So, it just feels "stuffy." I suspect that there's not much more to it than that. As far as mitigation, if you don't have the ability to ventilate, there's not

So, it just feels "stuffy." I suspect that there's not much more to it than that. As far as mitigation, if you don't have the ability to ventilate, there's not much you can do about it. I keep an exhaust fan in the window most of the most, when weather allows it. When it's not in there, if all the windows are fully closed, we can get up to 1,600 ppm in a single room if me and my wife are in it for a long time. (eg, sleeping.) With the exhaust fan, I haven't seen it read over 700 ppm, and seldom that high at all.