An airborne infection isolation (AII) room holds a negative pressure differential of at least 2.5 Pa relative to the corridor, delivers a minimum of 12 air changes per hour, and achieves that pressure by exhausting more air than is supplied – typically an offset of 120–200 m³/h, or 10–15% of the supply rate. Air is exhausted directly outdoors, and the room envelope must be tight enough that the leakage area matches the offset. Continuous pressure monitoring with a local visual indication is mandatory, because a room that has silently gone positive is more dangerous than no isolation room at all.
Why the offset, not the fan, creates the pressure
Room pressure is the product of two things: the volumetric offset between exhaust and supply, and the leakage area of the enclosure. The relationship is approximately Q = 0.827 × A × √ΔP, where Q is leakage in m³/s, A is effective leakage area in m² and ΔP is the differential in Pa. A typical door undercut of 12 mm across a 1.1 m wide leaf gives about 0.013 m², and with frame gaps and hardware the total effective leakage area of a well-built isolation room lands near 0.02 m². To hold 2.5 Pa across that opening the exhaust must exceed supply by 0.827 × 0.02 × 1.58 = 0.026 m³/s, which is about 94 m³/h.
Two conclusions follow. First, a leaky room cannot be fixed by a bigger fan: doubling the offset only raises the differential by a factor of four in the square-root relationship, so sealing pays far better than oversizing. Second, an over-sealed room becomes unstable, since a very small leakage area makes the pressure swing wildly whenever a door moves. Designers therefore engineer a deliberate, known leakage path – usually the door undercut – and seal everything else: ceiling penetrations, service risers, light fittings, sockets and the wall-to-slab junction.
Design criteria compared across critical room types
| Criterion | Airborne isolation (AII) | Protective environment (PE) | Operating room |
|---|---|---|---|
| Pressure relationship | Negative to corridor | Positive to corridor | Positive to all adjacent |
| Minimum differential | 2.5 Pa | 2.5 Pa | 2.5–5 Pa |
| Total air changes per hour | ≥12 | ≥12 | ≥20 |
| Outdoor air changes per hour | ≥2 | ≥2 | ≥4 |
| Supply filtration | MERV 14 minimum | HEPA 99.97% | HEPA or MERV 16 |
| Exhaust route | Direct outdoors, no recirculation | May return with HEPA | May return with filtration |
| Temperature and humidity | 21–24 °C, 30–60% RH | 21–24 °C, 30–60% RH | 20–24 °C, 30–60% RH |
| Pressure monitoring | Continuous with local alarm | Continuous with local alarm | Continuous |
| Anteroom | Recommended, sometimes required | Required for combined PE/AII | Not applicable |
Which pressure regime to use
- Use a negative room when the hazard is inside and must be contained: suspected or confirmed airborne infection, bronchoscopy, sputum induction, autopsy, or a laboratory handling infectious aerosol.
- Use a positive room when the occupant must be protected from the outside: severely immunocompromised patients, transplant recipients, or a sterile compounding room.
- Use an anteroom with cascading pressure when the patient is both infectious and immunocompromised. The room is positive to the anteroom, the anteroom is negative to both room and corridor, and neither requirement is compromised.
- Use a dedicated exhaust fan per room when reliability matters more than first cost, because a shared manifold lets one room's door movement disturb another's differential.
- Add HEPA filtration in the exhaust when the discharge point cannot be located at least 8–10 m from any air intake, operable window or pedestrian route.
- Choose a permanent installation over a portable HEPA unit when the room is in continuous clinical use; portable units are a surge measure and typically achieve equivalent performance only in rooms under about 40 m².
Design and commissioning sequence
- Fix the room air change rate first: for a 40 m³ single room at 12 ACH the supply airflow is 480 m³/h. Confirm the terminal device can deliver that without exceeding NC 35–40.
- Set the exhaust at supply plus the offset, so 480 + 120 = 600 m³/h in this example, and verify the offset is achievable within the balancing tolerance of the terminals.
- Locate supply diffusers at the ceiling above the foot of the bed and the exhaust grille low on the wall behind the patient's head, so airflow moves past the source and away from staff.
- Seal the envelope thoroughly, then measure the achieved differential and adjust the offset rather than the seal.
- Size the exhaust fan for the full static requirement including HEPA at final resistance, typically 600–750 Pa for a loaded filter, so the room stays negative at the end of the filter's life.
- Commission with a smoke tube or tissue at the closed door, then verify with a calibrated differential-pressure instrument reading 0–60 Pa with roughly ±1 Pa resolution. Repeat with the door opened and closed, and with the anteroom door cycled.
- Record the 99% contaminant removal time so clinical staff know the safe dwell interval: at 12 ACH it is about 23 minutes, and 99.9% removal takes about 35 minutes.
Keruite manufactures the exhaust fans, HEPA filter sections and air handling units used in isolation suites at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory; every rotor is balanced to G2.5 and each unit is performance-tested before shipment.
Why isolation rooms fail in service
The most common failure is not a design error but a control one. Rooms served by variable-air-volume terminals can lose the offset when the supply resets on cooling demand, so the sequence must hold the differential in priority over temperature. The second most common cause is envelope degradation: new cable penetrations, a replaced light fitting, or a ceiling tile left ajar can add more leakage area than the entire door undercut. Third is instrumentation error, where a pressure transducer sensing against a corridor that itself fluctuates reports a stable differential that does not exist. Finally, staff behaviour matters: propping a door open collapses the differential within seconds, which is why a visible local indicator, checked at every shift change, is worth more than any remote dashboard.
FAQ
How much negative pressure does an isolation room need?
At least 2.5 Pa relative to the corridor, measured with the door closed. Some jurisdictions require more, and design margin usually targets 5–10 Pa so that normal door movement and corridor fluctuation do not push the room toward neutral or positive.
Can an isolation room recirculate air?
Air from an AII room should be exhausted directly outdoors without recirculation. Where that is impossible, air may be recirculated only after HEPA filtration, and the discharge or return path must be documented and tested. Discharge outdoors should be at least 8–10 m from any intake.
How long must a room be left empty after an infectious patient?
Base the interval on the removal time. At 12 air changes per hour, 99% contaminant removal takes roughly 23 minutes and 99.9% about 35 minutes. At 6 ACH those times roughly double, so higher air change rates directly shorten room turnaround.
Is an anteroom mandatory for a negative pressure room?
Not universally. Many codes recommend rather than require one for standard AII rooms, but an anteroom is effectively essential where the room must be both protective and isolating, and it greatly stabilises the differential by breaking the direct connection to the corridor.