What Time of Year Is Radon Worse?
By Daniel SmokeDirector of Operations
Radon levels in a home are not constant. They rise and fall with the weather, the season, and how the house is operated. For most homes, radon is worst in winter. Understanding why helps you choose when to test and how to interpret a result. That way you are measuring something close to your home's realistic worst case rather than its best.
Why Radon Is Usually Higher in Winter
Two things push radon up in cold weather. First, homes are closed up and heated, so there is less fresh-air exchange to dilute indoor radon. Second, warm indoor air rising through the house creates a slight vacuum at the lowest level, the 'stack effect.' That actively pulls soil gas, including radon, in through foundation cracks and openings. Frozen or snow-covered ground can also cap the soil and channel more gas toward the warm building. Together these effects make winter the typical high-radon season.
Does That Mean You Should Only Test in Winter?
Winter testing gives you a useful worst-case picture, but radon can be elevated in any season, and you can test year-round. The EPA recommends testing whenever a home has not been tested recently rather than waiting for a particular month. If you test in warmer weather and get a result near the action level, it is wise to confirm with a follow-up test. Winter levels may run higher.
Short-Term vs. Long-Term Testing
A short-term test (typically 2 to 7 days) gives a quick snapshot and is common during real estate transactions, when timing matters. A long-term test (90 days or more) averages across changing conditions and gives a more representative picture of your year-round exposure. If a short-term test is done in summer, a long-term or winter follow-up helps confirm the true average.
Conditions That Affect Any Test
For a valid short-term test, closed-house conditions are usually required. Keep windows and exterior doors closed except for normal entry and exit, starting before and continuing through the test. Storms and high winds can also shift readings. Following the test protocol matters as much as the season, because an open-window summer test can understate your real exposure.
The Bottom Line on Timing
If you want the most cautious read, test during the heating season, when levels typically peak. But timing matters far less than actually testing. Radon is invisible and odorless, and measuring it is the only way to know your level. If you tested in summer and landed near the action level, schedule a winter follow-up before deciding on mitigation. Peerless Environmental supports radon testing across the Southeast year-round.
What Backs Our Work
Every Peerless Environmental location follows the same safety-first standard, so you get clear answers no matter which local team you call.
AHERA & EPA RRP Certified
AHERA certification for school and public building asbestos inspections, and EPA RRP certification for lead-safe work practices.
Clear Reports & Documentation
Results you can understand and share with owners, buyers, managers, or regulators.
Compliant Work Methods
Procedures aligned with EPA and state environmental regulations, from site preparation through final documentation and clearance.
Local Teams, Shared Standards
Market-level response backed by the Peerless Environmental process.