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 recipe 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?)
If
Carbon dioxide rises above
This Trigger fires every time the carbon dioxide rises above a value you specify.
Then
Turn on
This Action will turn a WeMo Insight Switch on. If the WeMo Insight Switch is already on, it will remain on.
Explore more great ways to automate WeMo Insight Switch and Netatmo Weather Station