I use something like this handheld air quality monitor. It's been close enough and useful enough to figure out how well my filter is working: https://www.alie

I use something like this handheld air quality monitor. It's been close enough and useful enough to figure out how well my filter is working: https://www.aliexpress.com/item/4000312301074.html?spm=a2g0o.productlist.0.0.37c13648L2Gy2g&algo_pvid=9e24c17f-5d44-40e7-89d5-81c943cca0ae&algo_expid=9e24c17f-5d44-40e7-89d5-81c943cca0ae-21&btsid=0bb0624616001459071324782e0b16&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_ https://www.aliexpress.com/item/4000312301074.html?spm=a2g0o...