A radon test is only as good as the conditions it’s conducted under. These common mistakes are the most frequent reasons a result doesn’t reflect a home’s actual typical level.
What makes these mistakes worth cataloguing is that none of them produce an obviously wrong answer. A test conducted badly returns a plausible looking number in the same format as a good one, with no indication that anything went astray. There is no error message, no out of range warning, and nothing on the laboratory report to tell you the detector spent three days next to an open window. The result is that testing errors do not present as errors; they present as decisions, either an unnecessary mitigation spend or a false reassurance that leaves a real problem in place for years.
- Keep devices away from drafts, vents, windows, and doors during placement.
- Maintain closed-house conditions strictly for the full test period.
- Unusual weather during a test period is a reason to consider retesting.
- Follow mailing/handling timelines exactly for short-term kits.
Placement Mistakes
Placing a device too close to a window, exterior door, or HVAC vent introduces air currents that can dilute the reading artificially. The EPA recommends placement in the lowest lived-in level, at least 20 inches above the floor and away from drafts.
The specifics of correct placement come from a small number of physical constraints, and knowing the reasoning makes them easier to apply than memorising the numbers. The detector should sit in the lowest lived in level of the house, since that is where concentrations are highest and where people actually spend time, which is why an unfinished storage basement is normally not the right choice while a finished basement bedroom is [1]. It should be at least twenty inches above the floor and below head height, roughly in the breathing zone rather than in the layer closest to the slab. It should be at least three feet from exterior walls, windows, and doors, and away from any obvious draft, because those locations sample locally diluted air rather than room air.
Several less obvious placements produce misleading results for reasons worth understanding. Kitchens, bathrooms, and laundry rooms are unsuitable because humidity affects charcoal adsorption, which is the physical mechanism a short term kit relies on. Heat sources, direct sunlight, and exterior wall surfaces that swing in temperature all affect device behaviour. Enclosed spaces such as closets, cabinets, and shelving units with a solid back sample stagnant air rather than the room. And the detector needs clearance around it, commonly around four inches from other objects, so that air circulates past it freely. A detector on a shelf pushed against a wall behind books is technically in the right room and functionally measuring the wrong thing.
Testing in an unfinished, rarely-used basement when the actual living space is on upper floors can also misrepresent your real exposure — test where people actually spend time, following your device’s specific placement guidance.
Closed-House Violations
Opening windows, running attic fans, or leaving exterior doors propped open during a short-term test period is the single most common cause of an inaccurate result — even brief ventilation can meaningfully lower a reading.
Closed house conditions exist for a straightforward reason: ventilation dilutes radon, so a house tested with windows open will report a level lower than the same house tested as it is normally lived in. The requirement is twelve hours of closed house conditions before a short term test of less than four days begins, maintained throughout. In practice this means keeping windows and exterior doors shut apart from normal brief entry and exit, and not running window fans, whole house fans, attic fans, or anything else that deliberately brings in outside air. Normal use of heating and air conditioning that recirculates indoor air is fine and expected.
The most common way this goes wrong is not deliberate. It is a mild weather week, someone opens the bedroom windows overnight out of habit, and the test period includes twelve hours of ventilated conditions that nobody records. Extended running of a range hood, bathroom exhaust, or clothes dryer is a subtler version of the same problem, and worse in one respect: exhaust only equipment lowers the pressure inside the house, which can temporarily increase soil gas entry as well as change the ventilation rate, so the effect is not a simple dilution. Where a test spans a period the household cannot fully control, the honest options are to note it and treat the result as indicative, or to rerun the test. Discarding an inconvenient result and keeping a convenient one is the mistake that follows from this one.
Weather extremes (high winds, storms) during a test period can also introduce unusual pressure differentials; if unusual weather occurred during your test window, consider retesting under normal conditions.
Handling and Timing Mistakes
Delaying mailing a short-term test kit to the lab after the test period ends can degrade sample integrity, particularly for charcoal-based devices sensitive to humidity and time. Follow the kit’s specific mailing instructions and timeline closely.
Charcoal kits have handling requirements that come directly from how they work. The device adsorbs radon from the air continuously from the moment it is opened, and it continues to do so until it is sealed, so the sealed pouch should not be opened until the detector is in its final position and it should be closed immediately at the end of the exposure period. Because the analysis is based on radioactive decay with a known half life, the laboratory needs accurate start and end times to work backwards to a concentration, and a kit returned with approximate dates or posted late is analysed against assumptions rather than facts. Most kits specify a maximum interval between the end of exposure and arrival at the lab, and exceeding it degrades the result whether or not the report says so.
Timing mistakes at a larger scale are more consequential than handling ones. Running a short term test during a period of unusual weather, such as high winds, a rapid barometric swing, or a storm, samples an atypical condition. Running one in mild open window weather samples close to the best case for the house. And the most consequential timing error of all is treating a single short term result as final. Standard practice after an elevated short term reading is to confirm before committing to mitigation, either with a second short term test or a long term one, because a single measurement can land high or low on weather alone. Retesting after mitigation is the mirror of the same principle, and it is the only way to establish that the system actually achieved something.
Testing during an unusually mild weather stretch and assuming it represents your winter (higher-risk) exposure is another common misread — see [[seasonal patterns]] for why season affects results.
How to Tell a Bad Result From a Real Change
When two tests on the same house disagree, the useful discipline is to work through the possible explanations in order rather than deciding which number you prefer. Start with placement, because it is the largest single source of variation and the easiest to reconstruct. Different levels of a house routinely differ substantially, and two tests in different rooms on different floors are not measuring the same thing at all. If the position is not recorded, that is itself the answer, and it is why writing down the room and the exact placement matters as much as writing down the number.
Next consider season and conditions. A winter test under stack effect and a summer test with periodic ventilation can differ several fold in the same room, with neither being wrong. Then consider device type and duration: a two day charcoal test and a ninety day alpha track detector are answering different questions, one about a specific window and one about an average, so a disagreement between them is expected rather than contradictory. Only after those three have been ruled out is it worth suspecting a genuine change in the house, and at that point the question becomes what changed, since foundation work, a finished basement, new exhaust equipment, air sealing, or a failed mitigation fan all move levels for identifiable reasons.
The way to make this diagnosis possible at all is to record enough at test time to reconstruct the conditions afterwards. Device type and manufacturer, exact start and end date and time, the room and the level, the placement position within the room, whether closed house conditions were maintained and any known breaches, and rough weather notes. That is a couple of minutes of writing that turns a bare number into something comparable. Without it, a disagreement between two results is simply unresolvable, and the usual outcome is that someone believes the lower one.
FAQ
What if I accidentally opened a window during my test?
This can invalidate the result. It’s best to restart the test under proper closed-house conditions rather than rely on a compromised reading.
What should I do if a window was opened during my test?
Note it and treat the result as an underestimate rather than a measurement, because ventilation dilutes radon and the number will read low. If the breach was brief and near the end of a multi day test, the effect is smaller than if it covered a full night early on, but neither case gives you a result you can act on with confidence. The reliable course is to rerun the test with closed house conditions properly maintained. What is worth avoiding is accepting a low result you know was ventilated, since that is the version of this mistake that leaves a real problem undetected.
Does the exact height of the detector really matter?
Enough to follow the guidance. The usual instruction is at least twenty inches above the floor and below normal breathing height, which places it in the air people actually breathe rather than in the layer immediately above the slab. Combined with the requirement to stay at least three feet from exterior walls, windows, and doors and clear of drafts and other objects, the point is to sample representative room air. A detector on the floor in a corner behind furniture is measuring a local pocket rather than the room.
Do I need to retest after installing a mitigation system?
Yes, and it is not optional in any meaningful sense, because it is the only evidence that the system achieved anything. Standard practice is a short term test shortly after commissioning, once the system has been running long enough for the house to reach a new steady state, with the usual window being within about thirty days. A long term follow up spanning the heating season is worth adding, since it confirms performance under the highest stack effect conditions the system will face. After that, retesting every two years and after any change to the system or the house is the normal cycle.
Following placement and closed-house instructions precisely is the single biggest factor in getting an accurate, actionable result.
References
- Radon in Homes, Schools and Buildings. U.S. Environmental Protection Agency
These statements have not been evaluated by the Food and Drug Administration. This information is not intended to diagnose, treat, cure, or prevent any disease. Content is for informational purposes only and is not medical advice; consult a qualified healthcare provider before starting any supplement. As an Amazon Associate we earn from qualifying purchases.
