What is Radon?

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.

Radon usually does not present a health risk outdoors because it is diluted in the open air. Radon can, however, build up to dangerous levels inside a house.

Share the information on this website with others (PDF, 252KB)


Radon Testing and Mitigation for Rental Properties

As of July 1, 2023, Montgomery County law requires radon testing for multifamily (including condominiums) and single-family rental housing, as well as disclosure and mitigation of radon hazards above a certain action level. This requirement is administered by the Department of Housing and Community Affairs. The DHCA website contains information for landlords and tenants:



The Radon and Home Building Law

As of October 1, 2016, County law requires that “a single-family home located in the County must be tested for radon before completing a sale of the home.” Learn more about the law as well as information for those building a new home:



What Areas are at Risk for Radon?

Montgomery County, MD has high levels of radon in the soil, and therefore, every home in Montgomery County should be tested for radon. 

The U.S. Environmental Protection Agency (EPA) has worked with state and federal geologists to develop maps which predict the potential indoor radon levels for every county in the United States. Those counties with the highest potential are designated as Zone 1; those with the lowest comprise Zone 3. 

Montgomery County, MD has been designated as a Zone 1 area which means there is a predicted average radon level at or above the EPA's 4.0 pico-Curies per liter (pCi/L) action level. (pCi/L is a measure of the amount of radioactivity in a known quantity of air.)  


Map of the Maryland Counties' radon zones. Montgomery County is in Zone 1 with the highest risk.
EPA designated radon zones as of 8/2016 - All homes should be tested for radon, regardless of zone designation


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.


Learn about Radon:


Useful Links

Share County radon information as a downloadable PDF (252KB):


If distributing large quantities of radon information, share it as a brochure (requires folding): PDF, 415KB

U.S. Environmental Protection Agency:

Additional Resources:


Meetings and Events

The Maryland Chapter of American Association of Radon Scientists and Technologists (AARST) offers valuable events, continuing education opportunities, and resources for certified professionals.