DIY radon testing: the right kit, the right placement, the right time

A do-it-yourself radon test answers the only question that matters — how much radon is in the air your household actually breathes — for between $15 and $45 in materials, provided the kit is deployed correctly. The two failure modes that ruin DIY tests are wrong-room placement (kitchens, bathrooms, drafty windowsills) and broken closed-house conditions (a window left open, a whole-house fan running), both of which the EPA's A Citizen's Guide to Radon (EPA-402-K-12-002) and EPA's Protocol for Radon and Radon Decay Product Measurements in Homes (EPA-402-R-92-003) call out as the most common sources of false readings.

Step 1 — Choose your kit type

The DIY market offers five practical device families. Each one trades cost against duration, precision, and how easily the result can be challenged later.

• Short-term charcoal canister (typically $15–25, lab fee included): activated charcoal adsorbs radon over 2–7 days, then a lab counts the gamma emissions from the trapped radon decay products. The default first-time test.
• Short-term alpha-track ($20–40): plastic film records alpha-particle strikes; etched and microscope-counted at the lab. Less humidity-sensitive than charcoal but less common in the short-term configuration.
• Long-term alpha-track ($25–45): the same film chemistry deployed for 90 to 365 days. Produces the single most representative reading an unmodified DIY workflow can deliver.
• Electret ion chamber: an electrically charged Teflon disc discharged by radon decay ions; the voltage drop is read by a calibrated reader. Largely a professional device, occasionally rented to homeowners.
• Continuous radon monitor (CRM) ($150–300 for consumer models such as Airthings, Ecosense, RadonEye): an electronic sampler that records an hourly time-series. A one-time hardware purchase with no per-test lab fee thereafter.

The five devices answer different questions. For a primer on the difference between a snapshot test and an integrated long-term reading, see short vs long-term radon testing — which to use when.

When to pick each kit

• First-time check or "is this a problem at all?" → short-term charcoal canister, 5–7 days.
• Most accurate single reading you can do at home → long-term alpha-track, 90+ days through at least one winter.
• Ongoing monitoring after mitigation, or in a known Zone 1 county → consumer CRM, deployed permanently.
• Already saw a high short-term result and want a fast confirm → a second short-term test (charcoal again is fine), per the EPA's two-test protocol.
• Real-estate transaction → not a DIY job (see Step 10).

Step 2 — Choose the lab

Every passive DIY kit (charcoal, alpha-track, electret) is just a sealed device until a lab reads it. The kit price almost always includes the lab fee, but the lab itself matters: only labs accredited under the National Radon Proficiency Program (AARST-NRPP) or the National Radon Safety Board (NRSB) are recognised by the EPA and most state radon programmes. About 20 US states also maintain their own radon-professional licensing lists; the state-by-state references are linked from our sources page.

Before you buy a kit, look at the manufacturer or retailer page for the words "NRPP-listed" or "NRSB-certified" alongside the lab name. If the kit ships from a hardware-store private label, search the lab name on the AARST-NRPP directory directly. A $15 kit processed by a non-accredited lab is worse than no test — you have a number that looks official but is not comparable to the EPA action level.

Step 3 — Placement (the most-screwed-up part)

EPA placement rules, drawn from EPA-402-R-92-003 and refined in ANSI/AARST MAH-2019, are precise. Most failed DIY tests fail here.

• Lowest livable level of the home. If your basement is finished or routinely used (guest room, home office, family room), test in the basement. If the basement is an unfinished crawlspace or storage only, test on the first floor.
• Height above the floor: 20 inches to 6 feet. Roughly chest height of a seated adult. Not on the floor, not on top of a tall cabinet.
• At least 4 inches from any interior wall. Radon concentration is slightly elevated immediately against wall surfaces; the EPA buffer rules out that artefact.
• At least 12 inches from any exterior wall. For short-term tests in particular — cold exterior walls drive convective currents that bias the reading.
• Not in a kitchen, bathroom, laundry room, sauna, or any high-humidity room. Humidity skews charcoal-canister results dramatically and corrodes electrets.
• Not in a drafty location. Not on a windowsill, not next to an exterior door, not within 3 feet of a forced-air supply or return register, not next to a wood stove or radiator, not in direct sunlight.
• Room must be one that’s used — or could be used — for living. A finished basement bedroom yes; a mechanical room no.

The placement rule does not change for the CRM — a $300 electronic monitor sitting on top of the refrigerator gives you the same wrong answer a $15 charcoal canister would.

Step 4 — Closed-house conditions

For tests of 4 to 7 days, closed-house conditions are still strongly recommended; for long-term tests of 90+ days, they are explicitly not required — the alpha-track integrates whatever conditions the household actually lives in, and that is the point.

Step 5 — Season

In most of the United States, winter is the highest-radon season because of the stack effect: warm indoor air rises and escapes the upper storeys, creating negative pressure at the basement and slab that draws soil gas (containing radon) into the house. EPA-402-K-12-002 notes that seasonal variation of 2 to 3 times is normal; some houses swing more. If you are doing a single short-term test and want the most conservative reading you can get, deploy it in winter under closed-house conditions. A summer short-term test that reads 3 pCi/L tells you very little about your winter exposure; a winter short-term test that reads 3 pCi/L is genuinely reassuring. The full seasonal discussion is in short vs long-term radon testing.

Step 6 — Duration

Short-term tests run a minimum of 48 hours; 2 to 7 days is the practical sweet spot. Charcoal canisters in particular have a published exposure window on the lab card; running longer than the window degrades accuracy because the charcoal also re-emits adsorbed radon. Long-term alpha-tracks run 90 days minimum, 90 to 365 days in practice. CRMs run continuously and report a rolling average; the longer they have been running, the more representative the displayed average becomes.

Step 7 — Ship to the lab promptly

Passive devices — charcoal, alpha-track, electret — need to reach the lab quickly. Charcoal canisters in particular have a half-life problem: the device is calibrated for a known turnaround, and every extra day in a hot car or warm post-room degrades the result. Most lab cards specify shipping within 24–72 hours of the test ending; use the prepaid envelope the kit ships with, and post the same day you stop the test. CRMs do not need to be shipped — the device displays its reading on the spot.

Step 8 — Interpret the result

The EPA reports radon in picocuries per litre of air (pCi/L). The action threshold is 4 pCi/L. The EPA explicitly notes that there is no known "safe" level — the 4 pCi/L line is the point at which the agency judges mitigation to be cost-effective at typical installed cost. The headline thresholds:

• Below 2 pCi/L: low. EPA recommends retesting every 2 years and after any major renovation.
• 2 to 4 pCi/L: EPA suggests considering mitigation, especially if a smoker lives in the home or if the living space is a basement.
• 4 pCi/L or higher: EPA recommends mitigation, but only after a confirmatory follow-up test — either a second short-term test (concurrent or sequential) or a long-term test, per the EPA Citizen's Guide two-test protocol. Do not mitigate on a single short-term reading.
• 10 pCi/L or higher: EPA recommends the follow-up be a short-term test (not long-term), so that you act within weeks rather than months at that exposure level.

For the underlying lung-cancer arithmetic at each pCi/L band, see radon lung-cancer risk numbers.

Step 9 — What about a continuous radon monitor?

Consumer CRMs (such as Airthings, Ecosense, RadonEye) sample air on a fixed interval and store the readings. The advantage is real: you see overnight spikes, you see seasonal swings, you see the effect of opening a window. The trade-off is that not every consumer CRM has been validated against the AARST measurement standards (ANSI/AARST MS-PC-2015) that lab-processed passive devices are calibrated to. Some CRMs publish accuracy data; others do not.

The practical rule: use a CRM for ongoing situational awareness, not as the single reading you make a mitigation decision on. If the CRM persistently reads above 4 pCi/L for weeks, that is strong evidence to commission a lab-processed passive test (charcoal short-term or alpha-track long-term) before paying for mitigation. After mitigation, a CRM is excellent for verifying the system continues to work between annual professional tests.

Step 10 — Tamper-resistance and real-estate

DIY kits are appropriate for personal information about your own home. They are not appropriate for legal disclosure in a real-estate transaction, school testing, multi-family building testing, or post-mitigation verification when a contract requires it. For those purposes, the relevant standard is ANSI/AARST MAH-2019 (homes) and the AARST Protocol for Conducting Measurements of Radon and Radon Decay Products in Homes for Real Estate Transactions, which require a NRPP- or NRSB-certified measurement professional, tamper-evident deployment, and (preferred) a continuous radon monitor whose hourly time-series can be reviewed for evidence of interference. The full transaction playbook is in selling a home with high radon.

DIY kit comparison

If the result confirms a problem

A confirmed reading at or above 4 pCi/L means installing a radon-reduction system. In slab and basement homes, the standard fix is active sub-slab depressurisation: a sealed suction point under the slab and a continuously-running fan vented above the roofline. National typical installed cost runs $1,000–3,000 per EPA-402-K-12-002; the resulting system reduces indoor radon by 50–99%. Mechanics, code requirements, and post-install verification are covered in sub-slab depressurisation explained. If the home gets its water from a private well in a granite or shale region, the test sequence and mitigation are different — see radon in well water.

Geographic context for your result

A 4 pCi/L reading in a Zone 1 county is exactly what the EPA map predicts; a 4 pCi/L reading in a Zone 3 county is unusual and worth a confirmatory test. The EPA Map of Radon Zones at the county level is summarised in EPA radon zones explained, and the highest-concentration counties are catalogued in highest-radon US counties. Whatever your county Zone, the EPA still recommends testing every home — the Zone is a population-level prior, not a per-house prediction.

Practical recommendation

For a typical owner-occupied home with no transaction pending and no prior radon test on file: buy a $15–25 short-term charcoal kit from a manufacturer whose lab is AARST-NRPP-listed, deploy it on the lowest livable level following the placement rules above, run it for 5–7 days in winter under closed-house conditions, and ship it to the lab the day you stop the test. If the result is under 2 pCi/L, set a 2-year calendar reminder. If the result is 2 to 4 pCi/L, consider a long-term alpha-track to see the year-round average. If the result is 4 to 9.9 pCi/L, order a long-term alpha-track and run it for 90 to 365 days; mitigate if the average comes back at or above 4 pCi/L. If the result is 10 pCi/L or higher, order a second short-term kit immediately and act on the average. For renovations that change the slab or HVAC, see radon retesting after renovation; for upper-floor units, see radon in apartments and condos.

The whole DIY sequence costs between $15 and $75 in materials and produces a number that is comparable to the EPA action level. The $7 county dossier at our order page tells you, before you test, what range your result is likely to fall in for your specific county — the EPA Zone, the state-survey average, and the local mitigator and measurement guidance.

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. Source for the 4 pCi/L action threshold, the 10 pCi/L expedited-retest rule, the two-test protocol, closed-house conditions, retest cadence, and the $1,000–3,000 mitigation cost range.
• US Environmental Protection Agency. Protocol for Radon and Radon Decay Product Measurements in Homes. EPA-402-R-92-003. Source for device placement (lowest livable level, 20 in–6 ft height, 4 in interior / 12 in exterior wall buffers, room-type exclusions) and closed-house condition rules.
• 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 and tamper-evident deployment standards.
• AARST-NRPP (National Radon Proficiency Program) certified lab and device directory — aarst-nrpp.com.
• National Radon Safety Board (NRSB) certified lab and device directory — nrsb.org.
• American Lung Association. Radon — consumer-facing summary of radon as a lung-cancer risk and the case for testing every home.

Related guides

• No. 01 · Test choiceShort-term vs long-term radon testing — which to use when
• No. 02 · EPA zonesEPA radon zones explained — what Zones 1, 2 and 3 actually mean
• No. 03 · MitigationSub-slab depressurisation explained — the standard radon fix
• No. 04 · Real estateSelling a home with high radon — the disclosure playbook