Air filter to catch radioactive particles - suggestions please

I live in an area which was well known for its mining industry. Nickel, silver, gold and lead were mined here. There are a number of granite and aggregate quarries too, some still active. About two hours drive away is a disused uranium mine. The soil is a mixture of light red clay, sand and boulders. Is it a high radon area? Looking at a radon map of Spain, I would say "no", but the hills to the north-west of me are.

Partly under my apartment block is a Spanish Civil War era air-raid shelter which hasn't been filled in. Also there are a series of fresh water wells, dating from the 19th century and again, not filled in, around the perimeter of the building. I think these are acting as reservoirs for radon. I would love if a proper radon survey was carried out, but it's a bit out of my price range to order one.

I'm just surprised how, in only 4 and a half hours, the reading dropped so dramatically.

Just a quick update.

I'm still testing the filters in the morning and in the evening, and there's been nothing significantly above background level during the time period since the last elevated readings.

Another week's gone past, and there's still no significant above background radiation to report......which is a good thing!

After 24hrs of no wind and light, sporadic rainfall, no sun and high humidity, the count is back up.

Indoor background averaged out at 0.14 microsieverts this morning, while the front face of the HEPA type filter was registering an average of 0.36 microsieverts.

Readings tonight are still elevated.

Indoor background - 0.15 microsieverts

Air intake side of HEPA type filter - 0.47 microsieverts