Does Radon Change With the Seasons?

A home that tests at 2.8 pCi/L in July might test at 6.5 pCi/L in January. Same house, same foundation, same soil. Nothing was renovated and nothing broke.

Seasonal variation is one of the most consequential and least understood aspects of residential radon, and it matters more in a climate like Central New York’s than almost anywhere else.

The Stack Effect

The main driver has a name: the stack effect.

Warm air rises. In winter, the air inside your home is dramatically warmer than the air outside — a 70-degree interior against a 15-degree January morning in Syracuse. That warm air rises through the structure and escapes through the upper portion of the building: attic penetrations, recessed lights, plumbing chases, gaps around the chimney.

Air leaving the top of a house has to be replaced. Replacement air gets drawn in at the bottom — which means through the basement, through the slab, through the perimeter joint and sump pit and floor cracks. Your house effectively becomes a chimney, and the foundation is the intake.

Soil gas sits directly beneath that intake. The greater the indoor-outdoor temperature difference, the stronger the effect, and the harder your home pulls radon out of the ground.

In summer, with a 75-degree interior and 80-degree exterior, the differential nearly disappears. The pump largely shuts off.

Frozen and Snow-Covered Ground

The second factor is specific to northern climates, and Central New York gets a full dose of it.

Radon generated in soil normally diffuses in all directions, and much of it escapes harmlessly into open air at the surface. When the ground freezes, and especially when snowpack sits on top of it, that upward escape route gets capped.

The gas is still being produced. It now has to go somewhere. The path of least resistance becomes the warm, depressurized structure sitting on that soil — your basement.

Syracuse averages well over 100 inches of snow annually and holds frozen ground for months. That is a long stretch of capped soil directing gas toward foundations rather than the atmosphere.

Sealed-Up Houses

The third factor is behavioral. From roughly November through March, Central New York homes stay closed. Nobody opens a window. There is minimal natural ventilation diluting indoor air.

In summer, even homes running air conditioning tend to get some window-open days in spring and fall, plus more door traffic. That air exchange dilutes accumulated radon.

Winter removes the dilution at exactly the moment entry rates are highest. More gas coming in, less air exchange to dilute it.

Shorter-Term Swings

Beyond the seasonal pattern, radon moves on shorter cycles.

Daily. Levels typically peak in the early morning hours, when the house has been closed all night and the temperature differential is greatest, then decline through the afternoon.

Barometric pressure. Falling pressure ahead of a storm reduces the atmospheric weight pressing down on soil, letting more gas escape upward. This is why testing during unsettled weather can produce readings that do not represent typical conditions.

Wind. Wind creates pressure differentials across a building. Depending on direction and configuration, it can either increase or decrease soil gas entry.

Rain and snowmelt. Saturated soil can seal the surface, temporarily pushing gas toward the foundation instead.

What This Means for Your Test Result

The practical implications are worth being direct about.

A summer result near the action level deserves a second look. If you tested in July and got 3.4 pCi/L, that home may well exceed 4.0 during heating season. It is not that the summer test was wrong — it accurately measured summer. But summer is not when your exposure peaks.

A winter result is closer to your worst case. A January test of 3.2 pCi/L in a Syracuse home is more reassuring than the same number in July, because it likely captured the high end of the annual range.

A high result at any time of year is a high result. Seasonality explains variation around the margin. If you tested at 15 pCi/L in August, that is not a summer artifact — that is a home that needs mitigation.

Long-term testing sidesteps the whole question. An alpha track detector left in place for a year averages across all of it. If you want to know your actual annual exposure rather than a seasonal snapshot, that is the way.

Should You Wait for Winter to Test?

Generally, no — with one exception.

Testing whenever you are thinking about it beats deferring until a season that feels optimal, because deferred tests have a way of never happening. A summer test that comes back at 9 pCi/L gives you actionable information in August rather than February.

Real estate transactions obviously cannot wait for a season either. You test when the contingency window says to test.

The exception: if you already tested in warm weather and landed somewhere between roughly 2.0 and 4.0 pCi/L, a follow-up during heating season is genuinely worth doing. That is the range where seasonal variation can move you across the action level, and knowing which side you are actually on changes what you do.

Mitigation Systems Handle All of It

Worth noting: an active sub-slab depressurization system is not seasonal. It maintains negative pressure beneath the slab continuously, so soil gas is drawn out and vented regardless of whether it is January or July, whether pressure is rising or falling, whether the ground is frozen.

That is a meaningful advantage over ventilation-based approaches. Opening basement windows lowers readings in August and does nothing in a Syracuse winter — precisely when levels peak. A fan-driven system works hardest exactly when the stack effect is strongest.

This is also why post-mitigation testing carries weight whenever it is performed. A system either holds levels down or it does not.

The Takeaway

Radon is not a fixed number for a house. It is a range, and where in that range you happen to measure depends on season, weather, and how your home is operating that week.

In Onondaga County — EPA Zone 1, long heating season, heavy snow — the winter end of that range is what matters most for exposure. A test result is a real data point, and knowing when it was taken is part of reading it properly.

Questions about your result or when to retest? Call (315) 234-2853 for certified radon testing across Syracuse, Cicero, Liverpool, Baldwinsville, Camillus, Manlius, and DeWitt.

Seasonal swings show up clearly in Baldwinsville, where newer Radisson-area construction is tightly sealed against winter air, and in Manlius, where fractured limestone bedrock gives soil gas an easy path upward once indoor pressure drops.