It's better to get a HVAC guy in and take some measurements. They measure the flow in the ventilation system and can tell you if it's enough or not. People die from CO but get headaches from high CO2 levels. Anyway, you trying to monitor the ventilation system is like the ventilation guy trying to monitor server performance.
If there is a real risk somewhere where people could die, you don't solve it with an alarm from amazon that you plug into the wall and hope it works and that someone will hear it and if they hear it they will know what it is and evacuate the building.