How to Test for Radon at Home: Kits, Steps & Results
Testing a home for radon is a weekend project that costs less than a tank of gas, and on the Colorado Front Range it is worth doing before anything else. Roughly half of the homes tested in Colorado come back at or above the EPA action level, because the granite and uranium-bearing soils under Denver, Boulder, and the foothills are prolific radon producers. Here is how to run the test correctly, read the number, and decide what to do with it.
Pick the right kit for your question
Radon is measured in picocuries per liter of air (pCi/L), and the device you choose depends on how fast you need an answer.
- Short-term charcoal kit (2–7 days): A sealed canister of activated charcoal that absorbs radon, then goes to a lab. Cheapest option at roughly $15–$30 including analysis, and the standard choice when you are under a real estate deadline. It gives a snapshot of the test window, not a yearly average.
- Long-term alpha-track kit (90 days to 1 year): A plastic film that records alpha particles from radon decay. About $30–$60. Because it runs across seasons, it produces a number much closer to your true annual exposure, which is what the lung-cancer risk actually tracks.
- Continuous electronic monitor (ongoing): A plug-in or battery device such as the Airthings Corentium Home or Wave that logs radon hour by hour, usually $150–$300. No lab, no waiting, and you can watch levels move with the weather. Cheaper consumer units drift over time, so treat a single reading as a screen rather than a lab-grade result.
For a home you plan to keep, most people start with an inexpensive short-term kit and, if it is anywhere near the action level, confirm with a long-term test or a monitor. Our breakdown of radon gas test kits compares specific models, and the cost of radon testing guide lays out DIY versus professional pricing.
Where to put the test
Placement decides whether the number means anything. Set the device in the lowest level you actually use, since radon enters from the soil and is heaviest near the ground. A finished basement rec room counts; an unfinished basement you never enter does not, so use the main floor instead.
- Position it 2 to 6 feet above the floor, in your breathing zone.
- Keep it at least 4 inches from exterior walls and 3 feet from windows, doors, and exterior openings.
- Stay away from drafts, HVAC supply registers, fireplaces, direct sun, and humid spots like the bathroom or over the kitchen sink.
- Do not test in a closet, crawl space, kitchen, or laundry room; those spaces read unrepresentatively.
The EPA’s placement rules are specific enough that a few inches matter. Our guide on where to place a radon test walks through room-by-room examples.
Run a short-term test step by step
- Set closed-house conditions 12 hours ahead. Keep all windows and exterior doors shut except for normal in-and-out. Run heating and air conditioning as usual, but do not run whole-house fans, window fans, or a fireplace with the flue open. This step is non-negotiable; skipping it can hide 30 to 50 percent of the real level.
- Open and place the kit, recording the start date and exact time on the card or app.
- Leave it undisturbed for the full window the kit specifies, keeping the house closed the entire time. Do not move the device or open it to check.
- Close and seal the kit at the end, note the stop time, and mail it to the lab the same day. Charcoal keeps absorbing and decaying, so a kit that sits on the counter for a week returns a low, wrong number.
- Read the result. Lab kits report back in about 3 to 10 days; a monitor shows the running average on its screen.
Test in the winter if you can. Cold-weather stack effect pulls more soil gas into the house, so heating-season readings run higher and give you the more conservative, protective number.
What the number means
| Result (pCi/L) | What the EPA says |
|---|---|
| 4.0 or higher | Fix the home. This is the EPA action level. |
| 2.0 to 3.9 | Consider mitigation; the EPA notes there is no safe level and reducing below 2 pCi/L is worthwhile. |
| Below 2.0 | Low risk. Retest every few years or after major renovations. |
There is no threshold below which radon is harmless; 4 pCi/L is where the EPA judges mitigation clearly cost-effective, and the World Health Organization uses a stricter 2.7 pCi/L reference. Radon is the second-leading cause of lung cancer in the United States after smoking, and the two together are far more dangerous than either alone. For a fuller read of the scale, see what counts as a safe radon level.
Confirm before you spend money on mitigation
A single short-term test can be thrown off by a passing storm or an open window you forgot about. If your first result is at or above 4 pCi/L, the EPA recommends confirming it before hiring anyone: run a second short-term test, or better, a long-term test that averages out the noise. Two elevated results, or one long-term result at or above 4, is a solid basis for action.
If the number holds, the fix is straightforward and permanent. A sub-slab depressurization system, a fan and vent pipe that draws soil gas out from under the foundation before it enters, typically runs $1,500 to $3,000 installed and brings most homes well under 2 pCi/L. Retest after installation to confirm it worked. See how radon mitigation works and the broader radon testing in Colorado overview for the full path from result to fix.
Frequently asked questions
Are cheap DIY radon kits accurate?
Yes, when used correctly. Charcoal and alpha-track kits sent to an accredited lab are the same technologies professionals rely on. The common errors are amateur ones: skipping closed-house conditions, poor placement, or mailing the kit too late. Follow the instructions and a $20 kit gives a trustworthy screen.
How often should I retest?
Retest every two to five years even after a low result, and always after finishing a basement, replacing an HVAC system, sealing the foundation, or major additions, since any of these can change how soil gas moves into the house.
Can I smell or see radon to know it’s there?
No. Radon is colorless, odorless, and tasteless, which is exactly why testing is the only way to detect it. There are no symptoms in the short term; the risk is cumulative lung damage over years of exposure.
Do I need a professional, or can I test myself?
For your own peace of mind, a DIY kit is fine. For a real estate transaction, most buyers and lenders want a test performed by a licensed radon measurement professional using a tamper-resistant continuous monitor under documented closed-house conditions, since the result can trigger price negotiations.
Sources: EPA, A Citizen’s Guide to Radon; CDC, About Radon; Colorado Department of Public Health and Environment, Radon; American Lung Association, Radon. This summarizes EPA and CDPHE guidance as of 2026; consult a certified radon professional for transactional or post-mitigation testing.