Comment by ekianjo

There's tons of things in the air. Dust (and many different types of dust at different levels, both in contents and size), other gases at the trace level - lack of ventilation not only increases CO2 levels but everything else, too. And then there's the people factor. Who says that people who have proper ventilation and CO2 control are the same as the ones who don't?

If they really want to do a robust study they need to do an intervention study with clear levels of CO2 accurately controlled and the rest of the air being identical for everything else, otherwise it's purely meaningless. (it's doable, by the way).