the requirements may rule each other out

I am considering buying a radon detector.  The one that has been recommended to me has these instructions for placement
>>The detector may be placed face up on a tabletop, counter, tip, or any flat surface where the ventilation slots will not be blocked. The detector must be kept free of dust. A proper airflow must be maintained through the detector to obtain an air sampling representative of the local enviror4enoment.<<

How  am I supposed to keep a flat surface free of dust if I can't cover it with anything?