I think a really hard limit on the max CO2 acceptable that most everyone would agree to would be where outside air starts to feel stuffy to humans. I have not f

I think a really hard limit on the max CO2 acceptable that most everyone would agree to would be where outside air starts to feel stuffy to humans. I have not found a good study on this but indoor air quality people say around 1000 ppm CO2 and you should start trying to increase ventilation. If the atmosphere gets too high it will be hard to keep indoor air below that without CO2 scrubbers. Over 750ppm(?) CO2 in the atmosphere and one gets the horrible situation where one goes inside a CO2 controlled space instead of outside to get "some fresh air". Edit: Here is a decent article on a study of some cognitive effects of CO2 levels (http://newscenter.lbl.gov/2012/10/17/elevated-indoor-carbon-dioxide-impairs-decision-making-performance/ http://newscenter.lbl.gov/2012/10/17/elevated-indoor-carbon-...). Actual study paper (https://ehp.niehs.nih.gov/1104789/ https://ehp.niehs.nih.gov/1104789/)