How to Use a Radon Test Kit Correctly

By Steve Crews · Last updated June 23, 2026

What kind of kit do you have?

The most common option is a short-term charcoal canister kit. A small canister of activated carbon sits open in your home for a defined period, absorbing radon from the air. You seal it, mail it to a lab, and a technician measures how much radon decayed in the carbon during the exposure window. Turnaround is usually five to seven business days.

Electret ion chambers and alpha track detectors are used for longer exposures, often 90 days to a full year. Those produce a time-averaged reading that is more representative of your actual long-term exposure than a 48-hour snapshot. If a short-term test comes back elevated, a long-term follow-up is the standard way to confirm the number before committing to mitigation. Our guide on short-term vs long-term radon tests explains when each type makes sense.

Before buying anything, check your state radon program. Many states offer free or reduced-cost kits through the health department or a university extension service. The result is just as valid as a commercial kit because the same accredited labs process them. Your state health department is the right first call, and the EPA maintains a state radon contact list on its website.

For a paid kit, look for one analyzed by a lab with AARST-NRPP (National Radon Proficiency Program) laboratory recognition. That means the lab follows a national measurement standard rather than its own internal process. Check price on Amazon

Closed-house conditions: the rule people skip

This is where most botched tests go wrong. Radon concentrations inside a home stabilize when the building is kept closed. Open the windows regularly and outdoor air dilutes the radon, producing a reading that is artificially low.

EPA guidance requires closed-house conditions for at least 12 hours before the test begins, and that closed state must hold throughout the entire test period. In practice:

• Close all windows and exterior doors on every floor, not just the level being tested.
• Turn off heat recovery ventilators (HRVs), energy recovery ventilators (ERVs), and whole-house fans that pull air from outside.
• Normal heating and central air conditioning can run as long as the system recirculates indoor air rather than drawing from outside.
• Avoid using a fireplace or wood stove, since combustion pulls replacement air in from outdoors.

Normal household activity is fine. Cooking, showering, running the range hood briefly, opening and closing interior doors: none of that disqualifies your test. The goal is keeping outdoor air out, not creating a sealed chamber.

Severe weather is worth avoiding: high winds and pressure swings affect how radon moves through soil and into the home, which can distort a short-term reading. A calm stretch of days is better.

Where to place the kit

Room choice. Test in the lowest level of the home you regularly occupy. A finished basement with a bedroom or home office is the right room. An unfinished basement that no one spends time in is not; test on the first floor instead. The reading should reflect actual exposure, not the single worst spot in the house.

Height. Place the canister at least 20 inches above the floor. Radon is heavier than air and tends to concentrate lower, so a kit placed directly on the floor can return an unrepresentative reading. A countertop, bookshelf, or desk is fine.

Distance from air movers. Stay at least 3 feet from exterior walls, windows, and doors. Keep the kit away from HVAC supply and return vents. Air moving past the canister biases how much radon the activated carbon absorbs.

Away from the sump pit. The sump is a major radon entry point. Do not set the kit directly over or immediately beside it. A few feet of distance is sufficient.

Somewhere undisturbed. Pick a spot where the canister will not be knocked over, picked up, or covered. A kitchen counter near active cooking is a poor choice; a corner of an unused table or a shelf in an adjoining room works well.

Running the test

The clock starts when you open the canister. Write the start date and time on the data card immediately. Short-term kits run for 48 to 96 hours; the range matters in both directions. Cut the exposure short and the lab has less signal. Run more than a day over and the decay math is off. If you exceed the window significantly, note it on the data card. Leave the canister in place throughout.

Sealing and mailing the kit

When the test period ends, close the canister exactly as the instructions say. Most kits have a replacement cap or seal. Place the sealed canister in the return pouch or box, then fill out every field on the data card: start date and time, end date and time, the floor level tested, whether closed-house conditions were maintained, and your return address. Incomplete cards can delay results or prompt the lab to qualify the report.

Mail it the same day you seal it. Radon’s short-lived decay products continue changing in a sealed canister, so delays between closing and mailing introduce measurement error. The prepaid mailer that comes with most kits is fine; just don’t let it sit on the counter for days.

What your result means

The U.S. EPA action level is 4 pCi/L (about 148 Bq/m3). At or above that, the EPA recommends mitigation. The agency also suggests considering action between 2 and 4 pCi/L because risk exists at any concentration and falls as the level drops. The U.S. Surgeon General has issued a national health advisory urging Americans to test their homes and reduce elevated levels.

For context: Health Canada uses 200 Bq/m3 (about 5.4 pCi/L) and the World Health Organization sets a reference level of 100 Bq/m3 (about 2.7 pCi/L). The different thresholds reflect each body’s weighing of health protection against practical achievability, not disagreement about the underlying risk.

A single short-term test is a useful screening tool, not a final verdict. If the result is above 4 pCi/L, the standard next step is a second short-term test or a long-term test to confirm the annual average. Our page on how to read radon test results explains what the numbers on your lab report actually mean and what to do next.

If your reading confirms that mitigation is warranted, hire a contractor credentialed through AARST-NRPP. In Canada, the equivalent credential is C-NRPP. Most state radon programs maintain a searchable list of certified mitigators by zip code. Typical costs run between $800 and $2,500 depending on foundation type and the work involved. Our guide to radon mitigation cost breaks down what drives the price up or down.

FAQ

Does the brand of test kit matter? Much less than placement and closed-house conditions. Activated charcoal works the same regardless of who made the canister. What matters more is that the analyzing lab holds AARST-NRPP laboratory recognition. A well-placed kit from an accredited lab gives a trustworthy result regardless of the brand name.

What if I forgot to keep the house closed during the test? Discard the test and start fresh. A test run with windows open will read low, giving false reassurance. Order a new kit and plan the test for a stretch of days when you can keep the house closed comfortably. Running the test correctly during a slightly warm week is more useful than a failed test during perfect weather.

Can I test in more than one room at once? Yes. Placing kits in the finished basement and on the first floor simultaneously gives you a fuller picture and a sanity check if one reading seems off. Use the result from the lowest regularly occupied level when deciding whether to mitigate.

What if I mail the kit a day or two late? A short delay introduces measurement error because radon decay products continue evolving in the sealed canister. Note the actual seal and mail dates on the card so the lab can flag it. A delay of more than two days means treating the result as approximate and considering a follow-up test.