The project described 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.
Project is Open Source/Public Domain Full code and EDA files are on the project sites below.

Warning: The system is mains powered (100VAC – 240VAC) so if you are replicating the project you should be very careful when wiring up and enclosing the board to ensure it meets all safety guidelines, and if it is going into a public space or factory environment it will need to be installed and tested by a qualified electrician. The PCB is designed for a standard industrial 158 x 90 x 65 mm Plastic Enclosure.