Blog

Updated September 30, 2026.

Direct answer: Put the radon test on the lowest level you use or could use as living space—in a bedroom, living room, or similar room, not a kitchen, bathroom, closet, crawl space, or furnace room. Keep the device at least 20 inches above the floor, at least 3 feet from exterior doors and windows, at least 1 foot from an exterior wall when there are no nearby openings, and at least 4 inches from other objects. Avoid HVAC vents, fans, heat, and sun; follow closed-house rules for short-term kits.

Placement is the most controllable part of a home radon measurement. A good lab kit in the wrong spot produces a misleading number. EPA home-test guidance and AARST’s MAH protocol target breathing-zone air in lived-in space—not a mechanical closet or a draft under a supply vent.

Which Floor to Test

Test the lowest level that could be used regularly, finished or unfinished: basement, walkout lower level, or first floor when there is no below-grade living area. Do not stop at upper floors if a usable level exists below—radon concentrates low in the building envelope.

Pick a room people actually occupy (bedroom, family room, den, home office). If crawl-space moisture sits under that level, a crawl space humidity monitor documents adjacent conditions; it does not replace a radon kit.

Height and Clearances

EPA’s measurement device protocol and AARST Table 3.3 agree on minimum clearances:

• Floor: At least 20 inches (50 cm) above the floor.
• Openings: At least 3 feet (90 cm) from exterior doors, windows, or other outdoor openings.
• Exterior wall: At least 1 foot (30 cm) from the exterior wall when no nearby openings apply.
• Objects: At least 4 inches (10 cm) from furniture, other detectors, or surfaces beside the kit.
• Height: Table height is typical; suspended devices may sit about 6–8 feet high in the general breathing zone.

Do not set the kit on the slab, in a sump pit, or tight to a foundation wall—those are entry pathways, not room-average air. Stay away from supply and return vents; EPA flags drafty locations near HVAC, not a single fixed inch count.

Closed-House Rules (Short-Term Tests)

EPA’s 2024 Home Buyer’s and Seller’s Guide still defines closed-house conditions for short-term work: windows stay closed, doors stay closed except normal entry and exit, and you do not run equipment that pulls in outdoor air (small brief exhaust fans and fans on an installed radon system are exceptions per EPA).

• Close windows and outside doors at least 12 hours before starting.
• Hold closed-house conditions for the full test when the kit requires it.
• Skip starting a sub-four-day test during severe storms or high winds if instructions warn you.

Rooms to Avoid

Do not test in closets, stairways, hallways, crawl spaces, sumps, kitchens, bathrooms, laundry rooms, furnace rooms, or unheated garages. EPA lists these as non-representative or high-velocity, high-humidity zones. Follow manufacturer instructions when they are stricter.

Multiple Tests and Mitigation Follow-Up

One device on the lowest lived-in level is the usual first screen. Test each sleeping area on different levels if people sleep there. Do not average simultaneous room results—each reading stands alone. Address any room at or above EPA’s action level of 4.0 pCi/L. Before you spend on mitigation, check whether homeowners insurance covers radon mitigation on your policy.

Post-mitigation verification uses the same placement rules; match the pre-test room when you can. Where soil air and wet crawl conditions overlap, radon work and drying are separate budgets—see crawl space dehumidifier cost for the moisture side. Operators who publish exact placement steps earn cleaner answers in AI search; an AI citation quick scan or tracking AI citation rate measures visibility, not dose.

Frequently Asked Questions

Should I test for radon in the basement or on the first floor?

Test in the basement if it is the lowest level you use or could use regularly—that is where concentrations are usually highest. If you have no basement or crawl-space living area, test on the first floor. If people sleep on multiple levels, test each sleeping floor per EPA guidance, not only the lowest point.

Can I put a radon test on my nightstand?

Yes, if the nightstand is in a bedroom on the lowest sleeping level. That height is inside the breathing zone and reflects long nightly exposure. Confirm the kit is at least 3 feet from windows and exterior doors, at least 1 foot from an exterior wall when no openings are nearby, and at least 4 inches from lamps or clutter on the stand.

My basement has multiple rooms—where should I put the test?

Place the device in the basement room you occupy or plan to finish for living use—office, bedroom, or family space—not storage-only zones. If the whole level is unfinished storage, test in the most central lived-in candidate on that level, then retest after you convert space to regular use.

How far from a window does the test need to be?

Keep the detector at least 3 feet (90 cm) from exterior windows, doors, or other openings to outdoors. That distance reduces skew from outdoor air mixing during entry and exit. If the room has no exterior openings, stay at least 1 foot from the exterior wall instead.

Can I run ceiling fans or the HVAC during the test?

Follow your kit instructions and EPA closed-house rules. Short-term tests generally require closed windows and no fans that import outdoor air. Normal heating and cooling operation is usually acceptable, but do not point supply vents at the detector or run whole-house fans that change building pressure during a short-term exposure.

What if my result is above 4 pCi/L?

EPA’s action level for follow-up is 4.0 pCi/L. Confirm with a second test—often a long-term measurement or another short-term under closed-house conditions—before committing to mitigation. Fix placement errors before you treat a bad reading as a structural radon problem.

The EPA’s radon testing protocols are the foundation for nearly every state radon program, real estate testing standard, and measurement professional certification in the United States. Understanding what the EPA actually requires — not what a neighbor summarizes or a contractor implies — lets you evaluate whether your testing was conducted correctly and whether your results are defensible.

EPA Radon Testing Guidance: Overview

The EPA’s primary radon testing guidance documents for homeowners are the A Citizen’s Guide to Radon and the Home Buyer’s and Seller’s Guide to Radon. For measurement professionals, the EPA Radon Measurement Proficiency Program established the original certification framework that NRPP and NRSB now administer. AARST-ANSI measurement standards (MAMF, ANSI/AARST Series) build on EPA guidance and are the operative standards for professional measurement.

Closed-House Conditions: The Core Protocol Requirement

Closed-house conditions are the most important and most frequently violated component of EPA’s short-term testing protocol. Without closed-house conditions, short-term results are not comparable to the EPA action level and may significantly underestimate actual radon levels.

What Closed-House Conditions Mean

• Keep all windows and exterior doors closed during the test and for 12 hours before the test device is placed
• Do not operate whole-house fans, attic fans, or similar ventilation during the test or the 12-hour pre-test period
• Normal HVAC operation is permitted — heating and cooling systems that recirculate interior air (central HVAC) may run normally
• Ceiling fans: permitted
• Kitchen and bathroom exhaust fans: minimize use; brief operation for cooking or showering is acceptable, but extended operation creates pressure differential
• Fireplace dampers: keep closed if not in use; do not operate fireplaces during the test

Why Closed-House Conditions Matter

Open windows introduce large volumes of outdoor air (~0.4 pCi/L radon) that dilute indoor radon levels to artificially low values. A home with actual indoor radon of 8.0 pCi/L may test at 3.0–4.0 pCi/L with windows routinely open — below the action level, creating a false impression of safety. This is why summer testing with open windows is unreliable and why EPA requires closed-house conditions for short-term tests.

Device Placement Protocol

EPA’s placement requirements for radon test devices:

• Level: Lowest level that is used or could be used as living space — including unfinished basements if they could be finished, and basements where family members spend any time regularly
• Height: At least 20 inches above the floor, within the breathing zone
• Wall distance: At least 12 inches from any wall
• Avoid: Drafts from windows, doors, HVAC supply/return vents, exterior walls; direct sunlight; humidity sources (sump pits, laundry, showers)
• Do not place in: Kitchens, bathrooms, crawl spaces, or closets
• Placement in room: Central to the room is ideal; near where occupants typically spend time is the goal — not in an uninhabited corner

EPA’s Two-Test Decision Protocol

For initial testing of a home, EPA’s protocol is designed to balance speed against accuracy:

If Initial Short-Term Test is Below 4.0 pCi/L

No immediate action required. Retest in 2 years. If the result was between 2.0 and 3.9 pCi/L, consider a long-term test to confirm the annual average. Levels in this range may be higher during different seasons or weather conditions.

If Initial Short-Term Test is 4.0–7.9 pCi/L

Follow up with either a second short-term test or a long-term test before making a mitigation decision. The rationale: a single 48-hour test in this range has sufficient measurement uncertainty that a confirmatory test substantially improves confidence in the true annual average level. If the confirmatory test also comes in at or above 4.0 pCi/L, mitigate. Average the two short-term results if using two short-term tests.

If Initial Short-Term Test is 8.0 pCi/L or Higher

EPA recommends proceeding to mitigation without waiting for a confirmatory test. At this concentration, the risk from continued exposure during a second test period is not justified by the additional certainty. Contact a certified radon mitigator and schedule installation.

The 4.0 pCi/L Action Level: Context and Criticism

EPA’s 4.0 pCi/L action level was established in the 1980s based on risk assessments available at the time and the practical cost of mitigation. It has not been formally revised since, despite significant advances in radon health research.

Notable context:

• The World Health Organization reference level is 2.7 pCi/L (100 Bq/m³) — lower than EPA’s threshold
• EPA itself notes that “radon levels less than 4 pCi/L still pose a risk, and in many cases may be reduced” — the 4.0 level is an action threshold, not a safety threshold
• EPA’s own long-term goal is to reduce indoor radon to outdoor air levels (0.4 pCi/L), acknowledging that any indoor radon above background carries some risk
• Several European countries have adopted national reference levels of 1.4–2.7 pCi/L (50–100 Bq/m³)

For practical purposes: the 4.0 pCi/L threshold determines when EPA recommends mandatory action. Below 4.0 pCi/L, mitigation is EPA-recommended for consideration between 2.0–3.9 pCi/L and is always a personal risk decision.

EPA Guidance on Specific Testing Situations

Real Estate Transactions

EPA recommends testing in real estate transactions. When time is limited (common in transaction timelines), short-term tests are acceptable. EPA provides guidance that real estate tests should follow closed-house protocols strictly and that buyers should understand the limitations of a single 48-hour test relative to annual average exposure.

Post-Mitigation Testing

EPA recommends testing 24+ hours after system activation (if continuous monitor) or placing a charcoal canister at least 24 hours post-activation for a minimum 48-hour test. If results are still at or above 4.0 pCi/L, contact the mitigator — the system requires adjustment.

New Construction (RRNC)

EPA recommends testing new homes with RRNC passive systems after at least 60 days of occupancy, under normal living conditions (closed-house protocol not required for this application — the extended occupancy averages conditions sufficiently). If results are at or above 4.0 pCi/L, activate the passive system with a fan.

Frequently Asked Questions

What is EPA’s recommended radon action level?

EPA’s action level is 4.0 pCi/L — the concentration at which EPA recommends mitigation. EPA also recommends considering mitigation at 2.0–3.9 pCi/L, acknowledging that radon at those levels still poses meaningful cumulative health risk, particularly for households with smokers or young children.

Does EPA require professional radon testing?

EPA does not federally mandate professional testing for homeowners conducting their own radon assessments. DIY test kits from NRPP- or NRSB-certified labs are EPA-endorsed for homeowner use. Professional certified testing is required for real estate transactions in some states, for regulatory compliance contexts, and when chain-of-custody documentation is needed.

What is the difference between EPA and WHO radon action levels?

EPA’s action level is 4.0 pCi/L (148 Bq/m³). The WHO reference level is 2.7 pCi/L (100 Bq/m³). The WHO level reflects more recent health evidence and is lower. In practice, both acknowledge significant risk below their respective thresholds — the difference is where each organization draws the line for recommended action.

Installing a radon mitigation system is not the final step — testing after installation is. A mitigation system without post-mitigation testing is a system with no verified outcome. You do not know it is working; you only know it is running. Post-mitigation testing is how you confirm the system achieved its purpose, satisfy warranty requirements, and establish a baseline for future monitoring.

When to Conduct Post-Mitigation Testing

EPA and AARST recommend testing within 24 hours of system activation if using a continuous electronic monitor, or placing a short-term charcoal canister at least 24 hours after the fan is turned on and running it for a minimum of 48 hours. The 24-hour delay allows the system to fully pressurize the sub-slab zone and reach equilibrium — results taken in the first few hours of operation may not represent steady-state performance.

Do not wait weeks or months to test. The purpose of post-mitigation testing is to verify the system is performing as designed while any issues are still under the contractor’s workmanship warranty. Most warranties require the homeowner to conduct post-mitigation testing and notify the contractor of any elevated results within a defined window.

What Test to Use for Post-Mitigation Verification

A 48-hour charcoal canister is the standard post-mitigation test for residential applications. It provides a fast result (3–7 business days after mailing) and is the test type most commonly specified in contractor warranties and real estate disclosure contexts.

Closed-house conditions are required during the 48-hour test period and for 12 hours prior to device placement. This is the same protocol used for any short-term test — windows closed, no whole-house fans, normal HVAC operation permitted.

Some contractors conduct post-mitigation testing themselves using a professional continuous monitor, providing same-day or next-day results. This is faster but has a potential conflict of interest — a contractor confirming their own work. For independent verification, homeowners can place their own charcoal canister alongside the contractor’s device.

Where to Place the Post-Mitigation Test Device

Same placement protocol as any residential radon test:

• Lowest livable level of the home — typically the same floor where the pre-mitigation test was conducted
• Breathing zone: at least 20 inches above the floor, at least 12 inches from any wall
• Away from drafts, HVAC vents, sump pits, and exterior walls
• Undisturbed for the full 48-hour period

Do not place the test directly under the suction point, adjacent to the riser pipe, or in the mechanical room where the fan vibration could affect the device. Place in the living area or bedroom where occupants actually spend time — the location most relevant to actual exposure.

What Post-Mitigation Results to Expect

A properly installed single-suction-point ASD system with good sub-slab aggregate and thorough slab sealing typically achieves:

• 85–99% reduction from pre-mitigation levels
• Final levels of 0.3–1.5 pCi/L for most homes
• Homes that previously tested at 20+ pCi/L commonly achieve 0.5–2.0 pCi/L post-mitigation

EPA’s target is below 4.0 pCi/L. Most certified mitigators target below 2.0 pCi/L as a workmanship standard, and well-executed installations on homes with good aggregate routinely achieve 0.5 pCi/L or lower.

What If Post-Mitigation Results Are Still Elevated?

A post-mitigation result at or above 4.0 pCi/L indicates the system is not achieving adequate reduction. This is not uncommon — national data suggests that 10–15% of initial installations require a callback before achieving target levels. The most common causes:

• Insufficient suction field coverage: One suction point is not covering the full slab footprint — additional suction points needed
• Unsealed bypass pathways: Gaps in the slab sealing — expansion joints, floor drains, sump pit, pipe penetrations — allow radon to enter the home directly without passing through the sub-slab vacuum zone
• Fan undersized for aggregate conditions: Post-construction reveal that sub-slab fill is denser than the diagnostic suggested — a higher-capacity fan or additional suction point is needed
• Block wall contribution: In CMU block basement homes, the block wall cores are a radon pathway that sub-slab depressurization alone does not address
• Test conditions violated: Radon post-mitigation tests that are conducted with windows open or whole-house fans running produce unreliable results that may understate or overstate actual levels

Contact your mitigator immediately when post-mitigation results are elevated. Most reputable contractors include workmanship warranties of 1–5 years that cover exactly this scenario — returning to add suction points, improve sealing, or upgrade fan capacity at no additional charge if results do not meet the contractual target.

Ongoing Retesting After Mitigation

EPA recommends retesting a mitigated home every 2 years. Reasons:

• Fan performance can degrade over time — a failing fan that still runs may not generate adequate suction
• Foundation settling creates new cracks and pathways that the existing system may not fully address
• Remodeling work — finishing a basement, adding a room over a crawl space — changes the building’s pressure dynamics
• Real estate transactions require current test results even in mitigated homes

The U-tube manometer provides daily visual confirmation that the fan is generating suction, but it does not measure actual radon concentration. Monthly manometer checks plus biennial radon tests is the recommended maintenance protocol for a mitigated home.

Frequently Asked Questions

How soon after radon mitigation can I test?

Place the test device at least 24 hours after the mitigation system is activated and run it for a minimum of 48 hours under closed-house conditions. Testing in the first few hours of system operation may not reflect steady-state performance. Most contractors recommend testing within the first 30 days of installation.

What radon level should I expect after mitigation?

Most properly installed ASD systems achieve 0.3–2.0 pCi/L regardless of the pre-mitigation starting level. EPA’s target is below 4.0 pCi/L; reputable contractors target below 2.0 pCi/L as a workmanship standard. If your post-mitigation result is at or above 4.0 pCi/L, contact your contractor immediately — a callback is almost always covered under the workmanship warranty.

Do I need to retest after radon mitigation every year?

EPA recommends retesting every 2 years in a mitigated home. Annual retesting is not required but provides more frequent confirmation of continued system performance. A long-term alpha track detector left in place for 90 days to a year provides the most accurate ongoing average.

My post-mitigation test shows 3.5 pCi/L — is that acceptable?

It is below EPA’s 4.0 pCi/L action level, but it is also not what a well-performing system typically achieves. Most single-point ASD installations in homes with standard aggregate conditions reach 0.5–2.0 pCi/L. A result of 3.5 pCi/L suggests the system may have room for improvement — additional sealing, a second suction point, or a fan upgrade. Discuss with your contractor whether a callback is warranted.

The difference between a short-term and long-term radon test is not just duration — it is what each result actually tells you. A 48-hour test gives you a snapshot of radon during specific conditions. A 90-day test gives you a seasonal average. A year-long test gives you the most accurate picture of your true annual exposure. Understanding when each applies prevents both under-reaction to real risk and over-reaction to a weather-influenced spike.

Short-Term Tests: The Screening Tool

Short-term radon tests run from a minimum of 48 hours up to 90 days. The most common residential short-term test is the activated charcoal canister, run for 48–96 hours under closed-house conditions.

How Charcoal Canister Tests Work

An activated charcoal canister absorbs radon gas from the surrounding air during the exposure period. At the end of the test, you seal the canister and mail it to a laboratory. The lab measures gamma radiation emitted by radon decay products that have accumulated in the charcoal, calculates the average radon concentration over the test period, and reports the result in picocuries per liter (pCi/L).

Short-Term Test Accuracy and Limitations

Short-term results are inherently variable because radon levels fluctuate by 30–50% day to day in many homes, driven by:

• Barometric pressure: Low pressure pulls more soil gas into the home; high pressure suppresses it
• Temperature differential: Greater indoor-outdoor temperature difference strengthens stack effect and increases radon draw
• Wind: Wind pressure against the house affects sub-slab pressure dynamics
• Precipitation: Rain saturates soil, reducing gas permeability and temporarily suppressing radon entry
• HVAC operation: Forced-air systems can both dilute and redistribute radon within the home

A single 48-hour test during an unusually high-pressure, warm, dry period may significantly underestimate actual levels. The same home tested during a cold snap with falling barometric pressure may read 30–50% higher than average. This variability is why EPA guidance does not recommend making final mitigation decisions solely on a single short-term result in the 4.0–8.0 pCi/L range.

When Short-Term Tests Are the Right Choice

• Initial screening: If you have never tested your home, a short-term test is the fastest way to identify whether a problem may exist
• Real estate transactions: When time constraints (contract deadlines) prevent long-term testing, short-term tests are universally accepted with appropriate disclosure
• Post-mitigation verification: After installing a radon system, a 48-hour charcoal test placed at least 24 hours post-installation verifies the system is working; EPA recommends this within 24 hours of system activation
• Initial high-result screening: If the initial test returns 8.0 pCi/L or higher, EPA recommends proceeding to mitigation without waiting for a confirmatory long-term test — the risk is sufficient

Long-Term Tests: The Accurate Baseline

Long-term tests run for a minimum of 90 days; one-year tests are the gold standard. The standard device is an alpha track detector — a small card with a clear plastic film (CR-39 or similar) that records microscopic damage tracks from alpha particles emitted by radon decay products over the exposure period. At the end of the test, the lab chemically etches the film and counts the tracks under a microscope, calculating average radon concentration.

Why Long-Term Tests Are More Accurate

By averaging radon levels across multiple seasons — or ideally a full year — long-term tests smooth out the barometric, temperature, and weather-driven variability that makes short-term results uncertain. A 90-day winter test captures the highest-radon season and provides a reasonably conservative estimate of annual average. A full-year test captures all seasonal patterns.

Studies comparing matched short-term and long-term measurements in the same homes consistently show that short-term tests, when compared to annual averages, overestimate the annual average in about half of cases and underestimate it in the other half — with individual test variance of ±40–50% common. Long-term tests reduce this uncertainty substantially.

When Long-Term Tests Are the Right Choice

• Confirming a short-term result in the 4.0–8.0 pCi/L range: Before investing $1,000–$2,500 in mitigation, a long-term confirmation test establishes that elevated levels are chronic rather than a test-period anomaly
• Establishing a baseline in a new home: A one-year test after moving in provides the most accurate picture of actual exposure
• Routine monitoring in a mitigated home: An annual alpha track detector run year-round provides ongoing confirmation of system performance
• Research or legal purposes: Situations requiring the highest-accuracy radon measurements

EPA Decision Protocol: Which Test When

Continuous Radon Monitors: The Third Option

Continuous electronic radon monitors (Airthings Wave, Corentium, RadonEye) provide real-time radon readings and running averages. They do not replace lab-analyzed test kits for official measurements but offer ongoing visibility into radon fluctuations that neither charcoal canisters nor alpha track detectors can provide.

Continuous monitors are most valuable for:

• Monitoring a mitigated home between formal retests
• Understanding diurnal and seasonal radon patterns in your home
• Detecting rapid changes that indicate fan failure or new entry pathways
• Confirming that closed-house conditions during a short-term test are being maintained

Consumer-grade continuous monitors have measurement uncertainty of ±10–20% at low radon levels and are not accepted as certified measurements for real estate transactions or regulatory compliance. They are monitoring tools, not certification tools.

Frequently Asked Questions

Which radon test is more accurate — short-term or long-term?

Long-term tests are more accurate representations of actual annual average radon exposure because they average out the weather- and pressure-driven fluctuations that make short-term results variable. A 90-day or one-year alpha track test provides a more reliable basis for mitigation decisions than a single 48-hour charcoal test.

Can I use a short-term test to decide whether to mitigate?

Yes, with caveats. If your short-term result is 8.0 pCi/L or higher, EPA recommends mitigation without a confirmatory test. If it is between 4.0 and 8.0 pCi/L, a follow-up long-term or second short-term test is advisable before investing in mitigation, to confirm the result is not an anomalous spike.

How long should I run a radon test?

Minimum 48 hours for a charcoal short-term test under closed-house conditions. For the most accurate annual average, run an alpha track detector for 90 days to one year under normal living conditions. Longer is more accurate.

Do I need closed-house conditions for a long-term radon test?

No. Long-term tests (alpha track detectors, 90+ days) are designed to run under normal living conditions — windows open in summer, closed in winter, normal HVAC operation. The extended duration averages out all of these variations. Closed-house conditions are required only for short-term charcoal tests (48–96 hours).

Radon testing is the only way to know whether your home has elevated radon levels. You cannot smell it, see it, or detect it with any sense — and the homes with the highest radon levels often show no correlation with geography, age, or construction style. The EPA estimates that 1 in 15 U.S. homes has elevated radon. Testing takes as little as 48 hours and costs $15–$30 for a DIY kit.

Why You Need to Test

Radon is the second leading cause of lung cancer in the United States after cigarette smoking, responsible for approximately 21,000 deaths annually according to the EPA. The risk is cumulative — it is the product of concentration and time. A home at 4.0 pCi/L poses roughly the same lifetime lung cancer risk as smoking half a pack of cigarettes per day. A home at 20 pCi/L — not uncommon in high-radon zones — roughly equals smoking two packs per day.

The only way to know your home’s radon level is to test it. No map, no neighborhood average, and no visual inspection can substitute for a measurement in your specific home.

Short-Term vs. Long-Term Radon Tests

Short-Term Tests (2–90 Days)

Short-term tests are the most commonly used initial screening method. The standard residential short-term test is a charcoal canister test run for 48–96 hours. Results are available within 3–7 business days after mailing the device to a lab.

• Duration: 48 hours minimum (EPA); 48–96 hours typical for charcoal devices
• Device type: Activated charcoal canister or electret ion chamber
• Conditions required: Closed-house conditions (see below)
• Best for: Initial screening, pre-purchase testing, post-mitigation verification
• Limitation: A single short-term test captures a snapshot — radon levels fluctuate with barometric pressure, temperature, and season. A short-term result may be higher or lower than the home’s true annual average.

Long-Term Tests (90+ Days)

Long-term tests provide a more accurate picture of the home’s actual annual average radon exposure. The standard device is an alpha track detector — a small card with a special plastic film that records radon decay particle tracks over time.

• Duration: 90 days to 1 year (one year is ideal)
• Device type: Alpha track detector
• Conditions required: Normal living conditions (no closed-house protocol)
• Best for: Confirming short-term results, annual monitoring, determining true annual average
• Advantage: Averages out seasonal and pressure fluctuations — provides the most accurate basis for mitigation decisions

EPA guidance: if a short-term test shows between 4.0 and 8.0 pCi/L, conduct a follow-up long-term test or a second short-term test before deciding on mitigation. If the initial short-term test shows 8.0 pCi/L or higher, proceed to mitigation without waiting for a confirmatory test — the risk is sufficient to act immediately.

Where to Place the Radon Test Device

Placement determines whether your result is meaningful. The EPA’s placement protocol:

• Level: Test in the lowest level of the home that is currently used or could be used as living space — even if you do not currently occupy it. If you have an unfinished basement you plan to finish, test there.
• Location within the room: Place the device in the breathing zone — at least 20 inches above the floor and at least 12 inches from any wall
• Away from drafts: Do not place near windows, doors, HVAC vents, or exterior walls where air movement can dilute results
• Away from humidity sources: Do not place near sump pits, laundry areas, or bathrooms — excessive humidity can affect charcoal canister performance
• Accessible but undisturbed: The device should be able to sit undisturbed for the full test duration — not in a high-traffic area where it might be moved

Closed-House Conditions

Short-term tests require closed-house conditions during the test and for 12 hours before the test begins. Closed-house means:

• All windows and exterior doors closed except for brief normal entry/exit
• No whole-house fans or attic fans running
• Normal HVAC operation is permitted (heating and cooling systems can run — they recirculate interior air)
• Ceiling fans are permitted
• Fireplace dampers closed (if not in use)

Closed-house conditions prevent outdoor air from diluting indoor radon to artificially low levels during the test. When conditions are not maintained, short-term results systematically underestimate actual radon levels — exactly the wrong direction for a safety measurement.

Interpreting Your Results

• Below 2.0 pCi/L: Below EPA’s average indoor radon level of 1.3 pCi/L if the home is new. No action required; retest in 2 years.
• 2.0–3.9 pCi/L: Between the national average and the EPA action level. Consider a long-term test to confirm. Some homeowners choose to mitigate at this level regardless, particularly if they have young children or smokers in the home.
• 4.0–7.9 pCi/L: At or above EPA action level. EPA recommends mitigation. Conduct a confirmatory long-term or second short-term test if time allows, then mitigate.
• 8.0 pCi/L or higher: Mitigate without waiting for confirmatory testing. At this level the health risk warrants immediate action.

DIY vs. Professional Testing

DIY test kits (charcoal canisters or alpha track detectors) purchased from hardware stores or online labs are the most cost-effective option for initial and ongoing screening. Cost: $15–$30 including lab analysis. Most state radon programs recommend purchasing from a lab certified by the National Radon Proficiency Program (NRPP) or National Radon Safety Board (NRSB).

Professional testing uses the same device types but is conducted and placed by a certified radon measurement professional. Professional testing is required or preferred in specific situations:

• Real estate transactions where the buyer requires a certified measurement
• Post-mitigation verification where the mitigator or a warranty requires professional confirmation
• Rental properties in states where landlord testing requirements specify professional measurement
• Situations involving litigation or insurance where certified chain-of-custody testing is required

How Often to Test

• Initial test: If you have never tested, test now — regardless of when you moved in or how long you have lived there
• After mitigation: Test within 24 hours of system installation (if using a continuous monitor) or place a short-term test 24+ hours post-installation; run for 48 hours minimum
• Routine retesting: EPA recommends retesting every 2 years even in mitigated homes — to confirm continued performance and catch new entry pathways from foundation settling or renovation
• After renovations: Any work that involves the foundation, basement, or significant changes to the HVAC system warrants a new test
• When buying a home: Always test — or require a recent test result — before closing

Frequently Asked Questions

How accurate are DIY radon test kits?

DIY charcoal canister kits analyzed by NRPP- or NRSB-certified labs are accurate to within ±10–15% under controlled conditions. This is sufficient precision for screening decisions. The larger source of variation is not the device itself but testing conditions — an improperly placed device or violated closed-house conditions introduce more error than the device’s inherent measurement uncertainty.

What time of year is best to test for radon?

Winter typically produces higher radon readings than summer — windows are kept closed, stack effect is stronger, and atmospheric pressure patterns tend to draw more soil gas into the home. Testing in winter gives a closer approximation of worst-case conditions. However, because any result at or above 4.0 pCi/L warrants mitigation regardless of season, the best time to test is simply now — not after waiting for an optimal season.

Can I test for radon myself or do I need a professional?

DIY testing is appropriate and recommended for the vast majority of homeowners. Purchase a certified short-term or long-term kit, follow the placement and closed-house instructions, and mail to the lab. Professional testing is required only for real estate transactions in some states, post-litigation measurements, or situations where certified chain-of-custody documentation is needed.

My neighbor’s home tested low — does that mean mine will too?

No. Radon levels vary dramatically between adjacent homes — sometimes between rooms in the same home. Differences in sub-slab aggregate, foundation type, construction methods, HVAC configuration, and soil permeability can produce completely different radon levels in homes built side by side. Your home must be tested independently.

Related Radon Resources

• EPA Radon Testing Protocol: Official Guidelines Explained
• Short-Term vs. Long-Term Radon Test: Which Do You Need?
• Where to Place a Radon Test in Your Home
• Post-Mitigation Radon Testing