CO2 ppm is the main number that research studies look at, but it's also a proxy for the "freshness" of the air. All the air quality metrics are correlated. Eg a lot of cheap CO2 meters measure something else like TVOC and convert it to eCO2 using a lookup table.

CO2 scrubbing would be better than nothing, but it's really expensive and won't improve other metrics like TVOC