I discovered the importance of fresh air last year because I have downstairs room in my apartment with no operable windows. I wondered why I always felt tired

I discovered the importance of fresh air last year because I have downstairs room in my apartment with no operable windows. I wondered why I always felt tired when I worked from there. Anectodal, but it turns out it was just a matter of air quality. Now I keep my windows open as much as possible. It was above 32 degrees recently and so I opened my windows and keep them open. I now prefer to be cold with fresh air than warm with bad air. This is really an unfortunate consequence of modern building systems, that all buildings are more or less isolated ecosystems circulating HVAC air.