Operating Rooms & Negative-Pressure Isolation Rooms in Hospitals: The 3-Stage Filter System
Operating Rooms & Negative-Pressure Isolation Rooms in Hospitals: The 3-Stage Filter System
After the COVID-19 pandemic, Vietnamese hospitals invested more heavily in negative-pressure isolation rooms. Alongside these, ISO 5/7 operating rooms, ICUs, and microbiology lab rooms all need specialised air filtration. This article rounds up the technical requirements for the four most important room types in a modern hospital.
1. The Standard Operating Theatre
General requirements
An operating theatre is a high-sterility area — any bacterium that lands in a wound can cause an SSI (Surgical Site Infection), prolonging treatment and raising mortality.
Common international standards: DIN 1946-4 (Germany), ASHRAE 170 (US), HTM 03-01 (UK). Vietnam references these for international-grade hospitals.
OR classification
| OR class | Surgery type | Cleanroom class | Filter train |
|---|---|---|---|
| Class I (Ultra-clean) | Organ transplant, joint-replacement orthopaedics | ISO 5 under LAF | Pre G4 + F9 + HEPA H14 + ceiling LAF |
| Class II (High) | Cardiac, bone-marrow transplant, paediatric surgery | ISO 7 | Pre G4 + F8 + HEPA H13 |
| Class III (Standard) | GI, obstetric, general surgery | ISO 7 in operation | Pre G4 + F8 + HEPA H13 |
| Class IV (Septic) | Infected surgery (amputation, peritonitis) | Negative pressure, ISO 7 | Pre G4 + F8 + HEPA H13 on both supply and exhaust |
Air velocity and flow
- ACH: 20-25 per hour for standard ORs, 40+ per hour for joint-replacement orthopaedic ORs.
- LAF velocity: 0.25-0.35 m/s at the operating plane (lower than pharma cleanrooms to avoid drying the wound).
- LAF coverage: covers the operating table, minimum 3×3 m.
Temperature and humidity
- Temperature: 19-24°C (flexible per surgeon's preference).
- Humidity: 30-60% RH.
- Differential pressure: +15 Pa relative to the surgical corridor.
2. Negative-Pressure Isolation Room
When are they needed?
- Isolating patients with suspected or confirmed airborne infections: TB, measles, chickenpox, COVID, MERS, SARS, Ebola, monkeypox.
- Hazardous-sample collection rooms.
- Autopsy rooms for infectious cases.
Design principles
Pressure inside the room is lower than the corridor (-15 Pa or below). When the door opens, air flows "into" the room rather than out — keeping pathogens contained.
Standard structure
- Anteroom between corridor and isolation room — at -5 Pa vs. corridor, +10 Pa vs. main room.
- Airtight doors with gaskets on both ends.
- Glass observation window to monitor from outside.
- Pass boxes for food, medicines, and samples.
Filter train
Supply air:
Pre G4 → Medium F8 → HEPA H13 (ceiling terminal box).
Exhaust air:
HEPA H14 at the exhaust inlet (in-line HEPA) → dedicated exhaust fan → discharge stack rising 3 m above the roof.
The exhaust HEPA is more important than the supply HEPA — it is the last barrier preventing pathogens from escaping. Every 6 months, integrity test and seal the housing when replacing it (to prevent technician exposure).
Safe HEPA replacement procedure
- Decontaminate the room before shutting down the system.
- Bag-in/Bag-out housing: the HEPA housing has an integrated PE bag enclosing the filter.
- Dispose per hazardous-waste protocol.
- Integrity-test the new HEPA before returning to service.
3. ICU (Intensive Care Unit)
Requirements
- ICU patients are immunocompromised — the environment must be cleaner than a regular ward.
- Filter train: Pre G4 + Medium F8 + HEPA H13.
- ACH: 12-15 per hour.
- Temperature: 22-24°C; humidity: 40-60%.
- Pressure: +5 Pa relative to the corridor.
Negative-pressure ICU
For ICU patients who need both critical care and respiratory isolation. The configuration combines ICU + negative-pressure room — capex runs 1.5-2× a standard ICU.
4. Hospital microbiology lab (BSL-2 / BSL-3)
BSL-2 (Biosafety Level 2)
- Routine bacteriology, blood culture, GI microbiology.
- Filter train: Pre G4 + Medium F8 in the AHU; BSC class II biosafety cabinet with internal HEPA H14.
BSL-3
- TB, respiratory viruses, SARS, COVID, category-B bacterial pathogens.
- Negative-pressure room at -25 Pa.
- Pre G4 + Medium F8 + HEPA H13 on supply; HEPA H14 bag-in/bag-out on exhaust.
- Exhaust fan with backup (UPS + generator) — must not stop while occupants are inside.
- Class II BSC with dual HEPA (supply + exhaust) ensures no air escapes the cabinet.
BSL-4
- Vietnam currently has no civilian BSL-4. This level is for Ebola, Marburg, Lassa.
- Requires a "suit" lab — technicians wear positive-pressure suits; lab exhaust passes through double HEPA + thermal destruction.
5. HVAC system for a general hospital
Overall structure
- Dedicated AHU per zone:
- OR AHU (5-8 rooms per AHU).
- ICU AHU.
- Negative-pressure isolation AHU.
- Surgical corridor AHU.
- Outpatient AHU.
- 70-80% recirculation for outpatient and corridor areas.
- 100% fresh-air for ORs and negative-pressure rooms.
Sensors and BMS
- Differential-pressure sensors in every room — alarm on ±5 Pa deviation from setpoint.
- Temperature, humidity, and online particle counters in ORs and ICUs.
- BMS integrated with the EMR system — logs everything for post-incident traceability of SSIs.
6. Validation and maintenance for OR / isolation rooms
Recommended frequency
- Daily: check differential pressure, temperature, humidity.
- Weekly: clean ceilings, floors, walls with disinfectant.
- Monthly: check ΔP across each filter; clean Pre Filters.
- Quarterly: clean and inspect Medium Filters.
- Every 6 months: DOP/PAO test HEPAs (Class I ORs and isolation rooms).
- Annually: comprehensive re-qualification, particle count per ISO 14644.
- Every 3-5 years: replace HEPAs.
Smoke airflow visualisation test
- LAF ORs: every 6 months — verify laminar flow is even with no reverse eddies.
- Isolation rooms: annually — verify air flows "into" the room from the anteroom.
7. Real-world case: COVID-era negative-pressure isolation rooms
During 2020-2022, dozens of Vietnamese hospitals converted regular wards into emergency negative-pressure isolation rooms. Lessons learned:
- You cannot "rig" negative pressure with a lone exhaust fan — you need the full supply - exhaust - bag-in/bag-out HEPA chain.
- Exhaust HEPA H14 is mandatory — without it, pathogens dispersed into the hospital's surrounding air.
- Anterooms cut staff infection risk by 5-10×.
- UPS-backed exhaust fans — a power outage in an isolation room is a life-threatening event.
8. Standard OR + isolation investment for new hospitals
Indicative costs (equipment included):
- Class II standard OR: 2-4 billion VND per room.
- Class I ultra-clean OR: 4-8 billion VND per room.
- Negative-pressure isolation room: 1.5-3 billion VND per room.
- Standard ICU: 1-2 billion VND per bed (ventilator, monitor, HVAC included).
Conclusion
Operating theatres, negative-pressure isolation rooms, and ICUs are the three backbone components of a modern hospital. Investing properly in the HVAC - HEPA - ULPA system reduces SSI rates, protects healthcare workers, and prepares the hospital for future outbreaks.
About Green Filter
Green Filter supplies HEPA H13/H14, ULPA, bag-in/bag-out housings, Medium F8/F9, and Pre Filters for operating rooms, negative-pressure isolation rooms, ICUs, and BSL-2/BSL-3 labs. Products come with full EN 1822-1:2009 certificates and PAO scan tests.
📞 Contact Green Filter for filter consulting for your hospital project: [insert hotline / email / website]
See also: What is a cleanroom? · HEPA/ULPA validation and replacement · BSL-2/BSL-3 microbiology and research labs.