I am relying on 2 purple air sensors 10 miles away in opposite directions for an outside estimate. They are usually within 10%, and we are in the same valley, s

I am relying on 2 purple air sensors 10 miles away in opposite directions for an outside estimate. They are usually within 10%, and we are in the same valley, so should be close. I also see when I stop the furnace fan, levels spike. Then drops when I turn on again...