Why not look at bare HVAC sensors used for office building air quality monitoring? - https://new.siemens.com/global/en/products/buildings/hvac/sensors/air-qual

Why not look at bare HVAC sensors used for office building air quality monitoring? - https://new.siemens.com/global/en/products/buildings/hvac/sensors/air-quality.html https://new.siemens.com/global/en/products/buildings/hvac/se... - https://buildings.honeywell.com/us/en/brands/our-brands/bms/field-devices/iaq-sensors https://buildings.honeywell.com/us/en/brands/our-brands/bms/... Granted, they're not meant for standalone use but at least you're not wondering whether the vendor is reputable or whether the sensor does what it says it does. For example, at a casual glance, the Siemens QPA2002D VOC and CO2 sensor seems to be available for <$500 and comes with a fairly comprehensive datasheet.