What Is a Radon Test? A Plain-Language Explainer
If you have just learned that radon is a thing and your real-estate agent or home inspector is talking about testing for it, this is the explainer that connects the unfamiliar terminology to what actually happens. Radon testing is a well-established science, the consumer technology is mature, and the homeowner decision once results come in is straightforward. This guide walks through what a radon test measures, the technologies used, the protocol that produces accurate results, and how to read what comes back from the lab.
InspectandTest is an independent editorial team. We are not certified radon measurement professionals. This guide synthesizes EPA, AARST, and NRPP guidance.
What a radon test measures
Radon-222 is a radioactive gas produced by the decay of radium-226 in soil and rock. It seeps into homes through foundation cracks, slab penetrations, and other entry points, and accumulates indoors to concentrations that can be hundreds of times higher than outdoor air. Long-term exposure to elevated indoor radon is the second leading cause of lung cancer in the United States behind smoking.
A radon test measures the average concentration of radon gas in indoor air over a defined exposure period. The result is reported in picocuries per liter (pCi/L) in the U.S., or becquerels per cubic meter (Bq/m3) in Europe. The EPA’s action level is 4.0 pCi/L (148 Bq/m3); levels above this threshold warrant mitigation. Levels between 2.0 and 4.0 pCi/L are considered worth addressing per the EPA’s secondary recommendation; the World Health Organization recommends 2.7 pCi/L as a tighter threshold.
The test does not measure individual molecules of radon directly. Instead, it measures one of three downstream signatures of radon’s radioactive decay, which the lab or monitor converts back to air concentration through calibrated correction factors.
Three test technologies
Charcoal canisters (short-term, 2-7 days)
The most common consumer test. A small canister of activated carbon is placed in the home for a 2-to-7-day exposure period. Activated carbon absorbs radon gas during the exposure; after sealing the canister, the lab measures gamma radiation from the trapped radon and its decay products on a sodium iodide detector. The measured radioactivity is converted to average air concentration through calibrated correction factors that account for exposure time, ambient conditions, and shipping delay.
Pros: inexpensive ($15 to $50 per kit including lab fee), fast initial deployment, accurate when used per instructions.
Cons: single integrated number for the exposure period rather than continuous data, sensitive to humidity outside calibrated range, results take 1-2 weeks after mailing.
Alpha-track detectors (long-term, 90+ days)
A thin film of polymer material is placed in the home for 90 days or longer. Alpha particles from radon decay strike the film and create permanent micro-damage trails. After exposure, the film is etched in the lab and the tracks are counted under microscopy. Track count converts to average air concentration.
Pros: best technology for long-term integrated averages, captures seasonal variation, more accurate over the long-term than short-term tests.
Cons: slow (90+ days minimum), single number rather than time-series data.
Continuous radon monitors (real-time)
Reusable digital devices (AirThings View Plus and View Radon, Corentium Home, RadonEye RD200) use real-time alpha pulse counting electronics to detect each individual alpha particle from radon decay and report rolling averages on a phone app and on-device display. Our radon detector buyer’s guide walks through the specific models.
Pros: live data starting hours after placement, meaningful averages within 24 to 48 hours, ongoing visibility into how levels change with seasons and conditions.
Cons: higher upfront cost ($150-300), single-time accuracy slightly below long-term integrated tests but multi-day averages are equivalent to short-term canister readings.
When each test is appropriate
Pre-purchase real estate transactions: short-term charcoal canister or short-term continuous monitor measurement, per the protocols specified in your purchase contract. Most contracts call for 48-to-96-hour testing with closed-house conditions.
First-time homeowner screening: short-term charcoal canister to establish baseline, OR continuous monitor for ongoing visibility. The continuous monitor pays back versus repeated short-term testing within two screenings.
Confirmation of elevated short-term result: long-term test (alpha-track) over 90 days, or continuous monitor watching for at least 30 days. Confirms the average condition before committing to mitigation.
Post-mitigation verification: short-term test 24-48 hours after mitigation activation, then re-test annually thereafter to confirm the system is still working.
Annual or biennial re-testing: short-term test every 1-2 years, or continuous monitor running continuously.
Test protocol that gives reliable results
- Place the test in the lowest occupied level of the home. For real estate transactions, this is typically defined in the contract.
- 20+ inches off the floor.
- Away from drafts (open windows, exterior doors, supply registers within 12 inches).
- Away from heat sources (radiators, fireplaces, direct sunlight).
- Not on or against an exterior wall.
- For short-term tests, maintain closed-house conditions: keep windows and exterior doors closed for 12 hours before and during the test. Doors used for normal entry/exit are acceptable.
- For continuous monitors, leave in place full-time once positioned.
- Note start and end times on the test paperwork (for short-term) or in the app (for continuous monitors).
- Mail short-term tests immediately after exposure ends; do not delay.
Reading your results
A typical lab report or app reading shows the average concentration in pCi/L, the exposure period (start and end dates), and sometimes confidence intervals.
Below 2.0 pCi/L: below the EPA action level. The home is in the low-risk zone. No mitigation required, but no level is risk-free; if you have lung-cancer risk factors (smoker household, COPD, immunocompromise) and are committed to minimizing exposure, mitigation can be considered even at low levels.
2.0 to 4.0 pCi/L: EPA recommends “consider mitigation.” Reasonable threshold to act on, especially for households with children, elderly residents, smokers, or other high-risk factors. The WHO uses 2.7 pCi/L as the conservative threshold; many real-estate contracts use this range as the negotiation trigger.
4.0 to 10 pCi/L: EPA action level exceeded. Mitigation strongly recommended. Confirm the result with a second test (long-term or continuous monitor) before contracting for mitigation; short-term results in this range can be biased high by atypical conditions.
10 to 20 pCi/L: substantially elevated. Mitigation should be a priority. Single-test confirmation is sufficient; do not delay.
20+ pCi/L: very high. Move quickly to mitigation. Consider basement sleeping arrangements as a temporary measure pending mitigation.
What to do next
If your result is below 2.0 pCi/L: re-test every 2 years or use a continuous monitor for ongoing visibility. Levels can shift with foundation changes, HVAC changes, or seasonal extremes; what was acceptable last year may not be next year.
If your result is between 2.0 and 4.0 pCi/L: confirm with a second test in a different season. If the average across multiple tests stays in this range, mitigation is reasonable; if it bounces between high and low values, the average may be acceptable.
If your result is above 4.0 pCi/L: schedule mitigation with an NRPP- or NRSB-certified mitigator. Our mitigation cost guide walks through pricing and vetting. Most credible mitigators offer a written warranty that post-mitigation levels will be below 4.0 pCi/L.