Radon levels are generally higher inside homes during the winter months. Radon is a colorless, odorless, and tasteless radioactive gas that results from the natural breakdown of uranium found in soil and rock. This gas seeps up from the ground and enters a home through cracks in the foundation and other openings. Because the gas cannot be detected by human senses, it can build up to dangerous levels unnoticed. Radon exposure is the second leading cause of lung cancer in the United States. Understanding the seasonal increase is important for protecting your home and family.
Why Radon Levels Peak in Colder Months
The increase in indoor radon concentration during winter is primarily due to two factors: reduced ventilation and the thermal stack effect. During colder periods, homeowners keep windows and doors tightly closed to maintain heat, which dramatically reduces the exchange of indoor air. This “closed-house condition” traps any radon that enters the structure, allowing it to accumulate to higher concentrations.
The stack effect intensifies as the temperature difference between the inside and outside air increases. A home heated in the winter causes the interior air to rise and escape through upper-level openings. This escaping warm air creates a vacuum or negative pressure in the lower levels of the home, including the basement or crawlspace.
This negative pressure actively draws air, along with radon gas, from the surrounding soil into the house through the foundation. Frozen or snow-covered ground acts like a cap that prevents the radon gas from escaping harmlessly into the atmosphere. The path of least resistance becomes the lower-pressure environment inside the home. This continuous suction force draws more radon-rich soil gas into the living space, making winter the time when the highest levels are typically recorded.
The Health Risk of Radon Exposure
Radon’s danger to human health lies in its radioactivity and subsequent decay into solid radioactive particles. When a person inhales radon gas, these microscopic particles become lodged in the lining of the lungs. Once deposited, the particles continue to decay, releasing energy that damages the DNA in the lung tissue.
This cellular damage can eventually lead to the development of lung cancer after long-term exposure. Radon is responsible for an estimated 21,000 lung cancer deaths annually in the United States. While smoking remains the primary cause of lung cancer, radon is the leading cause among non-smokers. The risk is significantly higher for smokers, as the combined exposure creates a synergistic effect. The United States Environmental Protection Agency (EPA) recommends taking action to mitigate levels when the concentration reaches or exceeds 4 picocuries per liter of air (pCi/L).
Testing Your Home for Radon
Testing is the only way to determine the radon concentration within a home. Performing a test during the winter months is often recommended to capture a worst-case scenario reading. This provides the most accurate picture of maximum potential exposure because the conditions that lead to the highest levels—closed windows and the stack effect—are most pronounced during this time.
There are two main types of tests available: short-term and long-term. Short-term tests are conducted over two to seven days and are often used for initial screening or during real estate transactions. These tests provide a quick snapshot but are more susceptible to daily and weekly fluctuations. Long-term tests measure levels for 90 days or more, providing a more stable and reliable average of the home’s year-round radon exposure.
For accurate results, the device should be placed in the lowest lived-in level of the home, such as a basement or first floor. It must be kept away from drafts, windows, doors, and excessive humidity. Any measurement at or above the EPA’s action level of 4 pCi/L should prompt a homeowner to consider installing a mitigation system.
Basic Radon Mitigation Strategies
If testing reveals elevated radon levels, the most common and effective strategy is installing an Active Soil Depressurization (ASD) system. This system works by creating a negative pressure field beneath the concrete slab or foundation, which reverses the flow of air. A fan is connected to a vent pipe that extends through the foundation into the soil below, drawing the radon gas out.
The fan continuously sucks the gas from the soil before it can enter the house and safely exhausts it above the roofline, where it quickly dissipates into the outdoor air. While sealing visible cracks and openings in the foundation can help, sealing alone is rarely sufficient to lower high radon levels below the action threshold. The ASD system is the preferred method because it actively addresses the pressure difference that draws the gas into the home, providing a reliable, long-term solution.