You clearly haven't been to the US.

You mean bloodstream CO2, if not what other form?

It might just be slow enough that we don't notice. We know that increased CO₂ indoors leads to poorer performance from studies. A widely discussed concern is that outdoor CO₂ is increasing and that this could raise indoor baseline levels, meaning buildings might reach cognitively relevant concentrations more easily unless ventilation improves. The issue is not that today’s outdoor CO₂ levels are directly toxic; rather, higher outdoor concentrations reduce the “headroom” between outdoor air and indoor levels generated by occupants.