CO2 is actually not so nice for breathing, if you ever monitored a CO2 ppm meter indoors, where the air gets stuffy when too much CO2 from breathing accumulates. With the current trajectory, by the end of the century, opening the window won’t get you what we now consider fresh air anymore, the air will feel stuffy all the time. We already get less fresh air now than 50 years ago, where CO2 base levels were around 300 ppm, vs. 400 ppm now.