I recently stayed in an apartment that didnt have central air so I created a corsi-rosenthal box since the smoke from the Canadian wildfires were so bad.
After 30 days of continuous use, with very minimal periods of it being turned off, this is what the filters look like!
It's disgusting yet also so satisfying to see the filters get darker from debris, dust, and dirt.
Edit: typos
this is true. I pick up filters from shut down businesses and warehouses sometimes when people dont know what they cost and post them to fbm or craigslist and if they are higher than 13 I also put a filter for the bottom panel since I usually hang them blowing down with cords taped at the corners. even then they start to suck inward.