Where do you test for radon? Why test for radon in the lowest, finished, living area? - WNC Independent Labs
The concern is long term exposure, so the test should be conducted in an area where people will spend a significant amount of time (which is usually a finished living area).
The lowest level is used as radon comes up from the ground and as it travels up through the house, there are more opportunities for it escape the house, so you are typically going to have the highest concentration on the lowest level.
30 January 2022 Radon Testing FAQ