Thinking about CO2 levels isn't common sense until after you've been keeping an eye on one of these gadgets. After sitting in a meeting room for an hour, people may find it feels warm or stuffy or notice a slight headache, but they won't necessarily draw the connection that they really ought to open a window or door or adjourn the meeting for a few minutes. The importance of breathing fresh air becomes much more common sense after you've learned to tell the difference.