There are sufficiently many aspects of air quality (NOX, PM2.5, PM10, O3, SO2, CO, CO2, VOCs, humidity) that a basic trustworthy monitoring station would set yo

There are sufficiently many aspects of air quality (NOX, PM2.5, PM10, O3, SO2, CO, CO2, VOCs, humidity) that a basic trustworthy monitoring station would set you back at least $500. I'd say get a trustworthy temp/humidity/CO2 sensor, since those are the only ones you really emit yourself, and get the remaining data from your local environmental info agency - check e.g. aqicn.org . Edit: is your air filter a HEPA unit? (Does it look like a big car engine air filter?) Do you change the filter every 6 months? Those should help.