Yeah, there’s some interesting stuff on it for sure!
Here is a study showing the concentrations of CO2 at three large gyms:
https://www.researchgate.net/publication/329537304_Fitness_centers_demonstrate_CO2_concentration_levels_above_recommended_standards
We also know that with low/moderate CO2 increases, ability to concentrate and decision-making performance decreases:
Is CO2 an Indoor Pollutant? Direct Effects of Low-to-Moderate CO2 Concentrations on Human Decision-Making Performance - PMC.
And lastly, we know that mental fatigue directly affects RPE and ability to work hard over long efforts in endurance sports:
So as pointed out, no we’re not in perfectly sealed rooms, but especially if you’re in a small room (I’m in something like 11x11x9) and you’re working out many hours, it seems very reasonable that you’re going to be experiencing some of the effects of high CO2. Airflow and heat over the body is a huge consideration, of course; I’ve sat on a trainer in ~35f with no airflow and I was burning up. So yeah, airflow is key, but some exhaust / intake is likely worth it if you’ve got the option #MarginalGains