With respect to "stuffy" vs. "fresh" air, I don't know whether my sense of it is the same as others'. What I would call "stuffy" is probably affe

With respect to "stuffy" vs. "fresh" air, I don't know whether my sense of it is the same as others'. What I would call "stuffy" is probably affected by humidity, dust, odors, plants, etc. and not just CO2. That's fair. I'm not sure if I'm really sensing CO2 levels, or if I'm wholly reliant on those proxy factors that you mentioned. I know that when my wife turns on the gas oven downstairs I can definitely feel a difference even if I wasn't aware she was turning it on and the CO2 meter confirms the CO2 spike. But even then, maybe I'm just "sensing" it via proxy factors like smell. In practice it probably doesn't matter all that much. Those proxy factors are really good proxies. It's hard to think of real-world scenarios where they wouldn't be.