Air changes per hour
The number of times per hour a room's volume of air is supplied or removed, calculated as airflow in cubic feet per minute times 60 divided by room volume in cubic feet. Cleanroom, pharmacy, and isolation room documents use it as a minimum ventilation or filtration rate.
Also called ACH, ACPH, air change rate
What does air changes per hour mean?
Air changes per hour is airflow expressed against room size: cfm × 60 ÷ room volume in cubic feet. A 1,512 cubic foot room supplied with 756 cfm receives 30 air changes per hour, the minimum USP 797 sets for an ISO 7 compounding room.
Air changes per hour: how it works
An air change rate turns airflow into a measure of how quickly a room's air is replaced, so rooms of different sizes can be held to one rule. In a well-mixed room, the concentration of anything released falls by about 63 percent with each air change, so the rate sets how fast the room dilutes what is released in it and how quickly it recovers after a disturbance.
Where you meet the term
- In USP 797, which requires at least 30 air changes per hour in ISO 7 rooms and at least 20 in ISO 8 rooms.
- In the CDC guidelines for airborne infection isolation rooms, which list at least 12 air changes per hour.
- In a certification report, where measured airflow at each filter is added up and converted to a rate for the room.
How to check it
- Calculate the rate from design airflow. Multiply the room's supply airflow in cfm by 60 and divide by the room volume in cubic feet, measured to the room's finished ceiling. Owner check
- Measure airflow at every supply terminal. A certifier or balancer measures each terminal with a capture hood or velocity traverse, adds the results, and converts the total to air changes per hour. Technician
Figures you will see
| Condition | Figure | What it tells you |
|---|---|---|
| USP 797 ISO 7 room | At least 30 ACPH | Read against the model documentation |
| USP 797 ISO 8 room | At least 20 ACPH | Read against the model documentation |
| CDC airborne infection isolation room | At least 12 ACH, 2 of them outdoor air | Read against the model documentation |
| USP 800 hazardous drug storage room | At least 12 ACPH | Read against the model documentation |
Air changes per hour: what it is not
- Air changes per hour is not a measure of cleanliness. Two rooms at the same rate hold different particle counts when they release different amounts.
- A high air change rate does not fix poor air distribution. Supply and return placement decide whether air sweeps the room or short-circuits past the work.
Air changes per hour questions
Does room volume include the space above the ceiling?
No. Air changes per hour uses the volume of the room the air serves, measured to the finished ceiling. The plenum above a cleanroom ceiling is part of the air path, not part of the room being ventilated.
Are air changes counted on supply or exhaust?
The governing document decides. A positive room is judged on its filtered supply, and in a negative room exhaust runs above supply by the offset, so designing supply to the required rate satisfies either reading.
How long does one air change take?
At 30 air changes per hour one air change takes two minutes, and at 12 it takes five. In a well-mixed room a 100 to 1 reduction of anything released takes about 4.6 air changes.
Where this definition comes from
Written from standards body publication, government guidance material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- General Chapter 797, Pharmaceutical Compounding, Sterile Preparations2023 revision, official 1 November 2023, for sterile compounding: at least 30 air changes per hour of HEPA-filtered supply in ISO 7 buffer rooms and anterooms and at least 20 in ISO 8 rooms, with at least 15 of them supplied by the HVAC system through HEPA filters; a minimum positive pressure differential of 0.020 inches of water column between the buffer room and anteroom and between the anteroom and unclassified space, monitored continuously; rooms maintained at 20 degrees Celsius or cooler and 60 percent relative humidity or below; and certification of primary and secondary engineering controls every six months.
- Guidelines for Environmental Infection Control in Health-Care Facilities, Appendix B: AirCDC table of ventilation parameters: airborne infection isolation rooms with at least 12 total and 2 outdoor air changes per hour, air movement into the room, a pressure differential of at least 0.01 inches of water gauge (2.5 pascals), and all air exhausted directly outdoors unless recirculated within the room through HEPA filters; protective environment rooms with at least 12 air changes per hour, positive air movement, and HEPA filters of 99.97 percent efficiency at 0.3 micrometers.
- General Chapter 800, Hazardous Drugs, Handling in Healthcare SettingsEngineering controls for hazardous drugs: storage rooms and nonsterile containment secondary engineering controls externally vented, at least 12 air changes per hour, and negative between 0.01 and 0.03 inches of water column relative to adjacent areas; sterile hazardous drug buffer rooms of ISO 7 air quality with HEPA-filtered supply, externally vented, at least 30 air changes per hour, and negative between 0.01 and 0.03 inches of water column; an ISO 7 anteroom with at least 30 air changes per hour and at least 0.02 inches of water column positive to adjacent unclassified areas; and a containment segregated compounding area externally vented with at least 12 air changes per hour at the same negative pressure range.
- ISO 14644-3:2019, Cleanrooms and associated controlled environments, Part 3: Test methodsTest methods for cleanroom performance: air pressure difference, airflow volume and velocity, airflow direction and visualization, recovery time using the 100 to 1 ratio with a 10 to 1 ratio added in the 2019 edition for less clean rooms, installed filter system leakage, containment leakage, temperature, and humidity. Recovery testing is optional and its acceptance is agreed between customer and supplier.