[rhd]

I went down a similar journey over the last couple years (CO2, VOC, PM1.0/2.5/10 sensors). In my apartment I sit around zero for particulate with windows closed. I jump up significantly with windows open unfortunately (for my allergies) in the midwest.

I do wish my AC/Heat had more air exchange, similar to your space my CO2 levels rise dramatically after an hour or two of occupation with windows closed.