The EPA's A Citizen's Guide to Radon (EPA-402-K-12-002) tells homeowners to start with a short-term test because it gives an answer in days, then confirm any result at or above 4 pCi/L with a second short-term or a long-term test before paying for mitigation. Short-term measures what's in your air this week under closed-house conditions; long-term captures the seasonal swings that make a single week unrepresentative of a whole year.
The EPA's two-test protocol in one paragraph
Every recommendation in this guide flows from one source: the EPA's A Citizen's Guide to Radon, document number EPA-402-K-12-002. It defines a short-term test as any measurement of 2 to 90 days, a long-term test as 90+ days, and sets the action level at 4 picocuries per litre of air (pCi/L). The protocol is: run an initial short-term test under closed-house conditions; if the result is below 4 pCi/L, the EPA suggests retesting every two years (and after any major renovation); if the result is 4 pCi/L or higher, run a follow-up, either a second short-term test (faster, less accurate) or a long-term test (slower, more representative), and then average the two results before deciding to mitigate. The same document recommends that anyone with a result of 10 pCi/L or higher do the follow-up test as a short-term test, because waiting 90+ days at that exposure level is itself a risk.
The unit, pCi/L, is a measure of radon activity per litre of air. The EPA's action threshold of 4 pCi/L is not a "safe / unsafe" line; it is the point at which the agency judges mitigation to be cost-effective. See EPA radon zones explained for how the zones layer onto this individual-home protocol.
What a short-term test actually is
"Short-term" covers any test of 2 to 90 days, but in practice most consumer short-term tests run 2 to 7 days. Four device types qualify, and they fall into two families:
Passive devices (mail to a lab)
- Charcoal canister / charcoal liquid scintillation: the most common kit on the consumer market. Activated charcoal adsorbs radon from the air over a 2–7 day window; you seal the canister and ship it to a lab that counts the gamma emissions from the radon decay products. Cheap ($15–30 typical, postage included), reasonably accurate, but sensitive to humidity and to delays in shipping because charcoal also releases adsorbed radon over time.
- Electret ion chamber (short-term configuration): a small chamber with an electrically charged Teflon disc. Radon decay ions discharge the electret; the voltage drop is read by a calibrated reader. Mostly used by professional measurement providers.
Active devices
- Continuous radon monitor (CRM): an electronic device that samples air on a fixed interval (commonly hourly) and stores a time-series. The advantage is huge: you can see how radon swings overnight, you can detect test interference (windows opened, device moved), and you get a reading on the spot when you stop the test. Pro-grade CRMs cost $400–1,500+ and require annual calibration to ANSI/AARST MS-PC-2015 standards; consumer CRMs (Airthings, Ecosense, Radonova) typically run $200–350.
- Alpha-track (short-term configuration): alpha-track is more commonly a long-term device, but short-term alpha-track configurations exist for institutional use. Rare in consumer testing.
Closed-house conditions for short-term tests
Per EPA-402-K-12-002 and the EPA's Protocol for Radon and Radon Decay Product Measurements in Homes (EPA-402-R-92-003): for any short-term test of less than 4 days, the home must be in closed-house conditions for at least 12 hours before the test starts and for the entire duration of the test. "Closed-house" means windows shut, exterior doors closed except for normal entry/exit, HVAC operated normally, and whole-house fans / window fans switched off. Without it, a short-term result is not comparable to the 4 pCi/L action level. You've measured a ventilated house, not the worst-case stack-effect condition the EPA is trying to capture.
What a long-term test actually is
A long-term test is any measurement of 90 days or longer. The device of choice is the alpha-track detector: a small plastic chamber containing a strip of polycarbonate (CR-39 or similar). Alpha particles from radon decay products strike the plastic and leave microscopic tracks; the lab etches the plastic in a caustic solution and counts the tracks under a microscope. The integrated count over 90–365 days gives you an average pCi/L for the period the detector was deployed.
Long-term electret ion chambers also exist, with thicker electrets sized for 3–12 month deployments. They are used by some certified measurement providers but are uncommon in the DIY market.
Long-term tests do not require closed-house conditions for their entire duration; that would be unrealistic over 3–12 months. The detector simply integrates whatever conditions the household actually lives in, which is precisely why it's the more representative measurement.
Why long-term is more representative
Indoor radon is driven mostly by the stack effect: warm air rises and escapes the upper floors, the resulting negative pressure at the basement and slab pulls soil gas (containing radon) into the house. The stack effect is strongest when the indoor–outdoor temperature difference is largest; in most of the United States, that means winter. Houses that test at 2 pCi/L in July can easily test at 6–8 pCi/L in January.
The EPA's Home Buyer's and Seller's Guide to Radon (EPA-402-K-13-002) and the ANSI/AARST MAH-2019 protocol both note that seasonal variation of 2–3× is normal, with some houses swinging more. A single 48-hour test caught on the wrong week can dramatically under- or over-state the annual average that actually drives your long-term lung-cancer risk. That's the entire reason the EPA recommends a long-term follow-up wherever the homeowner has the time to do one.
Short-term vs long-term: head-to-head comparison
| Attribute | Short-term test | Long-term test |
|---|---|---|
| Duration | 2–90 days (most commonly 2–7 days) | 90+ days, often 90–365 days |
| Typical devices | Charcoal canister, electret ion chamber, continuous radon monitor (CRM) | Alpha-track, long-term electret ion chamber |
| Cost (DIY kit) | $15–30 | $25–45 |
| Cost (certified professional) | $150–300 per measurement | $150–300+ per measurement |
| Closed-house conditions required? | Yes: 12 hours before + full test duration | No: integrates normal occupancy |
| Captures seasonal variation? | No: a snapshot of one week | Yes: integrates winter and summer if deployed long enough |
| Result turnaround | Days (mail-in lab) or instant (CRM) | Weeks after retrieval |
| Best for | Real-estate transactions, initial screen, confirmation when initial result ≥ 10 pCi/L | Confirmation of a 4–10 pCi/L initial result; year-round risk assessment in occupied homes |
The decision tree
The EPA's protocol collapses into four cases, all measured against the 4 pCi/L action level.
- Initial short-term result < 4 pCi/L: no immediate action. EPA recommends retesting every two years and after any major renovation that changes the building envelope or HVAC. See radon retesting after renovation for what triggers a fresh test.
- Initial short-term result 4–9.9 pCi/L: follow up with a long-term test (preferred, more representative) OR a second short-term test. Average the two results. If the average is ≥ 4 pCi/L, mitigate.
- Initial short-term result ≥ 10 pCi/L: follow up with a second short-term test (not long-term, since waiting 90 days at that exposure is itself a risk). Average the two short-term results. If the average is ≥ 4 pCi/L, mitigate.
- Real-estate transaction: a different protocol applies (see next section).
Real-estate transactions: the separate EPA protocol
The EPA publishes a separate document for property transfers: Home Buyer's and Seller's Guide to Radon (EPA-402-K-13-002), which references the AARST Protocol for Conducting Measurements of Radon and Radon Decay Products in Homes for Real Estate Transactions. The transaction window (usually 10–30 days) rules out a long-term test, so the protocol allows short-term measurements but adds extra rigour to compensate:
- Two concurrent short-term tests, side by side (within 10–20 cm), 2–4 days each, results averaged. If the two devices disagree by more than ~50% and the average is near 4 pCi/L, the result is suspect and a retest is recommended.
- Or two sequential short-term tests, run back-to-back, results averaged.
- Or one continuous radon monitor deployed by a NRPP- or NRSB-certified measurement professional for 48–96 hours. The CRM's hourly time-series lets the professional check for tampering (windows opened, device moved), which is one reason CRMs are the preferred device for transactions.
Closed-house conditions still apply for the full duration plus 12 hours prior. The test should be conducted in the lowest livable level of the home that could be regularly occupied (basement bedroom yes, crawlspace no). For more on transaction logistics, see selling a home with high radon.
Tester interference is a real problem in transactions
Because a 4 pCi/L result can drive a $1,000–3,000 mitigation request, sellers have an incentive to ventilate the house during the test. Continuous radon monitors deployed by a certified professional are the highest-trust option because the hourly data exposes interference. ANSI/AARST MAH-2019 and the AARST real-estate protocol both treat tamper-evident deployment as standard practice for transactions.
DIY vs certified professional measurement
DIY consumer kits are perfectly adequate for the EPA's initial screen and follow-up at home; the labs that process them are accredited under the National Radon Proficiency Program (NRPP) or the National Radon Safety Board (NRSB). What DIY kits don't give you is a paper trail signed by a certified measurement professional, which is what most real-estate transactions, schools, multi-family buildings, and post-mitigation verification tests require.
A certified pro measurement, performed under ANSI/AARST MAH-2019, runs $150–300 for a single short-term test and includes the measurement, the closed-house verification, the device retrieval, and a signed report. For most homeowners doing a routine check on their own house, a $15–30 DIY kit followed (if positive) by a $25–45 long-term kit is the right path. For anything contractual (a sale, a purchase, or post-mitigation verification), pay for the certified professional. The fuller breakdown is in our DIY radon testing guide.
How to verify a "certified" pro
Look the company up in the public AARST-NRPP measurement provider directory or the NRSB directory. Both list current certification, device types the provider is authorised for, and the state. State-level certification also matters in the ~20 US states (including Illinois, New Jersey, Pennsylvania, Florida, Maine, and others) that license radon professionals separately.
Where to place the device
The EPA places the test in the lowest livable level: the lowest floor of the home that is either currently occupied or could reasonably be lived in. A finished basement with a guest room or family room counts; an unfinished crawlspace does not. The device should sit 20–60 inches (50–150 cm) above the floor, at least 4 inches (10 cm) from any wall, away from drafts, exterior doors, heat sources, direct sunlight, and high humidity (so not a bathroom or laundry room). The same placement rule applies to short-term and long-term tests.
For dwellings without a ground-floor connection (upper-floor apartments and condos), placement gets nuanced; see radon in apartments and condos.
If the follow-up confirms a problem
A confirmed reading at or above 4 pCi/L means installing a radon-reduction system, almost always active soil depressurisation (commonly called sub-slab depressurisation in slab homes). Typical national installed cost runs $1,000–3,000 according to the EPA's consumer guides; the resulting system runs continuously and reduces indoor radon by 50–99%. The mechanics, code requirements, and post-mitigation verification testing are covered in sub-slab depressurisation explained.
Post-mitigation verification is itself a short-term test, run 24 hours to 30 days after the fan is energised, per ANSI/AARST RMS-LB-2018 (mitigation standard) and the AARST measurement standards. Most mitigators include that follow-up test in the install price.
Cost summary
- Short-term DIY kit: $15–30, including return postage and lab analysis
- Long-term DIY kit (alpha-track): $25–45, including postage and analysis
- Consumer continuous radon monitor: $200–350 one-time hardware cost; no per-test fee thereafter
- Pro-grade CRM rental + deployment: $150–300 per test in a transaction context
- Certified professional measurement under ANSI/AARST MAH-2019: $150–300 for a single short-term measurement, signed report
- Mitigation (if the follow-up confirms ≥ 4 pCi/L): $1,000–3,000 typical national range, per EPA-402-K-12-002
The default we’d run
For an owner-occupied home with no transaction pending: order a $15–30 short-term charcoal kit, run it for 5–7 days under closed-house conditions, and send it to the lab. If the result is below 4 pCi/L, set a calendar reminder to retest in two years. If the result is 4–9.9 pCi/L, order a long-term alpha-track kit and deploy it for 90–365 days through at least one full winter; average it with the short-term and decide. If the result is 10 pCi/L or higher, order a second short-term kit immediately and act on the average. For any real-estate transaction, hire a certified measurement professional and let them run a continuous radon monitor for 48–96 hours; do not rely on a DIY kit because the result will be contested.
That sequence costs $15–75 in materials over a year and answers the question with the precision the EPA’s own protocol asks for.
Sources
- US Environmental Protection Agency. A Citizen's Guide to Radon: The Guide to Protecting Yourself and Your Family from Radon. EPA-402-K-12-002, May 2012 (most recent print revision). Source for the 2–90 day / 90+ day definitions, the 4 pCi/L action level, the 10 pCi/L expedited-retest rule, closed-house conditions, retest cadence, and the $1,000–3,000 mitigation cost range.
- US Environmental Protection Agency. Home Buyer's and Seller's Guide to Radon. EPA-402-K-13-002. Source for the real-estate transaction protocol summary and the concurrent / sequential / CRM testing options.
- US Environmental Protection Agency. Protocol for Radon and Radon Decay Product Measurements in Homes. EPA-402-R-92-003. Source for closed-house condition rules and device-placement requirements.
- American Association of Radon Scientists and Technologists (AARST). ANSI/AARST MAH-2019: Protocol for Conducting Measurements of Radon and Radon Decay Products in Homes. Source for current consensus measurement protocol, device placement, and tamper-evident deployment standards.
- AARST-NRPP (National Radon Proficiency Program) and NRSB (National Radon Safety Board) public certification directories, for verifying that a measurement provider holds current accreditation.
- American Lung Association. Radon. Consumer-facing summary of radon as a lung-cancer risk and the case for testing every home.
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