What Time of Year Are Radon Levels the Highest?

If you live in the Pittsburgh area, you may already know that Pennsylvania sits in one of the higher-risk radon zones in the U.S. But many homeowners wonder: is there a “radon season”? In other words, is there a part of the year when indoor radon levels tend to peak? And does that mean that’s the best time to test?

In this article, we’ll dive into what the research says, how seasonal and daily factors influence radon, and practical advice for Pittsburgh area radon testing.

1. What influences indoor radon levels?

Before we look at seasons, it helps to understand what drives radon levels indoors. Radon is a radioactive gas produced by the decay of uranium in soil, rock, and water. It seeps upward through pores and cracks in soil and can enter buildings through foundation cracks, gaps around pipes, crawlspaces, or other openings.

Several factors cause indoor radon levels to vary:

Ventilation and air exchanges

Homes that are tightly sealed or that have reduced ventilation allow radon to accumulate. Conversely, open windows, fans, or increased airflow tend to dilute it.
Temperature and pressure differentials

Differences in indoor vs. outdoor air pressure, and thermal gradients, can help “draw” radon from the soil into the home.
Soil moisture and freezing

Moisture content in soil can influence the pathways radon takes; frozen ground can act as a barrier (forcing radon to enter through other routes).
Building operation changes

Running HVAC, fans, opening or closing vents, and use of fireplaces or exhaust fans changes airflow patterns, which can affect radon concentrations.
Geology and foundation type

Local soil composition, underlying bedrock, the presence of soil fractures or voids, and how the foundation is built (slab, crawlspace, basement) all matter.

Because of all these interacting factors, radon levels inside a home can vary in space and time—even within a single day.

2. Seasonal patterns in radon levels — what does the research say?

A. Winter generally shows higher radon levels

Many studies and radon-industry sources report that indoor radon tends to peak during colder months. The reasons include:

In cold weather, homes are more tightly sealed (windows and doors closed), reducing fresh air exchange, so radon accumulates.
The “stack effect” is stronger in winter: warm indoor air rises and escapes out of upper levels, drawing air (and radon) from the lower levels (basement or soil) into the building.
Frozen or snow-covered ground can restrict some escape routes, increasing the pressure gradient that pulls radon indoors.
Soil temperature and moisture changes in the colder season can reduce radon dispersion in soil, meaning more radon is pushed into structures.

Because of these effects, testing during winter (often late fall through early spring) is commonly recommended by radon professionals.

One source puts it succinctly:

“Experts recommend testing for radon during the colder months because radon levels are historically higher.”

Another notes that “your indoor radon level tends to be higher during the winter months, but some things can cause radon exposure to be just as high during the summer months.”

B. But it is not universally true—some summer or shoulder season spikes

Not all homes follow a simple winter-higher, summer-lower pattern. Some homes see their highest radon levels during the warmer months or during transitional seasons. Reasons include:

Increased use of air conditioning may reduce ventilation in some homes, trapping radon.
Changes in wind patterns, barometric pressure, or local weather events (e.g., storms) can drive temporary spikes.
Household behaviors (closing windows, running exhaust fans differently) can change airflow in ways that elevate radon.
Some studies suggest that 25 % of homes may have their highest radon levels in summer.

One page cautions that a short-term test might miss these fluctuations, so a long-term test is recommended to assess a home’s average risk.

Another remark:

“Radon testing season traditionally slows during the summer months — but that does not necessarily mean radon gas levels in your home are lower.”

C. Scientific/academic observations

A deeper dive into research shows that radon emissions from soil often exhibit seasonal variation: studies in Germany found emissions higher in winter and spring compared to summer and fall.

However, the amplitude of variation depends on climate, soil type, and building structure, so the pattern seen in one region may not exactly match Pittsburgh’s.

Thus, while winter is often the “peak season,” it is not guaranteed that every house will show its maximum radon reading at that time.

3. What about daily (diurnal) variation?

Radon levels also fluctuate throughout the day:

The lowest concentrations often occur in the afternoon, when outdoor mixing is strongest, ventilation is greater, and pressure differences are minimized.
Highest concentrations tend to appear in early morning or overnight when ventilation is lower and pressure differentials are more pronounced.
These daily fluctuations can be 10 % or more of the average.

Because of daily and weather-driven swings, a short test (e.g., 2–7 days) may or may not capture a home’s “worst case” concentration. That is why long-term (3-12 month) tests are preferred for estimating average risk.

4. What is the best time to test in the Pittsburgh area?

Given all of the above, here is guidance tailored for radon testing in the Pittsburgh area:

Ideal testing window

Late fall through early spring (roughly October to March) is often considered optimal, because heating mode is on and windows tend to remain closed, which tends to maximize radon accumulation.
For many homes, December through February can yield some of the highest indoor concentrations.

But don’t ignore other seasons entirely

If you test only in summer or times when windows are wide open, you may underestimate your radon risk.
If a short test in a “low-radon season” shows low levels, you might still want a long-term test to be safe.
If your home exhibits high radon even during warmer months, that suggests a robust pathway for radon entry and is a red flag.

Use long-term testing when possible

A 3- to 12-month continuous monitor or alpha-track detector gives a better picture of the overall average radon exposure, smoothing out seasonal swings.
If you must use a short-term device, test during the colder months for more conservative (higher) results.

Retest if your home changes or seasons shift

After renovations, sealing, or HVAC changes, retest.
If your initial test was done during “off-peak” months, consider retesting in peak season.

Local considerations for Pittsburgh / southwestern Pennsylvania

The Appalachian geology and glacial soils in this region can make radon entry more probable.
Many homes in the area have basements (common in Pittsburgh area housing). Because radon generally enters from the soil into lower levels, basements and ground-contact floors are critical testing zones.
Given that winters in Pittsburgh are cold and heating is used for many months, indoor conditions tend to favor radon accumulation in that season.

In short: for homeowners in the Pittsburgh area, testing during colder months offers a safer “worst case” snapshot, but combining that with long-term or repeated testing gives the fullest picture.

6. Recommendations for Pittsburgh area homeowners

If possible, plan a test during late fall through early spring to catch radon levels during their likely maximum.
Use long-term testing if feasible — it smooths out daily and seasonal variation for a more reliable average.
Test the lowest livable level (basement, slab, crawlspace) where radon is most likely to enter.
Retest after major changes (sealing, renovations, HVAC updates).
If a short-term test during summer shows elevated levels, take it seriously — your home may have a strong radon entry pathway.

7. Why this matters for health and remediation

Because radon is the second leading cause of lung cancer (after smoking) among nonsmokers, identifying high radon levels is a public health priority. Testing in a season when radon is likely to be highest gives homeowners a conservative assessment — better to err on the side of caution. Mitigation systems (sub-slab suction, sealing, ventilation) are effective and can reduce indoor radon substantially.