Some fresh air.
I've placed a fan in a location that allows it to draw in air from outside -- when CO2 levels are too high, it clicks on. A second Applet turns it off once levels are back in the acceptable range. The level I chose is particularly high, so this is a great opportunity to enjoy a walk in the sunshine; you can just lower it if you're a Vampire. (...but then, why are you worried about CO2 levels?)