Radon Test Kits: Charcoal, Alpha-Track, or Monitor?
If you are shopping for a radon test kit, the real decision is which of three device types fits your situation: a cheap short-term charcoal canister that ships to a lab, a long-term alpha-track detector that averages months of exposure, or an electronic monitor that reads out on a screen or phone app. They measure the same gas but answer different questions. This page explains what separates them, what the numbers mean, and how to place a kit so the result is actually trustworthy.
What the number on the report means
Every kit reports radon (Rn-222) in picocuries per liter (pCi/L) on U.S. devices, or becquerels per cubic meter (Bq/m³) internationally, where 1 pCi/L is about 37 Bq/m³. The EPA action level is 4.0 pCi/L: at or above that, the agency recommends fixing the home. Between 2.0 and 4.0 pCi/L, EPA suggests considering mitigation. There is no fully safe level, because radon is the second leading cause of lung cancer overall and the leading cause among people who never smoked, per the EPA Citizen’s Guide to Radon, the CDC, and the American Lung Association. For a fuller breakdown of thresholds, see what counts as a safe radon level.
The three kit types at a glance
| Type | Exposure time | How it’s read | Typical cost | Best for |
|---|---|---|---|---|
| Short-term charcoal canister | 2–7 days | Mailed to a lab | $15–$40 incl. lab fee | Fast answer, real-estate deadlines |
| Long-term alpha-track | 91 days–1 year | Mailed to a lab | $25–$50 incl. lab fee | A true year-round average |
| Electronic continuous monitor | Ongoing | On-device or app | $150–$300 | Trends, post-mitigation checks |
Short-term charcoal canister kits
A charcoal canister holds activated carbon that adsorbs radon over a 2-to-7-day window, after which you seal it and mail it to the analytical lab in the prepaid envelope. Results usually come back in about a week, so the whole cycle runs roughly two to three weeks. This is the kit sold at hardware stores and the one most home inspectors use during a real-estate transaction, because it fits the tight timelines a contract imposes.
The trade-off is variability. A single short window can land well above or below the home’s true average, since radon rises and falls with barometric pressure, wind, and how the furnace is running that week. EPA advises confirming any short-term result at or above 4.0 pCi/L with a second test before you spend money on a mitigation system. Two short-term tests placed side by side, or a follow-up long-term test, resolve most borderline readings.
Long-term alpha-track detectors
An alpha-track detector is a small piece of CR-39 plastic in a filtered housing. Alpha particles from decaying radon leave microscopic pits in the plastic; after 91 days to a full year of exposure, the lab chemically etches the film and counts the tracks under a microscope to derive an average. Because the reading spans seasons, it captures the winter concentration spike that a July charcoal test misses entirely, which matters in a heating-dominated climate. If you already live in the home and have no closing deadline, this is the most representative kit for the price. It is also the slowest to give you an answer.
Electronic continuous radon monitors
Continuous monitors run indefinitely and show a live reading plus rolling 1-day, 7-day, and long-term averages, most now over a phone app, some with an adjustable alarm. Under the hood they use a passive ion chamber or an alpha-spectrometry detector; the better units settle to a stable long-term number within about 48 hours and hold accuracy over years of use. They cost more upfront but pay off if you want to watch how sealing a sump lid or running a fan changes the number, or to confirm a mitigation system is still working. Our radon gas detector guide compares specific monitor features in more depth.
Check the NRPP or NRSB listing before you buy
Two national programs vet radon devices and labs: the National Radon Proficiency Program (NRPP) and the National Radon Safety Board (NRSB). A kit or monitor that passed device-performance testing, and a lab that runs regular blind reference samples, will say so on the packaging or product page. If a cheap kit or off-brand monitor cannot point to either listing, treat its accuracy as unverified. This single check separates a reliable $20 canister from a mystery one.
Placement is where most home tests go wrong
Where you put the kit changes the result more than which quality kit you bought. EPA’s protocol:
- Test the lowest lived-in level of the home — a finished basement counts if it is used, otherwise the ground floor.
- Set the device at least 20 inches above the floor and at least 3 feet from windows and exterior doors.
- Keep it away from drafts, exterior walls, supply vents, direct sun, and sources of high heat or humidity.
- Avoid kitchens, bathrooms, and laundry rooms — moisture and exhaust fans skew charcoal readings.
- For any short-term test of four days or less, hold closed-house conditions: keep windows and exterior doors shut except for normal in-and-out, starting 12 hours before the test and continuing through it.
Long-term detectors are far more forgiving of a single open window because they average it out. For a step-by-step on siting, see where to place a radon test.
Why kit choice matters more on the Front Range
Colorado sits on uranium-bearing granite and soils, and much of the Denver metro and northern Front Range falls in EPA’s highest-risk radon zone (Zone 1), where the predicted indoor average exceeds 4.0 pCi/L. State health data has long put roughly half of Colorado homes above the action level — a far higher hit rate than the national average of about 1.3 pCi/L. That makes testing here less of a precaution and more of an expectation, and it is why a winter-inclusive long-term test or a continuous monitor often earns its cost in a Colorado basement. The EPA radon zones guide shows how the county maps are drawn, and the broader radon testing guide walks the full path from testing to mitigation.
Choosing, and what a high reading costs to fix
Pick by your constraint, not by price alone. Under a real-estate deadline, a NRPP-listed charcoal kit with a fast-turnaround lab fits the window. Living in the home long-term with no deadline, an alpha-track detector gives the most honest year-round number. Wanting ongoing proof — especially after mitigation — a continuous monitor amortizes over years. A confirmed result at or above 4.0 pCi/L is fixable: sub-slab depressurization, the standard system, typically runs $1,500–$3,000 and cuts indoor radon by 90% or more. Compare that to a kit costing under $50, and testing is the cheapest health decision in the house. For the full picture on fixing a high result, see what radon mitigation involves.
Frequently asked questions
Are hardware-store radon kits accurate?
Yes, when they are NRPP- or NRSB-listed and processed by an accredited lab, a $20 charcoal canister gives a valid short-term reading. The common failure is not the kit but the deployment — wrong room, too close to a window, or an open door during a short-term test.
Can one 48-hour test tell me whether to mitigate?
It can flag a problem but should not be the sole basis for spending on a system. EPA recommends confirming any short-term result at or above 4.0 pCi/L with a second short-term or a long-term test, because a single window can miss the home’s true average.
Do I need a new kit for every test, and do they expire?
Charcoal canisters and alpha-track detectors are single-use and do expire — charcoal loses adsorption capacity with age, so check the use-by date and don’t test with a kit that has been sitting in a garage for years. Electronic monitors are reusable indefinitely.
Charcoal kit or an electronic monitor for a Colorado basement?
If you want one honest number and plan to act on it, a long-term alpha-track or a repeated short-term test through winter beats a single summer canister, because Front Range levels climb during the heating season. If you want to watch trends and verify a mitigation system over time, an electronic monitor is worth the higher cost.
This page reflects current EPA and Colorado Department of Public Health and Environment (CDPHE) radon guidance and is general information, not medical advice. Front Range homeowners with a confirmed reading at or above 4.0 pCi/L can reach a vetted mitigation contractor through our contact page.