How to Test Radon: A Plain-Language Methodology Guide

We may earn commission from links on this page. Lead-form submissions are forwarded to local inspector partners. How we research and review.

Testing for radon in a home is one of the few hazard tests a homeowner can perform without specialized training, but doing it correctly requires more than dropping a canister on a basement shelf. Placement, closed-house conditions, exposure duration, retrieval, and lab submission each have rules. Skipping any of them produces results that mislead. This guide summarizes EPA and Colorado Department of Public Health and Environment methodology current as of 2026 — consult a certified radon professional for mitigation decisions. The walkthrough below covers how to test radon step by step using the methods available to homeowners and inspectors on the Colorado Front Range.

How to Test Radon: The Four-Phase Methodology

The procedure breaks into four phases:

1. Setup. Choose the right test type and location.
2. Closed-house preparation. Establish the controlled conditions the test requires.
3. Exposure. Run the test for the specified duration without disturbance.
4. Retrieval and analysis. Mail to lab or read the device.

The phases apply to all three common test types — short-term charcoal canister, long-term alpha-track, and continuous radon monitor — with method-specific differences within each phase. For background on which test type fits which goal, see the best way to test for radon guide in this batch.

Phase 1: Setup

Selecting the right test type is the first decision. For a real-estate transaction in Colorado, a continuous radon monitor (CRM) operated by a certified tester is the answer. For a first-time owner-occupied screening, a charcoal canister or consumer CRM works. For long-term baseline, an alpha-track detector exposed for 90 days or more produces the most representative annual average.

Location matters next. EPA places the test:

• In the lowest occupied level of the home.
• 2 to 6 feet above the floor.
• 3 feet from exterior walls, windows, and doors.
• 20 inches from interior walls.
• Away from drafts (heating registers, returns).
• Away from humidity sources (laundry, bathroom).
• Away from direct sunlight.
• In a frequently-occupied space if possible (bedroom, living area), not a closet or utility room.

For Front Range homes with finished basements used as living space, the basement is the right placement. For Front Range homes with crawl space rather than basement, the lowest occupied level is the main floor — but radon often enters through the crawl, so the main-floor reading still reflects realistic exposure. The Colorado radon pillar guide covers the regional setup considerations.

Phase 2: Closed-House Preparation

Short-term radon tests require closed-house conditions. This means:

• Exterior doors closed except for normal entry and exit.
• Windows closed for at least 12 hours before the test begins and throughout the exposure.
• HVAC operating normally — heating in winter, cooling in summer, fan as designed. No fresh-air-only ventilation modes.
• Whole-house fans, attic fans, and exhaust fans operated as normal but not running continuously to ventilate.
• Tightly-shut windows even on pleasant weather days during the test.

The closed-house requirement exists because open windows depress radon readings — and a real-world average should reflect typical lived-in conditions, not artificially ventilated ones. EPA closed-house protocol is what makes short-term test results comparable across homes.

For long-term tests (90 days+), closed-house is not required. The longer integration period averages out window-opening and ventilation variability.

Phase 3: Exposure

This is where the method types diverge in execution.

Charcoal Canister

• Open the canister and place it in the chosen location.
• Record start date, start time, and exact location on the chain-of-custody form.
• Do not move the canister during exposure.
• Exposure period typically 2 to 7 days; follow the manufacturer’s range.
• At end of exposure, seal the canister immediately, record end date and time.

Alpha-Track

• Open the detector and place it in the chosen location.
• Record start date and location.
• Leave undisturbed for 90 days to 12 months.
• At end of exposure, seal in the supplied envelope, record end date.

Continuous Radon Monitor (CRM)

• Power on the device per manufacturer instructions.
• Place in chosen location.
• For pro inspector use: a 48 to 96 hour exposure period is typical; the device logs hourly readings.
• For homeowner ongoing monitoring: the CRM runs indefinitely; sync to phone app for rolling averages.
• Avoid moving during the active test window.

For more on charcoal-canister methodology specifically, see the how-to-test-radon-levels guide.

Phase 4: Retrieval and Analysis

For charcoal canisters and alpha-track detectors, retrieval means sealing and mailing. Place the sealed device in the supplied prepaid mailer along with the completed chain-of-custody form. Mail within 24 hours of sealing for charcoal — the half-life of radon decay products means delays affect accuracy. Alpha-track is more forgiving; mail within a week.

Lab turnaround for charcoal is typically 3 to 5 business days from receipt. Alpha-track results come back in 2 to 3 weeks. Most labs email the result PDF.

For CRMs, retrieval is reading the display or syncing to a phone app. Pro inspectors generate a report from the device’s logged data, which becomes the deliverable to the buyer. Consumer CRMs offer ongoing visibility — homeowners can watch trends over weeks and months.

Interpreting the Result

The result arrives in picocuries per liter (pCi/L). EPA action levels:

• ≥ 4.0 pCi/L: Mitigation recommended. Contract a certified radon mitigation contractor.
• 2.0 to 4.0 pCi/L: Mitigation can be considered, especially for occupants with respiratory conditions or smokers. Confirm with long-term test.
• Below 2.0 pCi/L: No immediate action needed. Retest every two to five years.

For Front Range homes (EPA Zone 1), CDPHE data suggests roughly half of pre-2000 homes test above 4 pCi/L without mitigation. Higher results are common — 8, 12, 20 pCi/L results are routine in Douglas, Jefferson, and Boulder county basements. Mitigation systems reliably bring those numbers below 2.

Common Mistakes When Testing for Radon

Patterns that compromise results:

• Placing the canister too close to a wall, window, or HVAC vent.
• Opening windows during the test period.
• Running fresh-air ventilation modes during the test.
• Moving the canister mid-exposure.
• Forgetting to seal and mail the charcoal canister promptly.
• Testing on the upper floors only (radon concentration is highest in the lowest level).
• Testing only in summer when natural ventilation may depress readings — heating-season testing produces more realistic worst-case values.

For Front Range homeowners, the heating-season test (October through April) tends to capture peak concentrations. Summer testing in homes with closed-house discipline still produces usable results.

When to Retest

Several scenarios warrant repeat testing:

• After installing a radon mitigation system — verify operation within 30 days.
• Every two to five years in a previously tested home with results below the action level.
• After major foundation, basement, or HVAC renovation.
• Upon purchase of a previously tested home, to confirm prior result.
• If a long-term test produces unexpected results.

The two-to-five-year cadence is recommended by EPA because soil radon levels and home airflow patterns can change over time. A clear test in 2020 does not guarantee a clear test in 2026.

DIY vs Professional Testing

For owner-occupied screening, DIY works. For real-estate transactions, professional testing with a CRM is the norm — and increasingly the legal expectation. The reason is chain-of-custody: a homeowner-collected canister has no documentation of seal-integrity, no tamper detection, and limited credibility in negotiation. A certified radon tester operating a pro CRM produces a defensible result.

The how-to-test-home-for-radon walkthrough covers the homeowner-perspective testing in more depth, including the can-you-do-it-yourself question.

Front Range Specifics

Colorado’s Zone 1 designation shapes radon testing in a few practical ways. First, every Front Range home should be tested at least once, regardless of age. Second, results above 4 pCi/L are common and mitigation works reliably — Front Range mitigation contractors install hundreds of systems annually. Third, CDPHE provides discounted test kits (often $5 to $15) to Colorado residents through county health departments, which lowers the barrier to first-test-ever for new homeowners.

For Front Range buyers under contract on a home, including radon testing in the inspection contingency is standard practice. Inspectors typically charge $150 to $200 for the radon test as an add-on to the main inspection.

References

• EPA Citizen’s Guide to Radon — U.S. Environmental Protection Agency
• CDPHE Radon Program — Colorado Department of Public Health and Environment
• CDC Radon Topic — Centers for Disease Control and Prevention
• American Lung Association Radon — American Lung Association
• EPA Radon Test Kit Resources — U.S. Environmental Protection Agency

Front Range homeowners needing a certified radon tester for a transaction can reach out through our contact page for a vetted referral.