Air filter to catch radioactive particles - suggestions please

The air purifier's was on overnight and when I tested the filters this morning the HEPA type filter was not registering any hotspots, but overall was giving a reading of 0.36 microsieverts against a desk background reading of 0.15.

I really should not switch the purifier on and just see if the activity falls over time. I suspect it will as the activity is I think radon related.

Air purifier filters now back to background level of radiation after a few days of increased activity, I think due to the unsettled weather here leading to radon washout.

I'll keep checking twice daily and keep you all informed of any changes.

Yesterday the weather was cooler than usual, windier than usual and much less humid than usual. That changed during the night when the wind dropped completely and the humidity went back to the seasonal norm, then it rained briefly.

The results from my air sampling taken this morning. Results are the average after 10 minutes using a GeigerPal

Outside background - 0.11 microsieverts
Outside 150g mesh bag of activated charcoal held in a length of plastic drainpipe exposed to passing air - 0.25 microsieverts
Inside background - 0.16 microsieverts
Inside HEPA type filter from air purifier, air intake side, taking in air from open window - 0.36 microsieverts

Yesterday nothing was deviating from the background count, either inside or outside.

Oh, for more sensitive equipment!!!

Another experiment for your delight...

In my apartment building there is a ground level equipment store which is only accessed a few times a month. Today I took the air purifier down to it and left it running for one hour.

Before I did this I measured the background radiation level in my apartment using a GeigerPal. It was 0.15 microsieverts. The normal level.

I measured the radiation level on the HEPA type filter and it averaged out at 0.29 microsieverts. As it's filtering the air in a building which has a fair amount of sandstone and granite in its construction, no great surprise.

After the air purifier had been running for one hour in the ground level equipment store I checked the radiation levels.

The air intake side of the HEPA type filter averaged out at 2.93 microsieverts and the air outlet side of the HEPA type filter averaged out at 4.06 microsieverts

I think there is an issue of radon seeping up through the stone floor of the ground level equipment store. How serious this is, I don't know as I don't think you can directly convert these readings into picocuries per liter??

Update.

Some 4 and a half hours had elapsed since the last measurements so I measured again.

Room background radiation at 0.16 microsieverts.

The air intake side of the HEPA type filter now reading 0.45 microsieverts and the air outlet side of the HEPA type filter averaged out at 0.64 microsieverts

I'm really surprised at such a rapid drop in the count. What's going on??