I can't direct you to a reference. Most of what I wrote is based on two things: 1) My own experiments. You can monitor PM2.5 levels accurately with a PMS5003

I can't direct you to a reference. Most of what I wrote is based on two things: 1) My own experiments. You can monitor PM2.5 levels accurately with a PMS5003 ($40 on Adafruit) plugged into an ESP32 ($10 on Adafruit) and ESPHome + Home Assistant, or you can splurge and get a full suite of sensors (<$100). That's what I did because it's fun and educational. Expect to spend a few hours setting it up. It's well-documented, but it's not quite plug-and-play. Friday evening will likely suffice, but it might be a full weekend if you run into problems. Heck, if you want to monitor pressure drop across your filter, you can get a pair of pressure sensors ($7 each, or $20 for a fancier multisensor like a BME688). Or you can get an Apollo Air or an AirGradient for <$200. Or you can get something from a company like Ambient Weather for easy and fully integrated. Don't go for cheap on Aliexpress / Amazon / eBay, though, since many of these are fakes. Monitoring PM2.5 levels in my room, I would see a drop in filtration if I needed to replace a filter due to filtration issues. Fans also get louder when they're overloaded. That's a good sign I need to vacuum something. You could do acoustic monitoring too, if you were fancy :) 2) Basic theory. A good book on -- of all things -- acoustics will show you how to model air pressure as voltage, something like a filter as a resistor, and a fan as a source. That gives good intuition for how filters stack. 3) Reading web forums, where others did similar. That matches what I see and theory. From that: The best design I see (e.g. a Coway AirMega 200) has three layers: - Washable prefilter - Carbon filter - HEPA filter Just using it, I see most of the stuff caught by the prefilter, followed by carbon, followed by HEPA. It's visible. I see the same thing on things I've built (e.g. box fan + high MERV + super-cheap filter duct-taped together in my basement), and in things like vacuum cleaners. So there you have it. My comments were based on my intuition backed up by my measurements at my house. Your mileage may vary. I can well imagine these things being different in a rural home with all sorts of pollens versus a city home with pollution from cars. The washable prefilter is very thin. A sheet would fit without modification. But if it were me, I'd modify and use a full stack, because it's fun to make things :)