Radon is a colorless, odorless, and tasteless radioactive gas that forms naturally from the decay of uranium in soil, rock, and water. It is the leading cause of lung cancer among non-smokers and the second overall cause of lung cancer worldwide. Because radon is undetectable by human senses, testing indoor air is the only way to know if a home or building has elevated levels. However, the accuracy of radon test results depends heavily on the season and conditions under which the test is performed. Choosing the wrong time of year can lead to false negatives or inflated readings, both of which can be costly and dangerous. This article explores the best time of year to conduct radon testing, explains the factors that influence radon levels throughout the seasons, and provides practical guidance for obtaining reliable, actionable results.

Why Timing Matters in Radon Testing

Radon levels in a building are not static; they fluctuate daily and seasonally due to a variety of environmental and structural factors. The primary driver of radon entry is the pressure difference between the soil gas and the indoor air. When a house is heated in winter, warm air rises and escapes through the upper levels, creating a vacuum that pulls radon-laden soil gas through cracks in the foundation. Similarly, during summer air conditioning, the house can become negatively pressurized, also drawing in radon.

Seasonal changes in ventilation and soil moisture also play major roles. In colder months, windows and doors are kept closed, reducing the exchange of indoor air with outside air. This allows radon to accumulate to higher concentrations. In warmer months, increased natural ventilation—via open windows or exhaust fans—dilutes radon levels. Soil moisture from rain or snowmelt can plug soil pores for a time, temporarily reducing radon entry, but as the soil dries, radon migration resumes. Testing during a period of unusually low radon levels (e.g., a mild, breezy spring) may underestimate long-term exposure, while testing during a period of peak accumulation (e.g., a cold, calm winter) gives a more conservative—and often more accurate—picture of the highest risk period.

The U.S. Environmental Protection Agency (EPA) recommends that radon tests be conducted under closed-house conditions, meaning all windows are closed, exterior doors are opened only for normal entry and exit, and central heating or cooling systems operate normally. These conditions are most easily maintained during the heating season (winter) and, in many regions, during the cooling season (summer). However, winter is widely regarded as the optimal time for testing because it typically produces the highest and most consistent radon concentrations, reducing the chance of a falsely low reading.

The Best Time of Year for Radon Testing

The ideal window for radon testing runs from late autumn through early spring—essentially the heating season in most of the Northern Hemisphere. During these months, homes are sealed against the cold, and the stack effect is strongest. Testing in winter provides a “worst-case scenario” that helps homeowners make informed decisions about mitigation. If radon levels are acceptable under winter conditions, they are likely safe year-round.

Why Winter Testing Is Preferable

  • Homes are sealed: Closed windows, doors, and weather stripping minimize air exchange with the outdoors, allowing radon to accumulate to higher, more stable readings.
  • Higher stack effect: The temperature difference between warm indoor air and cold outdoor air creates a strong upward draft, pulling soil gas into the house more efficiently.
  • Consistent conditions: Winter weather patterns often produce stable barometric pressure and soil moisture levels, leading to less fluctuation in radon entry.
  • Aligned with EPA guidance: The EPA’s standard testing protocol for short-term tests specifically recommends conducting the test under closed-house conditions, which are easiest to maintain in winter.

That said, winter testing is not always feasible for homeowners in warmer climates or for those who cannot maintain closed-house conditions due to extreme cold or other factors. In such cases, a summer test under closed-house conditions (with air conditioning running) can serve as a reasonable alternative, but the results may be somewhat lower than a winter test.

Regional Variations and Exceptions

The “best” time can vary depending on climate, local geology, and building construction. For example:

  • Northern climates (e.g., Canada, northern U.S., Europe): Winter is the clear winner. Permafrost and deep snowpack can actually increase soil gas pressure, contributing to higher indoor radon.
  • Warmer regions (e.g., southern U.S., Mediterranean): Summer air conditioning can also create negative pressure and closed-house conditions. Testing in late summer or early autumn may be more practical, though levels tend to be lower than in winter.
  • Humid tropical climates: Radon levels may peak during the rainy season when soil is saturated and homes are sealed to keep out humidity. In these areas, testing during the wettest months might be as important as winter testing is in temperate zones.
  • Homes with crawlspaces or basements: These structures can have different radon dynamics. Basements may exhibit higher radon in winter due to the stack effect, while crawlspaces may see increased levels in summer when moisture drives radon out of the soil.

Because radon levels can vary significantly from region to region, it is advisable to consult local radon maps, state health departments, or EPA radon zone information for guidance specific to your area.

Short-Term vs. Long-Term Testing: Which Is Better?

Timing also depends on the type of test you use. Short-term tests (2–90 days) are designed to give a quick snapshot of radon levels under specific conditions. They are typically used for real estate transactions or as a first screening. Because they are influenced by day-to-day weather and seasonal variations, they should be performed during the period when radon is most likely to be elevated. For a short-term test, winter is the preferred season, and the test should be conducted with all windows closed.

Long-term tests (90 days to 1 year) provide an average radon level over several seasons, giving a more accurate picture of annual exposure. If you use a long-term test, the start date is less critical because the test will capture both high and low periods. Many experts recommend starting a long-term test in late autumn so that it covers the peak winter months. The World Health Organization and the EPA both endorse long-term testing as the gold standard for residential radon assessment. For more details, see the WHO radon fact sheet.

Pros and Cons of Each Test Type

Test Type Best Timing Pros Cons
Short-term (2–90 days) Winter (closed-house) Quick results; good for real estate transactions; inexpensive Can be skewed by unusual weather; only captures one season
Long-term (90 days – 1 year) Start in late autumn Average annual exposure; more reliable; less influenced by short-term fluctuations Requires patience; more expensive; device must be left undisturbed

For homeowners serious about radon safety, a long-term test started in late fall is the most robust approach. If a short-term test is necessary, repeating it in both winter and summer can give a sense of the range of levels, though mitigation decisions should be based on the higher reading.

Additional Tips for Accurate Radon Testing

Regardless of the time of year, following proper procedures is critical to obtaining valid results.

General Testing Best Practices

  • Maintain closed-house conditions: Keep all windows and exterior doors closed for at least 12 hours before and during the test. Exterior doors should be opened only for normal entry and exit.
  • Place the test device in the lowest livable level of the home (e.g., basement or ground floor). Avoid kitchens, bathrooms, laundry rooms, and areas with high humidity.
  • Position the device at least 20 inches above the floor and away from drafts, direct sunlight, and HVAC vents.
  • Do not disturb the test: Avoid moving furniture, opening windows, or running exhaust fans that could affect radon levels.
  • Follow manufacturer instructions exactly regarding exposure time, shipping, and lab analysis.
  • Avoid testing immediately after home renovations (e.g., new flooring, foundation repairs) or major weather events like floods or heavy rain, as these can temporarily alter radon entry.
  • Do not test during severe storms or high winds, which can artificially depressurize the house and skew results.

When to Retest

The EPA recommends retesting every two years, or after any significant structural change to the home, such as a new basement, a renovated foundation, or installation of a heat recovery ventilator. If you have a radon mitigation system, you should test at least annually to ensure it is functioning properly.

Understanding Radon Mitigation and Action Levels

The EPA has set an action level of 4 picocuries per liter (pCi/L) of air. If a test shows radon levels at or above 4 pCi/L, the agency recommends taking steps to reduce the concentration. For levels between 2 and 4 pCi/L, the EPA suggests considering mitigation, especially if a long-term test confirms consistent readings in that range. The World Health Organization recommends an even lower reference level of 2.7 pCi/L (100 Bq/m³).

Mitigation typically involves installing a soil depressurization system (also called a radon fan system) that draws radon-laden air from beneath the foundation and vents it safely outside. Other methods include sealing cracks in the foundation, improving ventilation, and installing a heat recovery ventilator. Professional radon mitigators should be certified by a national program such as the National Radon Proficiency Program (NRPP) or the National Radon Safety Board (NRSB).

Radon Testing and Real Estate Transactions

Home buyers and sellers are especially concerned with test timing. Most real estate contracts require a radon test as part of the home inspection. The test should be performed under closed-house conditions, and winter is ideal because it is easier to keep windows closed and because the radon level is more likely to reflect the worst-case scenario. However, a winter test that yields a high reading can become a negotiating point. Sellers may wish to test in advance and, if levels are high, install a mitigation system before listing. Buyers should consider conducting their own follow-up test after moving in, using a long-term device.

It is important to note that short-term tests conducted during a real estate transaction are only valid if the house has been closed for at least 12 hours prior to the test and during the test period. If the test is performed in mild weather with open windows, it may produce a falsely low reading, leading the buyer to unknowingly purchase a home with a radon problem. The EPA’s Home Buyer’s and Seller’s Guide to Radon provides detailed recommendations for these situations.

Myths About Radon Testing Timing

Several misconceptions persist about radon testing. One common myth is that radon is only a problem in older homes or only in basements. In reality, radon can enter any home, regardless of age or construction type, and can accumulate on any floor. Another myth is that winter testing yields artificially high results that are not representative of whole-year exposure. While winter levels are typically higher, they represent the period of greatest risk, and the EPA’s action level is based on the potential for lung cancer from long-term exposure—including winter months. Therefore, a winter test is conservative but not misleading. A third myth is that if you test in summer and the result is low, you are safe. Unfortunately, that low result may double or triple in winter, exposing occupants to levels above the action level for several months every year.

Conclusion

Timing is a decisive factor in radon testing accuracy. For most homes in temperate climates, the best time of year to conduct a radon test is during the winter heating season, from late November through early March. Testing under closed-house conditions during the coldest months yields the highest and most consistent radon readings, giving homeowners a true picture of peak exposure. However, regional climate, local geology, and the type of test used can influence the ideal timing. Long-term testing that spans multiple seasons remains the most reliable method, but if a short-term test is necessary, winter is the preferred season. By choosing the right time, following best practices, and using certified professionals when needed, homeowners can protect their families from the serious health risks of radon exposure. Regular testing every two years, or after any major renovation, ensures ongoing safety and peace of mind.

For more information, consult the EPA's state radon contact list to find local experts and resources.