Definition Air changes per hour is the volumetric airflow divided by the room volume, expressed per hour: ACH = (volumetric_flow_rate / room_volume) x 3600 An ACH of 6 means the room air is theoretically replaced six times in an hour. Typical values Classrooms: about 4-6 ACH (see school ventilation) Laboratories and fume hoods: 6-12 ACH […]