Why winter is the best time to test for radon

Radon is usually at its highest when your house is closed up and the heat is on. That makes the cold months - and January, which is National Radon Action Month - the best time to find out whether your home has a problem. This guide explains why radon rises in the heating season, what Action Month is, and exactly how to run a winter test that gives you a number you can act on.

Quick version: close windows and doors 12 hours before testing, place the kit on the lowest lived-in floor 2-7 feet up, run it at least 48 hours, and mail it the same day. Fix at 4 pCi/L or higher; consider fixing between 2 and 4.

The short answer

Radon is a soil gas, and a house in winter behaves like a chimney that pulls soil gas in. Warm indoor air rises and leaks out through the attic and upper floors; that lowers the air pressure at the bottom of the house, and the replacement air is drawn in from the ground through slab cracks, the gap where the floor meets the wall, sump pits, utility penetrations and crawlspaces. At the same time windows stay shut, so less outdoor air dilutes what comes in. The Minnesota Department of Health, which publishes tens of thousands of home results, puts it plainly: in winter, home heating systems tend to draw in radon from the soil, and radon levels tend to be higher in winter, though they can be high at any time of year.

That is exactly why winter is the best time to test. A test taken when radon is near its seasonal high is the conservative one: if the result is low in January, you can be confident the house is not hiding a problem. If it is high, you have found it during the months when your family is breathing the most indoor air. And practically, the test conditions every protocol asks for - doors and windows closed for 12 hours before and during a short-term test - are simply how people live in a cold month. Colorado's health department says it is always a good time to test as long as you can keep the house closed, and that some people find this easier in winter.

Why radon rises in the heating season

The stack effect

The bigger the difference between indoor and outdoor temperature, the stronger the upward flow of warm air through a house and the stronger the suction at its base. On a 10°F night with the furnace running, that pressure difference is at its largest of the year. Building scientists call it the stack effect; it is the same force that makes a fireplace draw. A home that is slightly depressurized relative to the soil will pull soil gas through every opening below grade, and radon rides along with it.

Exhaust appliances and combustion air

Furnaces, water heaters, wood stoves, fireplaces, clothes dryers and bath and kitchen fans all push air out of the house. In winter they run more, and every cubic foot they exhaust has to be replaced. In a tight house a meaningful share of that make-up air arrives through the foundation rather than through gaps around windows. That is one reason two otherwise similar houses on the same street can test very differently.

Closed windows, less dilution

In summer, open windows and screen doors bring in outdoor air that usually carries very little radon. EPA has long used about 0.4 pCi/L as the typical outdoor level and about 1.3 pCi/L as the average indoor level in US homes. Close the house up and that dilution largely disappears. This is also why a test run during a week of open windows in May can read misleadingly low.

Frozen and saturated ground

When the surface of the soil around a house freezes or is soaked with snowmelt, gas that would otherwise escape straight up into the open air is partly capped. The unfrozen, drier soil directly under a heated house becomes the easiest exit, so more of it ends up beneath and inside the basement slab. The size of this effect depends heavily on local soil and climate, but it pushes in the same direction as the other three.

How much higher is winter, really?

It depends on the house, the soil and the climate, and no single multiplier fits everyone. Long-term studies of the same homes measured across seasons commonly find heating-season averages above the annual average, with the gap largest in cold, continental climates and in homes with basements. Minnesota's statewide database, which tracks average results by season, shows the same rise-and-fall pattern every year. Some houses run the other way: homes with strong summer air conditioning, or with soil that dries and cracks in late summer, can peak outside winter. That is one more reason a single season's test should not be the last word if you are close to a threshold.

For context, the modeled long-term picture on this site, which averages across all seasons, already shows how common elevated levels are. The population-weighted US average predicted basement level is 1.8 pCi/L, and 1,220 of the 29,215 ZIP codes we cover are predicted at or above 4 pCi/L in the basement. Upstairs is lower: in the typical ZIP the predicted above-ground level is about 57% of the basement level (see basements vs. upper floors). Those numbers are model estimates of an area's long-term average, not your house - which is the whole point of testing.

National Radon Action Month: January

January is National Radon Action Month in the US. The EPA uses it to push the message that every home should be tested, alongside state radon programs, the Conference of Radiation Control Program Directors and the CDC. The timing is deliberate: it lands in the middle of the heating season, when houses are closed and radon is most likely to be at its seasonal high, and right after the holidays, when people are home.

The EPA's case for the month rests on a few numbers it repeats every year:

• Radon is the leading cause of lung cancer among non-smokers in the US and the second leading cause overall.
• EPA estimates radon is responsible for about 21,000 lung cancer deaths a year, about 2,900 of them among people who have never smoked.
• The EPA and the US Surgeon General recommend testing every home, regardless of zone, and fixing homes at or above 4 pCi/L; EPA also says to consider fixing between 2 and 4 pCi/L.
• Radon in a house that smokes multiplies risk: smoking and radon together carry a far higher lung cancer risk than either alone.

Many state radon programs time their own campaigns to January. Depending on the year and the state, that can mean free or discounted test kits from the state or county health department, poster and video contests, and proclamations. Colorado's health department, for example, offers discounted test kits and runs annual radon poster and video contests. Find your state radon program and look for an Action Month offer in late December and January, then check your state's predicted levels while you wait for the kit.

A winter testing plan that gives you a number you can act on

1. Pick the right test for the question

• Short-term test (2 to 90 days). Charcoal canisters and similar kits are usually exposed for 2 to 4 days. Cheap and fast, and the right first step for almost everyone. In winter a short-term result is a good screen because it catches the house near its seasonal high.
• Long-term test (more than 90 days). Alpha-track detectors and some electronic monitors average over months. Health Canada asks for a minimum of 3 months. Starting one in November or December means it runs through the heart of the heating season and comes back with a number that reflects how your family actually lives.
• Continuous monitor. An electronic monitor that reports hourly or daily averages. It shows the swings - nights versus afternoons, cold snaps versus thaws - and keeps working after the test, which is useful if you later install a system.

Our how to test for radon guide compares these in detail. The short version for winter: start with a short-term test now, and if it comes back between 2 and 8 pCi/L, follow up with a second short-term test or a long-term test before deciding on mitigation.

2. Set up closed-house conditions

For a short-term test, close all exterior windows and doors at least 12 hours before you start and keep them closed for the whole test. Normal coming and going is fine. Your furnace can run normally. Do not run a whole-house fan, and avoid fireplaces and wood stoves if you can, since they change the pressure balance in ways that may not be typical. In winter most of this happens on its own, which is the practical reason many people find it easier to test now.

3. Put the test in the right place

• The lowest level you use regularly as living space - a finished basement, or the ground floor if there is no basement. If you are testing for a home sale, use the lowest level that could be finished.
• Two to seven feet off the floor, on a table or shelf, in an open central spot.
• Not in a kitchen, bathroom, laundry room, closet or crawlspace, and not right next to an exterior wall, window, door, heat register or fan.
• Away from direct sunlight and anything that could be bumped by children or pets.

4. Watch the weather

Avoid starting a short-term test during a major storm or a stretch of unusually high wind; both can skew a two-day reading in either direction. A normal cold week is fine and is exactly what you want. If a blizzard hits mid-test, note it and plan a confirming test.

5. Mail it promptly and log the details

Charcoal kits keep decaying after you seal them, so ship to the lab the same or next day. Write down the exact start and stop date and time and the kit serial number. Many labs ask for them when you register the kit, and you will want them if you compare results later.

Reading a winter result

Results are reported in picocuries per liter (pCi/L). The EPA's action level is 4 pCi/L; it also recommends considering a fix between 2 and 4, since there is no known safe level. Because winter readings tend to sit at or near the seasonal high, here is how to read them:

• Below 2 pCi/L: reassuring, precisely because it was taken in the worst season. Retest every couple of years, and after any renovation that touches the foundation or HVAC.
• 2 to 4 pCi/L: worth a follow-up. A long-term test running through the rest of winter and spring will tell you where the house really averages.
• 4 to 8 pCi/L: confirm with a second short-term test or a long-term test. If the average of two short-term tests is 4 or more, EPA guidance is to fix the home.
• Above 8 pCi/L: confirm quickly with a second short-term test and start getting mitigation bids. There is no reason to wait for a full long-term test at that level.

A common worry is that a winter test "exaggerates" the problem. It is better framed the other way around: a summer test with the windows open can understate it. If you live in the house year-round with the windows closed for five or six months, the heating-season number is the one your lungs experience for much of the year. If you are close to a threshold and want the full-year picture, run a long-term test that spans both seasons.

If your winter test is high

Winter is also a good time to line up a fix. A certified mitigator can install a sub-slab depressurization system in a day in most houses, and the work does not depend on the weather the way exterior projects do (though roof-line exhaust runs are sometimes scheduled around snow). Get two or three bids that spell out the fan, the number of suction points, the pipe route and a post-installation test. Our mitigation guide explains how systems work, and radon mitigation cost by state collects published price ranges from state programs. Then test again within 30 days of the fan starting - ideally while it is still cold, so you know the system handles the worst case.

Selling or buying in winter

Real-estate radon tests are short-term by necessity, and winter closing schedules are no exception. A winter result is generally the conservative one for a buyer. Sellers who already have a mitigation system should have the post-installation result and the installer's paperwork ready. See radon and real estate for how testing, disclosure and repair credits usually work.

A simple January checklist

1. Check your area's predicted level on your state, county or ZIP page, so you know how urgent this is.
2. Get a test kit - look for a state or county Action Month offer first.
3. Close up the house for 12 hours, place the kit correctly, and run it for at least 48 hours.
4. Mail it the same day you finish; note the times and serial number.
5. If the result is 4 or higher (or 2 to 4 and you want it lower), confirm, then get mitigation bids.
6. Put a reminder in your calendar to retest next winter, or in two years at most.

Sources

• US EPA - National Radon Action Month information
• US EPA - Health risk of radon
• Minnesota Department of Health - Radon testing and levels (MN Public Health Data Access)
• Colorado Department of Public Health and Environment - Testing your home for radon
• Health Canada - Testing your home for radon
• US EPA - Radon (action levels, testing protocol, average indoor and outdoor levels)

Area levels on this site are predicted (model estimate) long-term averages from the 2001-2021 National Radon Exposure Model, not readings from any home. Only a test tells you your home's level. See the methodology.