Which sensors actually matter for indoor air quality?

For most homes: PM2.5 (combustion, cooking, wildfire smoke), CO2 (ventilation), humidity (mold risk), and VOCs (off-gassing, cleaning products). If you have a basement or live in a radon-prone region, add a dedicated radon sensor — that's the single biggest reason to step up to an Airthings Wave Plus or View Plus rather than a $70 generic monitor.