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'm just amazed it apparently took two people to think of taping 4 air filters together with a fan on top.
They may not have been the ones who first thought of it, but the first people to publish research that shows that taping 4 air filters together with a fan works decently compared to other, non-DIY filters (where "worked decently" means does a good enough job that it's worth doing if you want a filter but can't/won't buy or build a more complex filtering system, even if it isn't as good as "proper" filtering systems)
(Tangent: I'm reminded of the "Cox-Zucker machine", a random maths thing that Cox and Zucker made together because when Cox and Zucker met at university, they realised that their names, when combined in the traditional, alphabetical order, sounded "delightfully obscene". )