How Does Radon Testing Work: Protocol and Process Guide
How does radon testing work, from a procedural standpoint, is a question with a defined answer: the EPA protocol prescribes specific steps from device placement through laboratory analysis to result reporting. The chain of decisions matters because radon levels fluctuate over hours, days, and seasons, and the protocol is designed to produce a result that reflects the home’s typical exposure rather than a transient peak or dip. This guide walks through the process from initial decision to final result, with particular attention to how closed-house conditions, placement choices, and timing affect what the test measures. The summary reflects EPA, CDPHE, and American Lung Association guidance current as of 2026.
Part of our Radon Levels guide — start there for the full picture →
The Five-Step Radon Testing Protocol
Radon testing follows five sequential steps: protocol selection, placement, duration, analysis, and reporting. Skipping or compromising any step degrades the result. This guide is process-focused; the kit-mechanism deep-dive covers the underlying detection physics.
Step 1: Protocol Selection
The first decision is short-term vs long-term testing. Short-term tests run 2 to 7 days and produce a snapshot measurement under closed-house conditions. Long-term tests run 90 days to 12 months and produce an integrated annual-average measurement. Each has a place.
Short-term testing is appropriate for initial screening, real-estate transactions (where a 48-hour test is standard), post-mitigation verification, and any situation where a faster result is needed. Long-term testing is appropriate for confirming an ambiguous short-term result, for true annual-average characterization, and for homeowners not under time pressure.
EPA’s recommended approach for current homeowners is one of three sequences: a single long-term test; a single short-term test followed by mitigation decision if elevated; or two short-term tests run sequentially, with the average used for the mitigation decision. The two-short-term approach reduces the risk that weather or HVAC variability during a single test produces a misleading result.
Step 2: Placement
EPA prescribes placement in the lowest livable level of the home. For most Colorado homes that means a finished basement. If the basement is unfinished and unused, the test goes on the first floor in a regularly occupied room. The test should sit 20 inches to 6 feet above the floor, at least 3 feet from exterior walls and windows, at least 4 inches from other surfaces, and at least 12 inches from a ceiling.
The test should not sit in a closet, in a bathroom or laundry, near an HVAC supply register, near a fireplace, or in direct sunlight. It also should not be on top of an exterior wall or on a windowsill. The placement guidance is designed to capture a representative room-air measurement, not air patterns near specific entry or exit points.
For real-estate transactions, an inspector typically deploys the test in the basement on a small stand. For DIY testing, the homeowner places the kit per instructions and notes the placement on the chain-of-custody card.
Step 3: Duration and Closed-House Conditions
Short-term tests run 48 to 96 hours under closed-house conditions. “Closed-house” means exterior windows and doors are kept closed except for routine entry and exit. Whole-house fans, attic fans, and window air conditioners should not operate during the test. Forced-air HVAC heating and cooling can run normally. The closed-house requirement begins 12 hours before the test deployment and continues throughout the test period.
The reason is that radon levels inside a home are heavily influenced by the air-exchange rate. An open window during a test will produce an artificially low reading that does not reflect typical home conditions. Closed-house conditions standardize the test environment so results across homes and across times are comparable.
Long-term tests do not require closed-house conditions because they integrate over enough time to average out daily ventilation variability.
Step 4: Analysis
After the test period, the device is sealed and shipped to the analyzing laboratory. For passive devices (charcoal canisters, alpha-track detectors, electret ion chambers), the lab performs the physical analysis: the canister’s gamma emission is counted, the alpha-track film is etched and the tracks counted, or the electret’s voltage drop is measured. Each method produces a pCi/L (picoCuries per liter) measurement.
For continuous radon monitors (CRMs), analysis happens in real time inside the device. The CRM logs hourly pCi/L readings throughout the test period, and the average and standard deviation are reported when the test concludes. Many CRMs allow electronic upload of results without shipping the device anywhere.
Laboratory turnaround for short-term passive devices is typically 3 to 7 business days from receipt. Long-term alpha-track lab turnaround is 7 to 14 business days. CRM results are typically available immediately after the test concludes.
The Chain of Custody for Real-Estate Tests
For tests performed during real-estate transactions, chain-of-custody documentation matters. The certified measurement professional records device serial number, deployment time and location, retrieval time and location, and signs the chain-of-custody form. The form accompanies the device through any shipping or analysis steps. If the result is disputed later, the chain-of-custody documents the test’s integrity.
For DIY testing, the homeowner serves as the chain-of-custody steward. The kit includes a card or form on which to record placement details and timing. Filling out the card carefully matters because the lab uses the deployment time to apply decay-correction calculations for charcoal canister analysis.
Tampering with a test — intentionally or accidentally — can produce misleading results. The most common accidental tampering is opening windows during a short-term test, which biases the result low. Intentional tampering, where a seller during a transaction opens windows hoping to produce a favorable result, is one of the reasons real-estate measurement professionals use tamper-resistant CRMs that log hourly readings and detect anomalous patterns.
Step 5: Reporting
The test report includes the device serial number, deployment and retrieval times, calculated average pCi/L, the chain-of-custody record, and laboratory accreditation information (NRPP or NRSB). For homeowners performing their own testing, the report typically arrives by email and is also available through the lab’s online portal.
For real-estate-transaction testing performed by a licensed measurement professional, the report goes to both the inspector or measurement professional and is shared with the parties to the transaction per the inspection agreement.
What Happens After Results Arrive
The action threshold is 4 pCi/L. Above that, EPA recommends mitigation. Between 2 and 4 pCi/L, the agency notes that mitigation should be considered. Below 2 pCi/L is generally accepted as not requiring action, though periodic retesting is still recommended.
For results above 4 pCi/L, the next step depends on context. In a real-estate transaction, the buyer typically negotiates with the seller for mitigation. In an existing home, the homeowner contacts a NRPP- or NRSB-certified mitigation contractor for an evaluation and quote. The sub-slab depressurization guide covers the most common mitigation approach.
Differences Between DIY and Professional Protocol
For homeowners running their own short-term test, the protocol is essentially the same as professional measurement: select a device, place it correctly, maintain closed-house conditions, document deployment timing, and ship to an accredited lab. The differences are in chain-of-custody rigor, instrument selection, and result use.
DIY tests use single-use passive devices. Professional measurement professionals typically use CRMs for transactional testing because the immediate result and hourly data better support time-sensitive decisions. For routine homeowner testing without transaction pressure, DIY kits produce information of equivalent quality if deployed correctly. The result-use difference matters most for real-estate transactions where chain-of-custody and tamper-resistance become important.
Re-Testing After Mitigation
After a mitigation system is installed, EPA recommends a post-mitigation test to confirm levels have dropped below 2 pCi/L. The post-mitigation test runs the same short-term protocol — typically 48 hours under closed-house conditions — and produces a verified result. Effective mitigation systems routinely drop indoor levels from 10 to 20 pCi/L down to below 1 pCi/L.
Homeowners with installed mitigation systems should retest annually because mitigation systems include moving parts (fan) that can fail over time.
Pre-Test Preparation Checklist
Before deploying a short-term test, the homeowner or measurement professional should establish closed-house conditions starting at least 12 hours before deployment. This means closing exterior windows and doors and using them only for normal entry and exit. Whole-house fans, attic fans, and window air conditioners should be off. HVAC heating and cooling can operate normally; in fact, the test should reflect typical HVAC operation patterns rather than running with HVAC off.
Make sure the test location is free of obstructions and that nothing will be moved or disturbed during the deployment period. Note the deployment date and time on the kit’s chain-of-custody card. If using a CRM, verify the device’s battery or AC power and confirm the device is logging data. For passive devices, verify the device is unsealed (most kits ship sealed in plastic to prevent radon exposure during shipping).
Common Protocol Mistakes
Several mistakes commonly compromise homeowner testing. Opening windows during the test invalidates the closed-house assumption. Placing the test in a kitchen or bathroom (high-humidity areas) can bias results. Placing it near an HVAC supply register can produce locally low readings that don’t represent the room. Disturbing the test during the deployment period — moving it, breathing directly on it, or covering it — can affect the measurement.
The Colorado radon testing pillar outlines best-practice deployment for Front Range homes specifically.
What Different Result Ranges Imply for Confidence
Single-test result confidence varies with the absolute reading. A result of 12 pCi/L on a single 48-hour test produces high confidence that the home is above the action threshold; the measurement variability typical of short-term tests would still leave the true value well above 4 pCi/L even at the lower end of the confidence interval. A result of 3.8 pCi/L on a single test produces much less confidence; the true value could plausibly be 2 pCi/L or 6 pCi/L given typical short-term test variability.
This is why EPA recommends confirmation testing in the 2-4 pCi/L range. The decision threshold is close enough to the measurement that a single test cannot definitively place the home above or below the action level. Two tests averaged, or a long-term confirmation test, resolves the ambiguity.
Who Should Test
EPA’s position is that every U.S. homeowner should test, with priority for homes in Zone 1 (which includes all of Colorado), homes with basements or slab-on-grade construction, and homes occupied by children, elderly residents, or people with pre-existing lung disease. CDPHE echoes the recommendation for Colorado.
For most Front Range homeowners who have never tested, the protocol cost is modest — 15 to 30 dollars for a DIY kit, 100 to 200 dollars for a professional short-term test bundled with a home inspection. The result is information that drives a real and consequential decision.
References
- EPA radon testing protocols and devices — U.S. Environmental Protection Agency
- CDPHE radon testing program for Colorado — Colorado Department of Public Health and Environment
- CDC radon health and testing resources — Centers for Disease Control and Prevention
- American Lung Association radon information — American Lung Association
Frequently asked questions
How long does radon testing take?
Short-term tests run 48 to 96 hours under closed-house conditions, with lab turnaround adding 3 to 7 business days. Long-term tests run 90 days to 12 months. Continuous radon monitors produce results immediately at test conclusion. Total time from deployment to actionable result is typically one to two weeks for short-term testing.
Do I have to keep windows closed during the test?
Yes for short-term tests. Closed-house conditions — exterior windows and doors closed except for routine entry and exit — are required, starting 12 hours before deployment and continuing throughout the test. Long-term tests do not require closed-house conditions because they average over enough time to even out ventilation effects.
Where exactly should I place the test?
In the lowest livable level of the home, 20 inches to 6 feet above the floor, at least 3 feet from exterior walls and windows, away from HVAC supply registers, fireplaces, and direct sunlight. Avoid closets, bathrooms, laundries, and kitchens. The placement should reflect typical occupied-room air conditions.
What does the result mean?
The result is the average pCi/L (picoCuries per liter) over the test period. EPA's action level is 4.0 pCi/L; above that, mitigation is recommended. Between 2 and 4 pCi/L, mitigation should be considered. Below 2 pCi/L is generally acceptable, though periodic retesting is still recommended.
Front Range homeowners wanting professional radon testing can get in touch here to connect with a certified local measurement professional.
Test your home for radon
Radon is invisible and odorless, so a test is the only way to know your level. A short-term kit gives a first reading in days; a digital monitor tracks it over time.
| Product | Why | Buy |
|---|---|---|
 AirChek Charcoal Radon Test Kit | Lab fee usually included; EPA short-term protocol. | Amazon — $21.95 |
 First Alert RD1 Radon Test Kit | Widely stocked; 48-hour charcoal test. | Amazon — $17.95 |
 Airthings Corentium Home | Digital monitor with a long-term average. | Amazon — $142.50 |