Actually both are worth monitoring in different installations.
Carbon Monoxide is a gas that binds with the hemoglobin in red blood cells and doesn't allow them to transport as much Oxygen as they would need in order to support life. Too much CO...you're dead.
Carbon Dioxide is a gas that effectively reduces the partial pressure of Oxygen and can reduce the amount of Oxygen that you can get from the air to a level too low to support life. Too much CO2...you're dead. That was one of the major issues faced by the NASA astronauts on the Apollo 13 mission.
BTW, have you all noticed that hospitals have strict rules on how many plants a patient can have? That's because plants consume Oxygen and give off Carbon Dioxide constantly, but they produce a net increase in Oxygen only when photosynthesis is taking place. Since plants in patient rooms rarely get enough light (of the right wavelength) to give off Oxygen, they end up competing with the patient for the available Oxygen in the room.