The apartment I used to live in was so good for this ventilation thing, even with all the windows closed. The air conditioner was installed permanently in the wall and had a gap around it that let enough wind through to rattle my vertical blinds across the room. Never had to worry about high co2 during those trapped-inside Midwest winters! :p

Joking aside, I covered that thing with plastic and layers of blankets. Current house isn’t much better in that regard, the air leaks are just from everywhere, because it’s ancient. Costs a fortune to heat, so I keep it cold all winter. But at least I don’t have too much to worry about with co2 buildup.