A CO2 monitor is quite helpful with keeping a room properly ventilated. The CO2 levels found in many indoor spaces are high enough to cause drowsiness, headaches and poor concentration. Of course all you need to do is open a window, but you'd be surprised how fast levels rise again after you close the window. It's particularly bad in small rooms and cars.
At this point I really think cars should have a CO2 meter built-in. The space is so small that CO2 might come fast and drowsiness might explain a lot of accidents on long-distance travels.
