A Long-Term Detector That Moved to the Garage
A homeowner with a prior 48-hour result of 5.8 pCi/L in the basement orders a 91-day alpha track detector. You place it in the basement bedroom where she sleeps, on a shelf 5 feet above the floor, away from the exterior wall, and log a start date of March 3. She returns the detector on June 14, 103 days later. In the enclosed questionnaire she notes that the basement was gutted for a remodel from April 20 to May 18, that the detector was moved to the attached garage during that time to keep it from getting dusty, and that she slept upstairs in the interim. The lab reports 4.6 pCi/L and flags the exposure as extended. She asks whether this proves the basement is safe to finish as a rental unit.
1. Is the 4.6 pCi/L result a valid estimate of the basement bedroom's radon concentration? Explain.
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No, for three reasons.
- Location change. The detector did not stay where it was placed. It spent about four weeks in the garage. A detector measures radon at its own location, so the reported value is a time-weighted blend of a basement bedroom during a remodel and a garage - not a bedroom average.
- Exposure length. The planned exposure was 91 days; it was returned after 103. Any exposure beyond the calibration period adds uncertainty unless the lab applies a documented correction, and the report should state which.
- Conditions during the remodel. The basement was gutted, likely with more ventilation and open doors, which would lower the bedroom contribution.
The result cannot answer the question she is asking - whether a finished rental unit will be safe. It should not be reported as a basement bedroom result, and it should not be used to reassure a future tenant.
2. How does the garage exposure affect the number, and what information do you need to interpret it?
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The garage exposure pulls the result in an unknown direction. A garage is typically leakier and closer to outdoor air, so it likely reads lower than the bedroom, which would dilute the bedroom value. But a garage can also hold soil gas or sit on the depressurized side of the house.
Without knowing which, you cannot bound the bias.
What you need:
- The exact dates the detector was in each location.
- Whether the garage was attached, heated, or open to outdoors.
- Whether the basement was ventilated during the remodel.
- Occupancy during the bedroom period.
- How the lab handled the 103-day exposure.
The questionnaire is the key record, which is why it must be completed and reviewed before the number is interpreted to a client.
3. What do you recommend before the owner finishes and rents the basement unit?
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Wait until the remodel is finished and the unit is under normal occupancy. Then place new detectors on the lowest livable level of the future rental - two devices, or one device plus a CRM for cross-check - and run a long-term exposure of at least 90 days.
If she needs an answer before leasing, a 48-hour closed-house test is acceptable but is only a snapshot; say so explicitly.
Test each distinct foundation area, since the unit may have different slab and wall construction. If the result is 4 pCi/L or higher, mitigate before renting. At 2–4 pCi/L, discuss mitigation and continued monitoring.
Retain the original 4.6 pCi/L result with a note explaining the relocation, so the record is honest rather than quietly discarded. Also check any state or local requirement for landlord testing and disclosure before a lease is signed.