Hospital HVAC controls infection by supplying HEPA-filtered air at high rates, holding directional pressure between zones, and exhausting contaminated air through sealed pathways. Operating theatres typically receive 20 to 25 air changes per hour, isolation rooms run about minus 2.5 Pa, and protective environments run positive pressure with H13 to H14 supply filters.
Infection-control ventilation is not optional engineering taste; it is written into law and standards. ASHRAE 170 in the United States, HTM 03-01 in the United Kingdom, and the EN ISO 14644 and EN 13779 series in Europe set minimum air changes, pressure differentials, and filter grades that a hospital must meet to obtain and keep its licence. Accreditation bodies such as Joint Commission or national equivalents audit the records, not just the hardware, so a system that works but is undocumented is treated as non-compliant. The measurable targets are blunt: 12 or more air changes per hour in patient rooms, 20 or more in operating theatres, a pressure differential of at least 2.5 Pa between clean and dirty zones, and terminal supply filtration of H13 or better where aseptic work occurs. Designers translate these into duct sizes, fan pressures, and filter frames, then prove the result with particle counts and manometer readings during commissioning. The cost of getting it wrong is measured in surgical-site infections, which add 5 to 10 days to a stay and tens of thousands in treatment, so the ventilation budget is small against the clinical risk it controls. Training and a live logbook keep the cascade honest long after the installer leaves.
Why Air Management Is the First Line of Defence
In healthcare, the air itself carries pathogens. Droplet nuclei from coughing, aerosol from procedures, and spores from construction dust all travel through the ductwork if the system is wrong. Proper ventilation dilutes and removes these particles faster than they can settle, and pressure relationships decide which way air moves when a door opens. A single infected patient in an unprotected room can seed the corridor; a correct cascade keeps the hazard boxed in. The goal is not just comfort but measured infection risk reduction, expressed in air changes per hour and pressure differential in pascals.
Pressure Cascades and Room Classes
Hospitals grade rooms by the direction air must flow. The comparison below shows the three common schemes and their setpoints.
| Room type | Pressure vs corridor | Supply filter | Air changes per hour |
|---|---|---|---|
| Operating theatre | Positive +5 to +10 Pa | H14 laminar canopy | 20 to 25 |
| Protective environment | Positive +2.5 to +5 Pa | H13 / H14 | 12 or more |
| Airborne isolation | Negative minus 2.5 Pa | HEPA exhaust | 12 or more |
| General ward | Neutral 0 Pa | MERV-14 | 6 to 8 |
Doors must be self-closing and gaps minute, because a 10 Pa difference needs only a small leak to maintain; a propped door collapses the cascade instantly.
Filtration and Air Changes
Supply air to theatres and protective rooms passes H13 or H14 terminal filters, while exhaust from isolation rooms is pulled through HEPA before release to the roof. Fresh-air fraction is kept at 100% for high-risk zones, though general wards may recirculate up to 80% through HEPA return banks to save energy. ASHRAE 170 sets the minimum rates: 12 air changes per hour for patient rooms, 20 for operating theatres, and 15 for emergency departments. Each air change replaces roughly the room volume, so 25 changes per hour clears a contaminant to under 5% in about 7 minutes.
When to Use Each Strategy
- Use 100% outside air and negative pressure when the patient is a source, such as TB, measles, or aerosol-generating procedures.
- Use positive pressure and H14 laminar flow when the patient is the vulnerable one, such as bone-marrow or transplant units.
- Use HEPA recirculation in general wards to cut energy while still controlling droplet nuclei at MERV-14 or better.
- Use ultraviolet or bipolar ionization only as an add-on, never as a substitute for filtration and dilution.
- Upgrade to H14 terminal boxes when surgical-site infection data shows the theatre is above ISO Class 5 at the wound.
Commissioning and Ongoing Control
Commissioning measures pressure differential with a digital manometer, counts particles with a laser counter to confirm ISO Class 5 at the surgical field, and smoke-tests door airflow direction. Annual re-verification tracks filter pressure drop and recalibrates the cascade. Logbooks record each HEPA change and leak scan, because accreditation bodies require evidence, not assumption. Training staff to keep doors shut and filters current is as important as the equipment itself.
Worked Example: Sizing a Theatre Supply
Take an operating theatre of 40 m² and 3 m height, a volume of 120 m³. ASHRAE 170 asks for 20 to 25 air changes per hour, so the design supply is 2,400 to 3,000 m³/h. Delivering that through an H14 laminar canopy at 0.45 m/s needs a filter face area of about 1.85 m², met by a 1.2×1.8 m terminal module. The supply fan must overcome roughly 250 Pa of HEPA drop plus 150 Pa of duct and coil, so it is selected near 400 Pa at 3,000 m³/h. The adjacent anaesthetic storage room is kept at minus 2.5 Pa by exhausting 10% more than it supplies, about 300 m³/h, through a separate HEPA exhaust. A manometer on the door confirms the cascade: smoke pulled inward when the door cracks proves the negative pressure holds and the patient is protected from room air.
Common Mistakes to Avoid
- Recirculating isolation-room air because it spreads pathogens; isolation zones must run 100% outside air with HEPA exhaust.
- Propping self-closing doors, which collapses a 10 Pa cascade instantly and lets contaminated air cross zones.
- Trusting the filter label without a particle count, since a leaking gasket can destroy an H14 installation.
- Skipping annual re-verification, so a drifting pressure setpoint goes unnoticed until an outbreak investigation finds it.
- Using UV or ionization as a substitute for filtration, when codes still require dilution and HEPA as the primary barrier.
Keruite manufactures this equipment at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory; every unit is balanced to G2.5 and performance-tested before shipment.
FAQ
What HVAC setup prevents infection spread in hospitals?
Hospitals use HEPA-filtered supply air, 12 or more air changes per hour in theatres, and 100% outside-air dilution with directional pressure cascades. Isolation rooms run negative pressure at minus 2.5 Pa, while protective environments run positive pressure to keep contaminated air from entering the patient zone.
How many air changes per hour does an operating theatre need?
An operating theatre needs at least 20 air changes per hour under ASHRAE 170, with many modern theatres designed for 25 air changes per hour through a unidirectional H14 laminar-flow canopy. The high rate dilutes anaesthetic gases and aerosols and keeps the surgical field below ISO Class 5 particle counts.
What is the difference between positive and negative pressure rooms?
Negative pressure rooms at about minus 2.5 Pa contain airborne pathogens by pulling room air out through HEPA exhaust so contaminants cannot escape. Positive pressure protective environments push filtered air outward to shield immunocompromised patients from outside microbes, using H13 or H14 supply filters.
Do hospitals recirculate air or use 100% fresh air?
Most hospital HVAC systems use 100% outside air for isolation and theatre zones to prevent cross-contamination, while general wards may recirculate up to 80% through HEPA return banks. All recirculated air passes HEPA or MERV-14 filtration before re-entering occupied space to control droplet nuclei.