Key takeaways
- Air changes per hour (ACH) is the airflow into a room per hour divided by the room volume.
- Design standards such as ASHRAE 170 set minimum ACH for operating rooms, isolation rooms and protective environments.
- Higher ACH clears airborne contaminants faster: at 12 ACH, about 99% is removed in roughly 23 minutes under ideal mixing.
- Mobile HEPA units and far-UVC add "equivalent" clean air without changing the HVAC system.
The calculation
ACH is simple: take the volume of air supplied to or treated in a room each hour and divide it by the room's volume. A 5 × 6 m room with a 3 m ceiling has a volume of 90 m³. If 1,080 m³ of air is supplied each hour, the room has 12 air changes per hour.
The same arithmetic applies to a mobile air treatment unit. A unit moving 1,200 m³/h in that 90 m³ room adds around 13 air changes of treated air, although real performance also depends on how well the air mixes.
Common design values
Many healthcare facilities design to ASHRAE Standard 170, Ventilation of Health Care Facilities, or to national equivalents. Typical values include:
- Operating rooms: at least 20 total air changes per hour, including at least 4 of outdoor air, with positive pressure.
- Airborne infection isolation rooms: at least 12 air changes per hour with negative pressure.
- Protective environment rooms: at least 12 air changes per hour with positive pressure and HEPA-filtered supply.
Local codes may differ, and existing buildings often do not meet current values.
How fast contaminants clear
With perfect mixing, each air change removes a fixed share of what remains. CDC guidance tabulates the result: at 12 ACH, about 99% of airborne contaminants are removed in about 23 minutes and 99.9% in about 35 minutes. At 6 ACH, the same removal takes twice as long. Real rooms mix less than perfectly, so actual times are longer.
Equivalent air changes
Air cleaning does not have to come from the HVAC system. Recirculating HEPA units, air decontamination units and upper-room or far-UVC systems can all remove or inactivate airborne microorganisms. Their contribution is often expressed as equivalent air changes per hour (eACH): the ventilation rate that would achieve the same reduction.
ASHRAE Standard 241, Control of Infectious Aerosols, published in 2023, formalises this idea as equivalent clean airflow and allows different technologies to be combined to reach a target.
Practical points for hospitals
- Start from the room: volume, current supply and extract rates and pressure regime.
- Use mobile units to raise clean-air delivery in rooms that cannot be upgraded quickly.
- Place units to avoid short-circuiting air between intake and outlet.
- Consider noise, especially in patient rooms and neonatal units.
- Treat far-UVC and filtration as additions to ventilation, not replacements.
Related: PLASMAIR, Zener far-UVC
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References
- ANSI/ASHRAE/ASHE Standard 170. Ventilation of Health Care Facilities.
- ANSI/ASHRAE Standard 241-2023. Control of Infectious Aerosols.
- Sehulster L, Chinn RYW. Guidelines for Environmental Infection Control in Health-Care Facilities. CDC and HICPAC, 2003 (updated 2019), Appendix B, Table B.1.
This article summarises published guidance and research for general information and is not medical or engineering advice. Device selection and use should follow the manufacturer's instructions, local regulations and your facility's policies.