When I was a student I had a little room and one time I had a co2 monitor in my room for a few days. The monitor had 3 leds, green yellow and red. While sitting in the room for three consecutive hours (at the computer) the co2 would slowly rise from green to red and after opening the window for a while it was back at green. Now the interesting part is that I felt a strong correlation between tiredness and high co2 levels (I was surprised how strong this effect was) and that opening the window really did help. Since then I thought that every classroom needs such a device.