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
I built one back when I lived in California during fire season, and then again during the pandemic. They do such a nice job making the air less gross.
I think that's about what mine looked like after 3 or 4 weeks too...