ensure that radon is not entering the house is to conduct a radon test.

Water Supply

The potential radon risk from water is generally much less than from infiltration of radon through a building's foundation. Radon in your home's water is not usually a problem when the water source is surface water. A radon problem is more likely when the source is groundwater (e.g., private wellor a public water supply system that uses groundwater). If you are concerned that radon might be entering your home through the water and your water comes from a public water supply, contact your water supplier.

The radon in your water supply poses an inhalation risk and an ingestion risk. Research has shown that your risk of lung cancer from breathing radon in the air is much greater than your risk of stomach cancer from swallowing water with radon in it. Most of your risk from radon in water comes from radon released into the air when you use water for showering and other household purposes.

Building Materials

Certain building materials, most notably granite countertops, can be a source of radon. However, according to EPA, building materials rarely cause radon problems by themselves, and EPA believes that the existing data is insufficient to conclude that the types of granite commonly used in countertops are significantly increasing indoor radon levels.

Image of How Radon Enters a House
Natural Resources Canada.



Managing Radon in Your Home

Information on radon testing and what to do if a test indicates a radon problem exists in your home is summarized below. Much of this information is adapted from EPA's publication entitled "A Citizen's Guide to Radon.

All homes in Montgomery County should be tested for radon. Any home can have a radon problem, even those that are in an area of low "geologic potential" for radon. Soil maps are not accurate enough to identify where radon will not be a problem, and as a result homes in areas of low geologic radon potential can have high radon readings.

Radon Testing

There are two basic ways to test for radon: short-term testing and long-term testing. Do it yourself radon test kits of either type are available through the mail or from hardware stores and other retail outlets. Carefully read and follow the instructions for using the test kit regardless of the type used.

  • Short-Term Testing

The quickest results are achieved with a short-term test. Short-term tests remain in your home for 2 to 90 days, depending on the device. Charcoal canister detectors are the most commonly used short-term testing devices. Because radon levels tend to vary from day to day and season to season, a short-term test is less likely than a long-term test to tell you your year-round average radon level. But if you need results quickly, you can use a short-term test followed by a second short-term test to decide whether any remediation is needed to address high radon levels.


  • Long-Term Testing

Long-term tests remain in your home for more than 90 days. "Alpha Track" and "electret" detectors are commonly used for this type of testing. A long-term test will give you a reading that is more likely to tell you your home's year-round average radon level than a short-term test.

The amount of radon in the air is measured in picocuries per liter (pCi/L). Sometimes test results are expressed in working levels (WL) rather than in picocuries per liter (4 pCi/L = 0.016 WL).


EPA recommends that you fix your home if the radon level is 4 pCi/L or more. Because there is no known safe level of exposure to radon, EPA also recommends that you consider fixing your home if the radon level is between 2pCi/L and 4 pCi/L. EPA's estimate of 21,000 lung cancer deaths per year due to radon is based on the average radon concentration in American homes, which is about 1.3 pCi/L. The average concentration of radon in outdoor air is 0.4 pCi/L, or 1/10 of EPA's 4 pCi/L action level.

Image of Radon Test Kits

Order a Radon Test Kit 

Visit the National Radon Program Services website or contact a nearby home improvement store to see if they have them in stock.

EPA Recommended Testing Steps

  1. Do a short-term test. If the result is 4 pCi/L or higher, proceed to step 2 to be sure.

  2. Follow up with a long-term test or a second short-term test:

  • For a better understanding of your year-round average radon level, do a long-term test.

  • If you need results quickly, conduct a second short-term test.

The higher your initial short-term test results, the more certain you can be that you should use a short-term rather than a long-term follow-up test. If your first short-term test result is more than twice the EPA's 4 pCi/L action level, you should do a second short-term test immediately.

3.  If you followed up with a long-term test, fix your home if your long-term test result is 4 pCi/L or more.

If you followed up with a second short-term test, the higher your short-term results, the more certain you can be that you should fix your home. Consider fixing your home if the average of your first and second test is 4 pCi/L or higher.


Reducing High Radon Levels

There are several proven methods for reducing radon in your home, but the one primarily used is a vent pipe system and fan, which pull radon from beneath the house and vent it to the outside. This system, known as a soil suction radon reduction system, does not require major changes to your home. Sealing foundation cracks and other openings makes this kind of system more effective and cost-efficient. Similar systems can also be installed in houses with crawl spaces. Radon contractors can use other methods that might also work in your home. The right system depends on the design of your home and other factors.

EPA's Consumer's Guide to Radon Reduction provides information on how to select the right contractor to install a radon reduction system, what to look for in a contract, different types of reduction systems, and other information to ensure the system you install meets your needs.

Certified Radon Contractors

If you would like to hire a contractor to perform the testing or to install practices to reduce radon in your home, you should first contact one of the independent radon proficiency programs below. They have a list of the certified radon contractors serving your area. The websites for these organizations have a contractor search feature that enables consumers to find professionals certified in testing and/or remediation.

National Radon Safety Board (NRSB)

Phone: (866) 329-3474


American Association of Radon Scientists and Technologists

National Radon Proficiency Program
Phone: (800) 269-4174


Radon Regulations & Useful Links

Building Code for New Homes

Since 1995, all homes constructed in Montgomery County must be built to resist radon entry and prepare the building for post-construction radon mitigation, if neccessary. These requirements are contained in Appendix F of the International Residential Code (IRC).

In general, Appendix F establishes requirements for the construction of building foundations, the sealing of building envelope penetrations, and the installation of "passive" infrastructure to allow for the installation of an "active" radon mitigation system if a radon test indicates this is prudent.

For more information about the requirements of the building code, contact the Montgomery County Department of Permitting Services or call 311 (240-777-0311 outside of Montgomery County).

Buying or Selling a Home

There is no federal, state, or local law that requires the performance of a radon test when buying or selling a home. However, some lending agencies may require a radon test to be conducted in order to provide financing for a home loan. For a thorough discussion of the role of radon in buying and selling homes, see EPA's Home Buyer's and Seller's Guide to Radon.

Useful Links

U.S. Environmental Protection Agency:

National Radon Safety Board (NRSB)

National Environment Health Association (NEHA) - National Radon Proficiency Program