I always thought that you need to clean your air filter once a month to stop the dirt entering into your AC/heat unit. NOT THE CASE ANYMORE! I took out my air filter, and it was dirty, and placed the GMC-300 on top of it. The GMC-300 started rising from 18 to 54 them the beeping started. The readings were over 100, then 200+, them 400+. It level off around 500. I can not win, do I want clean air or do I want radiation free. I am killing myself without knowing it.
I have two Gmc-300 in front of my filter while the unit is running. Both GMC-300 are reading over 120 cpm. If I turn off the filter for about 2 hours the GMC-300 go back to 20 cpm. I think it is radon gas because radon gas is 9x heavy than air. Maybe you do not have radon gas in your house???