Super stupid. I have a small 9x7 American Freedom Units room that is fully enclosed with no ventilation (door and window). I usually have the window open. I was just on a meeting where I absent-mindedly shut the door with the window closed. Within about 45 minutes, my chest tightened up, I started breathing weird, my head started to hurt, and I started getting groggy. CO2 poisoning is no joke, people. I ran outside and was fine within 5 mins. I have CO2 monitors in multiple rooms, but not this one.