How to Test for Radon at Home
The charcoal kit sat on top of the dryer for three days, eight feet from a basement window somebody had cracked open because the laundry room gets stuffy. The lab read it, the slip came back at 0.9 pCi/L, and the house got filed under "fine." Nothing about the kit was defective — it just sampled the wrong air: a warm, drafty corner by an open window, during the one week of the year the house breathed the most. The kit did its job. The setup didn't.
Why a bad radon number is usually a bad setup
Radon comes up out of the soil, and it gets in because the house sits slightly under-pressurized relative to the ground beneath it. Warm air leaks out the top, the building pulls replacement air in through the bottom, and soil gas rides along through slab cracks, sump openings and utility penetrations. That pull peaks in winter, when the indoor–outdoor temperature difference is largest, and collapses on a mild day with the windows open.
So a result describes conditions, not just the house. The EPA puts the average indoor level in US homes near 1.3 pCi/L and outdoor air near 0.4 pCi/L — a detector parked in a draft is partly measuring outside and reporting a diluted number. Day-to-day swings of two to three times are ordinary as pressure drops or wind loads one wall. None of that is instrument error, and it's why short-term protocols specify a 48-hour minimum, a closed house and calm weather.
The four ways to measure, and what each returns
| Method | Runs for | What comes back | Typical cost | Use it when |
|---|---|---|---|---|
| Short-term charcoal kit | 2–7 days | One lab-averaged pCi/L | $15–30, lab fee usually included | First-ever check, or a fast answer |
| Long-term alpha-track kit | 90+ days, up to a year | One number close to your annual average | $25–40 | You want the number that describes the house |
| Consumer continuous monitor | Nonstop; first reading in 24–48 hours | Live pCi/L plus 7-day and yearly trends | $150–300 once | Ongoing tracking, seasonal swing, post-work checks |
| Certified professional test | 48 hours and up | Documented result from a licensed measurement pro | $150–300 | Real-estate deals, or a result that must hold up |
State radon programs often beat retail — several sell kits at $5–15 or hand them out free. Whatever you buy, the lab reading it should be listed by a national proficiency program (NRPP or NRSB), and monitors should say which one they're listed under. The kit-versus-monitor call is broken down in radon test kit vs continuous monitor.
Where it goes, when to run it, and what to do with the number
Placement is the part people improvise and the part written down most precisely. Following EPA measurement protocol and the ANSI/AARST standard it points to, put the detector on the lowest level you occupy regularly — a finished basement den counts, a crawl space doesn't — in a room you'd use, not a closet. Then five clearances: 20 inches to 6 feet above the floor, at least 4 inches from other objects, 1 foot from exterior walls, 3 feet from windows, doors and other openings, and 3 feet from vents, fans, fireplaces and heat sources. Skip kitchens, bathrooms and laundry rooms — humidity and air movement both skew charcoal.
Closed-house conditions mean windows and exterior doors stay shut for 12 hours before the test and throughout any run of 4 days or less. Normal in-and-out is fine; whole-house fans, window fans and open-window ventilation are not, and window AC should run on recirculate. Heating-season tests tend to be the honest ones, and short runs are worth postponing through storms or high winds.
Then the thresholds. The EPA action level is 4.0 pCi/L, with action also worth considering between 2 and 4 pCi/L; in metric, 1 pCi/L = 37 Bq/m³. One result at or above 4.0 is a trigger for a follow-up, not a verdict: a second short-term test averaged with the first if you need speed, or a 90-day-plus test for the better number. Under 2 pCi/L, most owners retest every couple of years. If the follow-up confirms an elevated level, the call goes to a state-certified radon professional — measuring and fixing are separate trades, and the fixing one isn't a guide topic.
Common mistakes
- Testing with the house open. Closed-house conditions run 12 hours before and through any test of 4 days or less; an open window drags the reading toward the 0.4 pCi/L of outdoor air.
- Putting the detector on the floor or a windowsill. The band is 20 inches to 6 feet up, 1 foot off exterior walls, 3 feet from windows and doors — sills and floors miss it in both directions.
- Treating one short-term number as the house's number. A 2–7 day average describes one week's weather; the 90-day alpha-track test is what lands near the annual average.
- Acting on a single elevated result. At 4.0 pCi/L or above the EPA's own instruction is a follow-up test — averaged second short-term run, or a long-term one — before anyone touches the house.
FAQ
How long does a home radon test need to run?
A short-term kit needs 48 hours minimum and commonly runs 2–7 days, with closed-house conditions 12 hours beforehand and throughout. A long-term alpha-track test runs 90+ days and lands much closer to your annual average. A continuous detector gives a usable first reading in 24–48 hours and keeps averaging after that.
Where exactly should I put the test kit?
Lowest lived-in level, in a room you actually use, 20 inches to 6 feet above the floor. Keep it 1 foot from exterior walls, 3 feet from windows, doors and vents, 4 inches from other objects — and out of kitchens, bathrooms and laundry rooms, where humidity and airflow distort the sample.
Do I need to test again if the first result was low?
A result under 2 pCi/L is reassuring, but it's still one week of weather; retesting every two years or so is common practice, sooner after work that changes sealing or ventilation. If you'd rather watch continuously than repeat kits, that's the case for a monitor — see Airthings vs Ecosense.
What do I do if the test comes back at 4.0 pCi/L or higher?
Run a follow-up before drawing conclusions: a second short-term test averaged with the first if you need speed, or a 90-day-plus test if you can wait. If it confirms the level, contact a state-certified radon professional. Kit and monitor options are compared in $15 vs $150.
General information about measuring radon and indoor air quality, not health or mitigation advice. Radon is measured in pCi/L; the US EPA action level is 4.0 pCi/L. If your readings are elevated, contact a state-certified radon professional rather than attempting mitigation from a guide, and take any health concern to a doctor. Prices and specifications vary by model, state and region.