I got Xiaomi purifyers, it had the best test results , < $100 used at ebay

air-quality can be measured with PM2.5 measurement devices,
I have a mobile one for < $50 , or just look it up for your region
from EPA (in USA)

indoor air CO2 increases when you are breathing in a small room a lot
e.g. at night it goes up to 1500 in my room and attracts mosqitoes
also < $50 for my device , they used it for classrooms during COVID-waves