[amluto]

My general theory is that HEPA makes sense for filtration where a single pass is all you get. So it makes sense for vacuum cleaners, air intakes, and such. But, for room air cleaning, what matters is the rate at which particles are removed, not the cleanness of the air coming out. So, for recirculating, something like a MERV 13 filter seems like a better choice: it lets more particles through on each pass, but it also restricts airflow less.

[dredmorbius]

PM2.5 particles are small enough that they pass through many materials.

You can use lower-efficiency (higher effective particle-size) filters ... but you'll just be blowing the bad stuff through them rather than removing it.

The net metric is how fast the room particulate level drops, which is the treatment goal. Flow rate is a red herring.

[amluto]

That’s why I suggested MERV 13 and not, say, MERV 8. If your filter removes a mere 50% of particles at a given size but moves twice as much air as a HEPA filter, you’re doing fine for a recirculating filter.