The most important rule during a radon test is maintaining closed-house conditions, meaning you keep all windows and exterior doors shut except for normal entry and exit. Breaking this rule is the single most common reason radon tests produce inaccurate results. A short-term test typically runs 2 to 7 days, and what you do (or don’t do) during that window directly determines whether your reading reflects the actual radon levels in your home.
Don’t Skip the 12-Hour Pre-Test Setup
Closed-house conditions need to begin at least 12 hours before the test device is activated or placed. That means closing all windows and exterior doors a full half-day before testing starts. Many people make the mistake of only closing up the house when the test kit goes down, but radon needs time to stabilize at its natural indoor level after outside air has been mixing in. If you aired out the house the morning of the test, your first several hours of data will reflect artificially low concentrations.
Don’t Open Windows or Exterior Doors
This is the rule people break most often, sometimes without realizing it matters. Cracking a window for fresh air, propping the back door open while grilling, or leaving the garage door up for an extended period all introduce outdoor air that dilutes radon concentrations. The result is a reading that looks lower than what you actually breathe day to day.
You can still come and go through your doors as you normally would. The Colorado Department of Public Health and Environment puts it simply: enter and exit as usual, but don’t leave the door open for a long time. A few seconds of opening a door to walk through it won’t compromise your test. Leaving it open for 20 minutes while you bring in groceries or let the dog run in and out could.
Don’t Run Exhaust Fans
Kitchen range hoods, bathroom exhaust fans, and whole-house fans all pull indoor air out of your home and draw replacement air in from outside. This creates the same dilution problem as opening a window. Connecticut’s radon testing guidelines specifically warn against operating exhaust fans or any ventilation system that brings outside air into the home during testing.
A quick bathroom fan during a shower is a gray area, but the safest approach is to leave exhaust fans off entirely for the duration of the test. If you have an attic fan or whole-house fan, keep it off. Ceiling fans that just circulate air within a room are fine since they don’t exchange indoor air with outdoor air.
Don’t Move or Disturb the Test Device
Once the radon detector is placed, leave it alone. Don’t pick it up, nudge it aside to make room on a table, cover it with anything, or relocate it to a different spot. Professional continuous radon monitors record interval data on humidity, barometric pressure, and concentration changes throughout the test. If someone moves the device or opens windows partway through, the data often shows a suspicious drop in radon levels that indicates tampering with closed-house conditions. This can invalidate your results entirely.
Charcoal-based test kits (the type you mail to a lab) are even more vulnerable. They’re sensitive to excessive humidity and airflow, and unlike electronic monitors, they provide no record of what happened during the test. That means there’s no way to distinguish a genuinely low reading from one caused by interference, so the lab simply reports whatever the charcoal absorbed, accurate or not.
Don’t Place the Kit in the Wrong Location
Where the test device sits matters as much as what you do around it. The CDC recommends placing the device at least three feet off the ground, in the middle of the room, on the lowest level of your home. “Lowest level” means the lowest livable area, which includes finished or unfinished basements but not crawl spaces. If you don’t have a basement, test on the first floor.
Avoid placing the kit in a kitchen, bathroom, or hallway. These spaces have unusual airflow patterns from exhaust fans, plumbing penetrations, and foot traffic that can skew readings. Don’t put it near exterior walls, windows, or doors either, since drafts in those spots don’t represent the air quality in the room overall. Keep it away from direct sunlight and heat sources like radiators or fireplaces, which can affect charcoal kits chemically and create localized air currents that distort the reading.
Don’t Change Your HVAC Habits
Run your heating and air conditioning the way you normally do. If you typically keep the thermostat at 72°F, keep it there. If you run the AC in summer, keep running it. The goal is to measure radon under your actual living conditions, not under artificial ones. The one exception: don’t switch to a setting or mode that brings in outside air, such as a fresh-air intake or economizer mode on some HVAC systems. Standard heating and cooling that recirculates indoor air is perfectly fine.
If you already have a radon mitigation system installed and you’re retesting to check its effectiveness, keep it running. Turning it off would measure the unmitigated level, not the conditions you’re actually living in.
Don’t Test During Severe Weather
Heavy storms, high winds, and unusual temperature swings can all temporarily change how radon enters your home. Storm systems bring low-pressure air that can draw more radon up through your foundation, while high winds alter pressure differences around the building’s exterior. These conditions can spike or suppress radon levels in ways that don’t reflect your typical exposure.
If a major storm rolls through during your test, it doesn’t automatically ruin the results, especially if the unusual weather only lasted a few hours of a multi-day test. But if you have the option to choose your testing window, pick a stretch of relatively normal weather. Avoid testing during periods of heavy rain, snow, sustained high winds, or temperatures that are far outside the seasonal norm for your area.
Don’t Run the Test Too Short
Short-term radon tests need a minimum of 48 hours to produce a meaningful average. Some charcoal kits allow testing periods up to 7 days. Pulling the kit early, even by a few hours, reduces the accuracy of your result because radon levels naturally fluctuate throughout the day and night. A full 48-hour minimum captures enough of those fluctuations to give you a useful number. If your test instructions specify a longer minimum, follow them.
Long-term tests (90 days to a year) give a more accurate picture of annual exposure, but short-term tests are the standard screening tool. Whichever type you use, the closed-house rules apply for the entire duration. That means if you’re running a 4-day test, you need 12 hours of pre-test closed conditions plus 4 full days of keeping everything shut. Planning the test around a period when you can commit to those conditions is the simplest way to get a result you can trust.