Aranet, the company from which folks are getting the nifty little CO2 monitors, is having another great sale that puts them at $166.83 AND you get a nice little pouch to carry one around (great to check the air quality of stores and restaurants!).
If you have the monitor already and want the pouch, it's the Square Bag from TopoDesigns, retails for $19.
When I got my first monitor, I was shocked to learn how much CO2 was building up in rooms at home, even with just one person. I started opening windows and watching the numbers go down.
Last night we turned on the AC and closed all the windows, and I woke up at 4am feeling off. The thought that popped into my head was, "the air is bad," and then I thought, "I can't possibly know that," but I got up and checked the monitor: CO2 was way too high. Opened the windows, went back to sleep.