[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.