When are radon levels highest?

If you are asking when radon levels are highest, you likely know enough about radon gas to understand that it is not something you want in your home at any time of year. Conducting a radon test is the first step in understanding your risk of radon exposure. Our advice? If you have never tested your home, go ahead and test, regardless of what time of year. Radon levels are almost always going to be higher in the colder winter months, so we also recommend conducting follow-up testing during the winter season to get a full picture of radon in your home year-round.

Why should you test for radon?

Radon is a known human carcinogen, the leading cause of lung cancer among non-smokers. This naturally occurring, radioactive, gas is created from the breakdown of uranium underground and seeps into buildings from small cracks in the foundation or plumbing. Testing for radon is the only way to know your risk of exposure.


Radioactive radon particles are harmful to your lungs when breathed. Its radioactive properties can damage or mutate lung cells, which can result in cancer. More than 21,000 people die from radon-induced lung cancer every year. Exposure to radon is preventable with proper testing and mitigation in homes and buildings.

When are radon levels the highest?

On average, radon levels are the highest in the colder months, or the heating season. Radon levels are naturally affected by the changing seasons, atmospheric pressure, and precipitation throughout the year. However, temperature fluctuations have the greatest impact on indoor radon levels due to the differences in pressure put on the home.

Why are radon levels higher in the winter?

Weather changes

Changing weather conditions can impact your indoor air quality. Various weather patterns are caused by atmospheric pressure changes. This can impact the air pressure in the soil as well, causing soil gases, including radon, to be pushed up toward the foundation of your home. These kinds of conditions could increase the possibility for radon and other soil gases to enter your home.

Snow barrier

The snow and ice also affect radon entry into buildings. When there is snow or ice surrounding the building, a barrier is created above the soil. Radon gas below the soil is then sealed under the ground below the foundation of the home. Radon, and other soil gases, will follow the path of least resistance. With a blanket of snow and ice surrounding your home, the path of least resistance is often cracks and openings in the foundation.

Thermal stack effect

A fundamental building science element is the thermal stack effect. This effect describes the movement of air inside and outside of the home due to natural laws of pressure. Cold air is more dense than warm air, meaning cold air falls and warm air rises. This law of pressure is always present, regardless of the season.

When a home or building is heated in the winter, warm indoor air naturally rises. Because warm air is less dense than cold air, it rises upward, escaping through the roof, vents, or other openings at the top of your home.

As warm air escapes, cold air is pulled in from below, much like a hot air balloon. The pressure difference creates a vacuum-like effect that sucks in the colder air from outside and from beneath the foundation.

Anything in the air below the foundation, regardless of the safety or quality of it, can be pulled into your home as a part of the process of the structure “breathing”. It is possible that hazardous soil gases are present, compromising your indoor air quality. Dangerous soil gas, including radioactive radon, can be sucked into homes and buildings at a faster rate during the colder months because of the thermal stack effect.

The thermal stack effect explains why radon levels are almost always higher in the winter. Simply put, outdoor air is being pulled into the home quicker and more frequently in the winter than in the summer. For this reason, the potential for being exposed to higher levels of radon in your home is greater in the colder winter months.

When Are Radon Levels Highest
Sealed Homes

When temperatures are more desirable, windows are opened creating more airflow throughout the home or building. Airflow can help dilute the radon gas buildup indoors and can improve your overall indoor air quality. Within tightly sealed buildings, there are few ways for gas particles to escape. Radon gas can then become more concentrated and build up to dangerous levels indoors.

Why test for radon in the winter?

Radon levels can and will fluctuate over time and with the changing seasons. Seasonal variability, stack effect, tightly sealed homes, and snowy barriers help us understand why radon and other soil gas levels are almost always higher in the colder months.

We have seen seasonal test results increase from a range of 1.8 – 2.2 pCi/L in the summer to a range of 28.0 – 32.0 pCi/L in the winter in the same building. The EPA recommends mitigation if the radon level is 4.0 pCi/L or higher. If you have only tested your home in the summer months, you may be unaware that your breathing air contains dangerous levels of radioactive radon in the winter.

The only way to know if your radon levels have fluctuated in the winter is to test. Also, if you have never tested or have not tested in the last five years, you should request a professional radon test as soon as possible.

Want to know the average radon test result near you?

Search your zip code below for the average reported radon test result in your area.

How to reduce your risk of radon exposure all year long

If your radon levels are elevated, installing a mitigation system is the next step. You will want to make sure your mitigation system is installed by a qualified professional who is certified and/or licensed. Unfortunately, mitigation systems can be completely ineffective if installed incorrectly or designed for a lower pressure level in the home.

  1. Test for radon in different seasons or conduct a long-term test to understand how radon levels fluctuate in your home.
  2. If levels are elevated, work with a qualified professional to install a radon mitigation system in your home.
  3. Have your mitigation system serviced annually by a qualified professional to ensure your system continues to function correctly.
  4. If you have a mitigation system, test every two years to ensure that you are continuing to be protected against radon exposure.

"A properly designed and installed mitigation system is essential in preventing exposure to cancer-causing radon gas. Unfortunately, many radon contractors fail to take the seasonal pressure differential variances within the home into consideration when designing the system, leading to the homeowner being unknowingly exposed to unsafe levels of radon during certain times of the year." 

- Kyle Hoylman, CEO of Protect Environmental

Is your radon mitigation system affected during the colder months?

If you had a mitigation system installed in the warmer months, test again during the winter season to make sure your system is continuing to keep you safe with the cold weather changes. If your mitigation system was designed for a lower pressure level during the warmer months, it could be essentially ineffective and elevated radon levels could still be in your home or building.

We recommend testing every two years, even if you have a mitigation system installed, because of these seasonal fluctuations. Consider testing in the colder months or conduct a long-term radon test to get a complete picture of the radon levels in your home year-round.

Key Takeaways:

  • Radon levels can and will be affected by seasonal variability.
  • Indoor radon levels are normally at the highest in the winter or colder months because of the thermal stack effect, a snowy barrier, and tightly sealed homes.
  • Cold temperatures increase the pressure within the home, meaning more air is being pulled in from the ground, which elevates the risk of radon entering the home.
  • Test your home and other buildings in the colder months to get a complete picture of radon exposure.
  • Test your home every two years to ensure your radon mitigation system continues to protect your home from radon in higher pressure conditions caused by colder temperatures.
Press Releases

Seasoned Due Diligence Professional Joins Protect Environmental

Kenneth Deemer to lead development and management of national operations, maintenance, and monitoring (OMM) services portfolio for Louisville-based environmental services company

Louisville, KY, February 11, 2022 – Protect Environmental announced today that Kenneth Deemer has joined the company as its Director, OMM Services. He joins the company with 18 years of experience in the environmental due diligence industry, where he managed programs related to environmental contaminants, including radon, asbestos, mold, and PCBs. Deemer also brings a strong background in environmental site assessment, having conducted hundreds of due diligence assessments on residential, commercial, and industrial sites for national lending clients. He received his Bachelor of Science Degree in Environmental Science from Allegheny College, and is a certified radon measurement and mitigation professional, an accredited asbestos inspector and management planner, and possesses the OSHA HAZWOPER 40 credential.

“I’m excited to be working with the recognized leader in providing radon and chemical vapor intrusion management solutions for clients across the country,” Deemer said. “Joining the team at Protect Environmental provides me with the opportunity to have a role in providing healthy and safe indoor environments in the places we live, work, and learn,” he added. 

As Director, OMM Services, Deemer will be responsible for developing and maintaining the national OMM services portfolio of the company by providing solutions for meeting long-term stewardship and continuing obligations for owners of property located on contaminated radon and chemical vapor intrusion sites. 

“Kenny’s experience will have an immediate positive impact for our clients,” said Kyle Hoylman, CEO of Protect Environmental. “As the regulatory environment continues to evolve, ensuring we are working with the most experienced professionals is important in assisting our clients with effectively managing their risks and liabilities,” Hoylman added.

About Protect Environmental

Protect Environmental is a national leader in the environmental services industry, focusing on radon and chemical vapor intrusion management. With a proven track record spanning 15 years that includes completed project work in all 50 U.S. states and 2 U.S. territories, the company delivers expert service from its trusted professionals to provide peace of mind protection to property owners seeking efficient and effective management of environmental risks and liabilities. For more information, call 502-410-5000 or click on to

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