Unless you're locked in an airtight space (or small closet, etc) high CO2 should not be a concern whatsoever.

Any normal living space is going to be fine. Even a relatively "tight" super-efficient modern home changes air over several times per hour (and have fresh-air injectors on the heating/cooling systems to introduce fresh air and prevent staleness). Even there, a house full of people wall-to-wall could not significantly alter the CO2 levels to a degree there is a health risk.

If you need something to worry about in the air, be concerned about diesel particulates in filthy city air.

source: former HVAC tech.

It might not be a health risk, but I found this study [1] interesting. It showed impaired decision making at CO2 levels that you could encounter indoors.

[1] "Is CO2 an Indoor Pollutant? Direct Effects of Low-to-Moderate CO2 Concentrations on Human Decision-Making Performance" . https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3548274/

That's true for major health effects, but research is showing that lower than previously assumed CO2 levels may affect cognition and health, especially in poorly ventilated indoor environments. https://www.researchgate.net/publication/311844520_Carbon_di...

What about a dancehall/disco packed with people dancing?

Oxygen consumption increases with activity, obviously. Air handling is rated on square footage, structure design and intended purpose. A home is going to carry a very different set of requirements than a factory floor or a restaurant (which often has high CFM exhaust hoods and requires a large amount of fresh-air infiltration to make up for exhausted air). Max occupancy permits as based on these factors, and often limited by egress (exits) plus a safety-factor is added to that. But that's about the limits of my expertise. Beyond this lies architects & permitting inspectors.

The calculation of human heat load and perception of temperature (sensible/latent heat, and psychrometrics in general) is a big part of system design. I'd have to break out the software and start asking specifics to get you an actual number but my gut feel is that a dance hall is going to easily pass the air quality tests.

Of course, somebody could pack 100 people into a low-ceiling windowless 200 ft^2 room without forced ventilation and have a problem with not just air quality but also heat (each person is approx 20W at rest, 200W w/vigorous activity), but I'm speaking in generalities about properly used residential & commercial space.

Are you implying that a responsible club DJ should be modulating the crowd’s activity level based on the rated CFM of the air handler as well as the current CO2 PPM?

In that particular case, have a drink instead of rubbing your stomach and making deep oooohhm noises.