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I use something like this handheld air quality monitor. It's been close enough and useful enough to figure out how well my filter is working:

https://www.aliexpress.com/item/4000312301074.html?spm=a2g0o...

PMS5003

also a cheap box fan and 20x20x1 merv 11+ rating filter should work wonders.

You can build your own air quality sensor with our instructions. Our sensor uses the same PM sensor like in the purple air. You can build it for <USD 20.

https://www.airgradient.com/diy/

BTW: Our main product is an air quality monitoring service specifically designed for schools. Since we are just starting, I would love to talk to parents, teachers etc. from schools that want to monitor their air quality better. Please contact me if you are interested to talk!