Start with the decision, not the device
Radon varies. A short measurement answers a time-sensitive question; a long measurement is more likely to reflect a year-round average. EPA describes short-term tests as lasting from two to 90 days and long-term tests as lasting more than 90 days. The instructions for the specific device control its minimum exposure time.
Closed-house conditions are not “seal the building unnaturally”
For a two-to-four-day short-term test, EPA’s real-estate checklist calls for closed-house conditions for at least 12 hours before the test and throughout the exposure. Windows stay closed, exterior doors open only for normal entry and exit, and fans or machines that bring in outdoor air do not operate. Heating and cooling run normally; follow the guide for the device and test type.
Placement should represent regularly used space
EPA’s real-estate guide says to test the lowest level that could be used regularly. Follow the device manufacturer or qualified tester’s instructions for distance from the floor, exterior walls, drafts, heat, humidity and direct sunlight. Do not move or cover the device.
What an hourly monitor can—and cannot—show
A continuous monitor can show changes over time and may help detect interference. An hourly pattern does not, by itself, prove that a particular rise came from soil, rain, HVAC operation or any other single cause. Interpret the valid test result, not a story attached to one point on the curve.
After a high short-term result
CDC advises a second test after a high short-term result, using either a short- or long-term device; if a second short-term test is used, it describes averaging the two results. A transaction can have its own timing and protocol, so use a qualified Florida-certified measurement business when a defensible third-party result matters.