The place to look is existing codes for ventilation. Exempli gratia: https://dos.ny.gov/system/files/documents/2020/09/2020-mcnys... (see PDF page 46). Regulations to enforce outside air being brought into human spaces already exist.
I have been in some office buildings in United States which had CO2 monitors in each meeting room, and the ventilation would engage to control CO2 below a set level. We would entertain ourselves by exhausting our lungs onto the sensors to trigger the ventilation system.
In terms of outside air, a lot of US cities I think would not benefit from that, all that much. Especially during certain parts of the day, with a lot of smog.
But regardless, all that entered my mind was "Once employers are required to add any form of scrubbing, and perhaps O2 injection, they'll over do it for optimal employee output." Regardless of whether it's helpful once the employee leaves the workplace.
I'm not against this, I'm just actually saying the regulation should be locally defined.
Scrubbing indoor CO2 is sensible only when you want to go below the outdoor CO2 level, not at levels above it.
The main question is: If your workplace, city, whatever forces you to work or live in an harmful/unhealthy environment, do you have any realistic course of action to improve the situation? In the US you would call this (gasp) regulation, I would call it a basic human right.
If we talk about stairways, nobody complains about building regulations that mandate handrails. CO₂ levels are not totally different.