(Assuming you mean AC) - HVAC systems are generally designed to keep air flowing through the building optimally, which may include control systems for attenuati

(Assuming you mean AC) - HVAC systems are generally designed to keep air flowing through the building optimally, which may include control systems for attenuating fans, exterior building air intake, and whatnot as a function of sensor measurements. I'm not sure how often CO2 is taken into account during building design or operation to be honest, aside from rough targets on designed occupancy rates and volumetric estimates of airflow. In the UK, most buildings are naturally ventilated. There's no mechanical ventilation moving air around automatically, you gotta open your window if you want airflow. In the winter this generally puts people between a rock and a hard place: either you are slightly cold and sitting in a stuffy roof, or you open a window and are REALLY cold and also go broke from the energy bills :P