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Put the detector in the lowest room you actually use, at breathing height, away from outside walls, draughts and heat sources. Close the house for the test period and leave the instrument alone. Read the long-term average, not the first reading; a fast result is a preliminary one. A plug-in unit such as the Safety Siren Pro4 or a battery unit such as the X-Sense XR0B-SR both work, provided you give them the weeks the physics requires.
What the detector is counting, and why that sets the waiting time
Radon is a gas produced by uranium decaying in soil and rock. It seeps into buildings through cracks, sumps and joints. The gas itself is not what does the damage; its short-lived decay products, called progeny, are solids that attach to dust and get inhaled. A radon detector counts the alpha particles those decays produce, either in a chamber or on a surface, and converts the count rate into a concentration in becquerels per cubic metre or picocuries per litre.
The count is statistical. Alpha emissions arrive at random intervals, so a short counting window gives a noisy estimate. The uncertainty falls with the square root of the number of counts, which is why a one-hour figure and a three-month figure are not the same kind of number. A detector that reports in ten minutes is showing you a preliminary estimate, not a verdict. The GZAIR unit states a first reading in 10 minutes and reliable data after 1 hour, which is a fair description of a fast instrument, but one hour still cannot average out the daily breathing cycle of a house.
Radon concentration swings with weather, stack effect and occupancy. It is usually highest at night and in winter, when the indoor-outdoor temperature difference is largest and the house is closed. A short test that happens to land in a windy week can read low. That is why the standard advice is to test over months, not minutes.
Placement: the lowest lived-in room, not the corner that is convenient
Radon enters low and accumulates where air is still. The lowest level of the building that people use is the place to measure. If you have a basement that is used as a bedroom, office or playroom, that is the priority location. If the basement is a sealed crawl space nobody enters, measure the ground floor instead, because that is where exposure happens.
Height matters less than air movement. Place the detector at roughly breathing height, on a shelf or table, at least a metre from an outside wall and away from doors, windows, vents, radiators and strong draughts. Do not put it in a cupboard, behind furniture or next to a dehumidifier. Do not put it in a kitchen or bathroom, where steam and aerosols interfere with the chamber. A plug-in unit needs a socket, so choose the socket for its position, not the other way round.
One detector in one room is a sample, not a survey. If the building is large or has several low rooms, the honest approach is to test the likely worst case first, then move the instrument if the result is low and you want confirmation. The Airthings Corentium Pro is portable and tamper resistant, which suits a technician moving between rooms or between buildings; a fixed household unit stays put.
The closed-house period, and what it does and does not simulate
A short-term test is normally run with doors and windows closed, except for normal entry and exit, for the duration of the test. The purpose is to measure the building as it is used in the season when radon is highest, not to trap a worst case artificially. Closing the house for a few days is a compromise: it removes the dilution from open windows, but it does not reproduce a whole winter of stack effect.
Practically, this means you should decide the test period before you start and then leave the instrument alone. Opening the house to air it out halfway through invalidates the average. If you must ventilate, note the time and treat the result as a lower bound. The listing for the X-Sense XR0B-SR states results in 24 hours and accuracy reaching 95 percent after 7 days or more, which is a useful way to think about it: the first day tells you whether to keep going, and the week tells you what you have.
Long-term tests, typically three months or more, are the ones that correlate with risk. If your short-term result is near a threshold, the correct next step is a long-term measurement, not a second short-term one in the same week.
Reading the screen: instantaneous, short-term and long-term are different numbers
Most household detectors show more than one figure. The instantaneous reading updates every few minutes and is the noisiest. The short-term average covers days. The long-term average covers weeks or months and is the one that should drive a decision. The table below sets out what the supplied listings publish, and where they are silent.
| Instrument | What the listing says it displays | Sensor or method stated | Time to first result | Alarm |
|---|---|---|---|---|
| X-Sense XR0B-SR | Real-time tracking; audible and visual alerts | Not stated in the listing bullets | 24 hours; 95 percent accuracy after 7 days or more, as stated | Sound and light when threshold exceeded |
| GZAIR | Real-time, 1-day and long-term averages; app with daily, monthly and trend views | Pulse ionization chamber, 30 cph/pCi/L sensitivity, as stated | 10 minutes first data; reliable after 1 hour, as stated | Not stated in the listing bullets |
| Safety Siren Pro4 | Real-time digital reading, short-term and long-term averages | Not stated in the listing bullets | Not stated in the listing bullets | Three audible modes plus visual alert |
| Airthings Corentium Pro | Not stated in the listing bullets | Not stated in the listing bullets | Not stated in the listing bullets | Not stated in the listing bullets |
The lesson from the table is that the published figures differ in kind. One listing quotes a sensitivity in counts per hour per picocurie per litre; another quotes a percentage after a week; another quotes nothing about the sensor. None of these is directly comparable without knowing the counting chamber volume and the calibration reference. Treat the published numbers as claims to be checked against the standard the instrument is certified to, not as a league table.
What to do with the number once you have it
Action thresholds are set by regulators, not by manufacturers, and they differ between countries. The common reference points are around 100 to 150 becquerels per cubic metre for a long-term average in a home, with remediation advised above that. The exact figure in your jurisdiction is the one to use; the detector’s job is to give you a defensible average, not to make the policy.
If the long-term average is above the reference, the next step is a professional assessment, not a bigger detector. Mitigation usually means sub-slab depressurization: a fan and pipe that draw soil gas from beneath the building and vent it above the roof, so the pressure under the slab is lower than indoors and the gas goes around the building rather than through it. Sealing cracks alone is rarely enough. A detector can verify that mitigation has worked, but it cannot substitute for it.
If the average is below the reference, keep the instrument running through a winter before you conclude the building is clear. Radon is seasonal. A summer average is the least informative number you can collect. For a wider view of which instruments suit which building, see how to choose a radon monitor and radon monitors for basements.
The maintenance nobody mentions, and the limits of the instrument
A radon detector has little to maintain, but it is not maintenance free. The chamber must stay clear of dust and insects. Batteries must be replaced before they fail, because a gap in the record is a gap in the average. The listing for the X-Sense unit states a 2-year battery life and low-power notifications, which addresses the common failure mode; a plug-in unit such as the Safety Siren Pro4 depends on mains power and will stop measuring in a power cut unless it has a battery backup, which the listing does not state.
Calibration is the other quiet issue. A detector that has drifted reads consistently wrong, and you cannot tell from the screen. The GZAIR listing states that its sensors are calibrated and that no lab fee is required; the Airthings listing states AARST/NRPP certification. Those are different kinds of assurance. Certification is a statement about the instrument and the professional who deploys it; a factory calibration is a statement about the sensor at the point of manufacture. Neither removes the need to replace or recalibrate an instrument after years of service, and the listings do not publish a recalibration interval.
Finally, know where the instrument stops being the right answer. A radon detector does not measure carbon monoxide, mould, or general air quality, and it is not a substitute for a professional survey when a property transaction or a mitigation decision is at stake. If you need a broad indoor air picture, a radon detector is one instrument among several; see the radon detector comparison for the range of options and radon test kits for the low-cost laboratory route.
The short version
Place the detector in the lowest lived-in room, at breathing height, away from draughts and heat. Close the house for the test period and leave the instrument alone. Read the long-term average, not the first number on the screen. If the average is above your jurisdiction’s reference level, call a mitigator; if it is below, run it through a winter before you stop. The instrument is the easy part; the discipline of placement and patience is what makes the number worth having.
Frequently Asked Questions
How long do I need to leave a radon detector running before the reading means anything?
For a decision, a long-term average of three months or more is the standard. A week gives you a useful indication, and a day tells you whether to keep going. The listings differ: the X-Sense unit states 24 hours for a result and 95 percent accuracy after 7 days or more, while the GZAIR unit states a first reading in 10 minutes and reliable data after 1 hour. Those are preliminary figures, not long-term averages.
Where exactly should the detector go?
The lowest room that people actually use, at breathing height, at least a metre from an outside wall, away from doors, windows, vents, radiators and draughts. Not in a kitchen, bathroom or cupboard. If you have a basement bedroom, that room comes first. If the basement is unused, measure the ground floor.
Do I have to keep the windows closed the whole time?
For a short-term test, yes, apart from normal entry and exit. Opening the house dilutes the radon and gives you a lower number than the building would produce in normal closed use. If you ventilate during the test, note it and treat the result as a lower bound.
My detector shows a high number one day and a low number the next. Is it broken?
Probably not. Radon concentration varies with weather, wind and the indoor-outdoor temperature difference, and the instantaneous reading is statistically noisy. That is why the long-term average is the number to act on. A single spike is not a reason to panic or to move the instrument.
Can I use a radon detector to check that my mitigation system is working?
Yes, as a verification tool, but it does not replace the mitigation system. Sub-slab depressurization is the usual fix, and a detector run for a full season after installation will show whether the average has fallen. A short test immediately after installation is not enough to confirm performance.
What does the detector not measure?
It does not measure carbon monoxide, carbon dioxide, mould or general air quality, and it is not a substitute for a professional survey when a property transaction or a mitigation decision is at stake. It measures radon and, depending on the sensor, its alpha-emitting progeny. Treat it as one instrument with one job.