An open source CO2 monitoring project from Roving Dynamics:
The project described below uses a MH-Z16 or MH-Z19 CO2 sensor and a DHT-22 (or DHT-11 if less accuracy is required) to measure the Temperature and Humidity. It has a 4 line by 20 character LCD Display to show the current readings and status, a warning alarm and two relays which can be triggered on a low CO2 (Generally above 1000 ppm) normally to switch on an extractor fan and a high level (4000 ppm) which will trigger a warning device such as an external alarm. There are two models I used the 0 to 5000 ppm device here but the code will be the same for the 0 to 10000 ppm model.
Project info at rodyne.com.