This is relatively minor, but it would it be possible not to show a cookie notice to people loading the site from countries such as the US where these notices are not required?

Interesting! I hadn't realized people had started using the term this way!

That's a box fan CR box

A filter with PC fans isn't a "CR box" -- a Corsi-Rosenthal box is specifically a design based on box fans. For example, Wikipedia, US Davis, Clean Air Crew, and the Corsi-Rosenthal Foundation mention only box fans.

Good point! I'm used to houses with the older systems (steam, forced hot water) that are common in the Northeast and wasn't thinking about this effect.

Efficiency in terms of fraction of removed particles wouldn't decrease, but because CFM will decrease efficiency in terms of CADR will too.

Ceiling fans are generally reversible, so you could have the ceiling fan blowing air up and mount the filters for airflow in the opposite direction. I haven't tested this configuration though!

There is actually this study from the national labs that indicate filters hardly need replacement at all. I think it is even worse than razorblades that grow dull - filters last forever!

That study is looking at nuclear facilities, but I'm not sure it generalizes to environments with more particulates in the air. In a dusty enough environment your filter will surely get clogged up to the point where you're not able to move much air through it!

Sure!

I do have my question about your design (and GPT o4-mini-high is pretty skeptical), wondering whether the fan really draws all the air through the filters.

When I tested the prototype it worked well: https://www.jefftk.com/p/ceiling-air-purifier

Now that I know more about how these fans move air, I think it would work even better if the filters extended slightly lower.

Sure!