How a radon test kit works
A radon test kit measures radon gas in picocuries per liter (pCi/L), either with a passive detector you mail to a lab or a continuous monitor that reports the number itself. Most kits need the house closed up for the test period, since open windows dilute the reading.
Maybe a kit showed up in the mail after an order, or a real estate agent asked for a result before closing on a house. Either way, the box itself gives no clue what it's doing while it sits on a shelf for a week or two. This post covers the different kinds of radon test kits, how each one actually detects the gas, where to put one, and what to do with the number once it comes back.
Key Takeaways
- A radon test kit works by either absorbing or counting radon decay (a lab-analyzed canister or track detector) or by measuring it electronically in real time (a continuous monitor).
- Short-term kits run 2 to 90 days; long-term kits and continuous monitors run more than 90 days and catch the seasonal swing a short test misses.
- The EPA's action level is 4 pCi/L. Below 2 pCi/L, it doesn't recommend any step.
- Every kit works best on the lowest level you actually use, at least 20 inches off the floor and away from drafts, doors, and windows.
- A test kit measures. It doesn't fix a high reading, and only a mitigation contractor's vent system does that.
How a radon test kit detects the gas
Most radon test kits sold for home use are passive: no battery, no display, just a material that reacts to radon while it sits in the room. A charcoal canister absorbs radon onto activated carbon over the test period, then goes to a lab, which measures the radioactivity left in the charcoal. An alpha track detector works differently: a small plastic film records the path of each alpha particle radon gives off as it decays, and the lab counts the tracks under a microscope after the test ends. An electret ion chamber measures a drop in static charge on a charged disk as radon ions strike it.
A continuous radon monitor skips the lab. It counts decay events electronically and reports a number directly, the same way the rest of a Measured Air kit reports CO2 or PM2.5. The tradeoff is upfront cost against turnaround: a lab-analyzed kit is the cheaper one-time test, and a continuous monitor is the one that keeps reading after the first result.
Short-term and long-term radon test kits
A short-term radon test kit runs 2 to 90 days with the house closed up: windows shut, exterior doors used normally, no fans pulling in outside air. It's the kit most home inspections use, since it fits inside a closing timeline.
A long-term test, or a continuous monitor left running, covers more than 90 days. The EPA's citizen's guide to radon recommends a long-term test for the most accurate picture of year-round exposure, because radon levels shift with the seasons: a closed, heated house in winter pulls more soil gas in through the lowest level than the same house with the windows open in spring. A single short-term kit run in July can understate what the same room sees in January.
Where to place a radon test kit in your home
The lowest level you spend real time in is the right floor for the test. A finished basement used every day qualifies. An unfinished one you visit twice a year doesn't, and the first floor is a better stand-in for the air a household actually breathes.
Height and distance matter more than most people expect. The EPA's placement guidance calls for at least 20 inches off the floor, out of direct sun, and clear of exterior walls, windows, and doors, since any of those can pull in a gulp of outside air and knock the number down for reasons that have nothing to do with the house. A shelf in a closet or a corner of a spare room beats a windowsill every time. Once the kit is placed, leave it alone and keep the house closed up for the rest of the test period; opening a window on day four of a seven-day test produces a number that fits neither condition.
Reading your radon test kit results
A lab-analyzed kit comes back as a single number in pCi/L, averaged over the test period, usually by mail or a lab portal within a week or two of shipping the detector back. A continuous monitor shows the number on a dashboard as it accumulates, and a Measured Air kit keeps that history alongside CO2, PM2.5, humidity, and VOCs, so a January radon average can sit next to last summer's once there's enough data to compare.
Either way, the EPA sets 4 pCi/L as the point where it recommends fixing a home, with 2 to 4 pCi/L worth a second look and anything under 2 pCi/L needing no action. More on what the number means, and how it moves with the seasons, is in radon levels in a home.
A continuous monitor against a single test kit
A single-use kit answers one question once: what did radon average over this specific window. That's exactly what a home sale needs, and it costs less upfront than a continuous monitor.
A continuous monitor answers a different question: what does radon do across a whole year, room by room, as seasons and ventilation change. The real tradeoff is patience. Radon needs months of data to read reliably, unlike CO2 or PM2.5, which respond within hours. If you need a number before a closing date, a short-term kit still does that job faster. If you want to know what your own house does across a winter and a summer, a monitor left running is the one that can tell you.
What to do if your radon test kit reading is high
At 4 pCi/L or above, the EPA recommends hiring a qualified radon mitigation contractor, who typically installs a vent pipe and fan system that draws soil gas out from under the foundation and releases it above the roofline. At 2 to 4 pCi/L, the usual next step is a retest, since a single reading can vary by season and even by week.
Radon exposure is linked to lung cancer risk, a conclusion that comes from the World Health Organization's own guidance on radon and health, not from anything a test kit itself can tell you. A kit's job stops at the number. What that number means for a given person's health is a question for that guidance, or a doctor, not for the canister on the shelf.
Radon next to the rest of a home's air
Measured Air makes kits that report on the air people actually breathe: PM2.5, CO2, VOCs, humidity, and radon, each next to the outdoor air at your address. On the homeowners page, every reading stays in history, which is useful for radon specifically, since one month rarely tells the whole story the way a year of months does.
Radon is the slowest reading of the bunch. A CO2 spike tells you something changed by morning. A radon average takes a season to mean anything. For more on testing every reading at once, see how to test indoor air quality.
FAQ
How do radon test kits work? A passive kit, such as a charcoal canister or alpha track detector, reacts to radon gas over the test period and goes to a lab for analysis. A continuous monitor counts radon decay electronically and reports the number itself, without a lab step.
How long does a radon test kit take? A short-term kit runs 2 to 90 days with the house closed. A long-term kit or continuous monitor runs more than 90 days, which gives a more reliable picture since radon shifts with the seasons.
Where should I put a radon test kit? On the lowest level you regularly use, at least 20 inches off the floor, away from drafts, doors, and windows. If the basement isn't lived in, test the first floor instead.
Are radon test kits accurate? A kit used correctly, placed away from drafts and left for its full test period, gives a reliable reading for that window of time. The bigger source of error is usually placement or a shortened test, not the detector itself.
Can an air quality monitor replace a radon test kit? A continuous monitor that reports radon, including a Measured Air kit, can replace repeat short-term tests over time, but it needs months of data to be reliable. A single-use kit still answers faster if you need a number before a specific date, such as a home sale.
What happens after a radon test kit comes back high? At 4 pCi/L or above, the EPA recommends a qualified mitigation contractor, who installs a system to vent soil gas from under the house. Between 2 and 4 pCi/L, the usual next step is simply retesting.
The bottom line
A radon test kit works by either absorbing or counting radon decay in a lab-analyzed canister or track detector, or by measuring it directly in a continuous monitor. Short-term kits answer in days; long-term kits and continuous monitors take more than 90 days but catch the seasonal swing a short test misses. Whichever kit you use, the number that matters is pCi/L, and the EPA's line for action sits at 4 pCi/L.
Placement decides the result as much as the kit does: lowest level you use, away from drafts, left alone for the full period. Order a kit sized to your home and start a radon baseline alongside the rest of what's in your air.