Air quality is not so great right now at 11 am on August 19, 2020 with particulate matter (PM 2.5) ranging up to 150 locally*. I used a phone linked sensor to measure PM 2.5 around my house this morning and got some surprising results. Inside PM 2.5 averaged 40 ppm. When I went outside the readings started to drop going below 20 ppm. This made no sense until I removed the sensor from my pants pocket and the results went up to 90. I tried this a few more times (see attached graphic) and got similar results.
The polyester material was acting as a fairly effective filter. I then alternately covered the sensor with paper towel layers, first 2 layers then 1. Removing the paper towels sent readings up to about 100 ppm and restoring the paper layer brought the readings back down to 20 (2 layers) and 40 (1 Layer). One other observation was seeing a difference in whether the pants pocket zipper was open or closed. The readings were lower when the zipper was closed, so having a mask that fits well is important if you really want that extra protection.
So if you are wondering if you could use paper towels as an inner liner for a cloth mask instead of buying the commercial and mostly unavailable paper filter liners sold together with some cloth masks, maybe this info will help you decide.
I may test other materials later, but this is dependent on an ambient air quality that I hope does not last.
* https://www.purpleair.com/gmap?&zoom=13&lat=37.92674868882685&lng=-122.28011027597898&clustersize=23&sensorsActive2=10080&orderby=L&nwlat=37.988670840901264&selat=37.864774345189225&nwlng=-122.43795291208738&selng=-122.12226763987059
