[kccqzy]

What makes outdoor air "fresh" compared to indoor air? You said that the temperature and humidity of indoor air are preserved. So is it just CO2 concentration? Would installing a chemical CO2 scrubber have an effect similar to an ERV system then?

[NavinF]

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

[runjake]

CO2 and chemical off gassing from indoor items. Eg: manufacturing chemicals from furniture, natural gas, cooking fumes, etc.