A central vacuum cleaning system 3 has a central vacuum unit 17 with a motor 10, a motor compartment 19, and a vacuum chamber 25. The system 3 also has a control circuit 1 for controlling power to the motor 10, and one or more airborne contaminant sensors 15 for sensing contaminants within the system 3. The sensors 15 may be located in pipes 31 motor compartment 19, vacuum chamber 25 and hose handle 34. Vacuum air may be drawn through aperture 37 in wall valve 38.