Since indoor air quality/IAQ is a composite metric, I'd say that some of the interesting things to measure are: PM2.5/PM10 - particles from combustion, exh

Since indoor air quality/IAQ is a composite metric, I'd say that some of the interesting things to measure are: PM2.5/PM10 - particles from combustion, exhaust, traffic (wheels against asphalt) etc CO, NOx - from traffic exhaust CO2 - from humans VOC - volatile organic compounds, IIRC correlated with cancer and lots of unpleasant stuff. Comes mostly from paint and manufactured stuff (furniture etc) at home. I'm about to make a small sensor thingee that measures these plus relative humidity (too dry at home, 30--50% should be good, we're at 10-15%), temperature (16-20 when sleeping), and for fun barometric pressure, ambient light. For this I'm going to use a Honeywell HPMA-115S0, and IIRC BME680 and some other stuff. If interesting I can have a closer look. Battery-operated devices are a little bit of a challenge since often some of these sensors internally use a heating element for a long duration, so they consume quite a lot of power. The Plantower looks interesting, thanks for the link.