A hospital is the most demanding HVAC environment in commercial design. Multiple cleanliness levels, infection-control requirements, redundancy mandates, and patient-safety considerations all converge in the air system. NABH 5th Edition (Indian standard), FGI 2022 (American), and ASHRAE 170-2021 (also American) form the consolidated reference base.
This pillar covers space classification, pressure relationships, air change minimums, isolation room design, and the OT-specific airflow regimes that determine whether a hospital’s most critical spaces certify.
Hospital space classification
Three macro categories drive the HVAC requirements:
Category 1: Sterile / surgery
Operating theatres (OT), procedure rooms, sterile pharmacy. Highest cleanliness. Positive pressure relative to corridor.
Category 2: Patient care
ICU, isolation rooms, burn units, ward rooms. Variable pressure (some positive, some negative).
Category 3: General hospital
Corridor, dining, public space, laundry, kitchen. Standard commercial HVAC.
Pressure relationship matrix
NABH 5th Edition + ASHRAE 170 specify the pressure relationship for every hospital space type:
| Space | Pressure relative to corridor | Min ACH outside air | Total ACH |
|---|---|---|---|
| OT — General | +20 Pa positive | 4 | 20-25 |
| OT — Bone marrow | +20 Pa positive (laminar flow) | 4 | 25+ |
| OT — Cardiac | +20 Pa positive (laminar flow) | 4 | 25-30 |
| Procedure room (cath lab) | +15 Pa positive | 3 | 15 |
| Sterile pharmacy | +15 Pa positive | 3 | 15 |
| Patient room (general) | +5-10 Pa positive | 2 | 6 |
| Patient room (immune-compromised) | +20 Pa positive | 4 | 12 |
| Airborne infection isolation (AII) | -20 Pa negative | 2 | 12 |
| Burn unit | +10 Pa positive | 4 | 12 |
| Endoscopy procedure | -10 Pa negative | 2 | 12 |
| Decontamination room | -10 Pa negative | 4 | 6 |
| Soiled utility | -10 Pa negative | 2 | 10 |
| Laboratory | -10 Pa negative | 2 | 10 |
| Pathology laboratory | -10 Pa negative | 4 | 12 |
| Triage/treatment | +5 Pa positive | 2 | 6 |
| Kitchen | -5 Pa negative | 2 | 8-15 |
| Soiled holding | -10 Pa negative | 2 | 10 |
| Toilet (public) | -5 Pa negative | 2 | 6 |
ACH values are minimums. Higher ACH may be required by site-specific infection control plans.
Operating theatre design
OTs are the most demanding hospital HVAC environment. Three airflow regimes:
Regime A: Conventional mixing
Supply diffusers + return grilles in the OT ceiling. Air mixes throughout the room. ACH ~25.
Pros: simple; flexible.
Cons: contamination can re-circulate; bacterial counts higher than alternatives.
Regime B: Laminar / unidirectional flow
HEPA-filtered air supplied at 0.30-0.45 m/s downward over the surgical zone. Sweeps contaminants away.
Pros: lowest bacterial count over the surgical zone; ASHRAE 170 cleanliness score.
Cons: laminar flow mainly works directly under the diffuser; off-axis cleanliness lower; cold draft on patient/staff.
Regime C: Conditioned mixing with supplemental laminar
Mixed flow in room, plus a small laminar supply over the surgical zone (often 1.2 m × 1.2 m source, 0.30 m/s down).
Pros: balance of comfort + bacterial control.
Cons: complex.
For Indian projects, Regime A is most common (NABH-acceptable). For high-surgery-volume centres or specialised surgery (cardiac, ortho), Regime B is often required by surgeons.
OT temperature: 18-22 °C typical (cooler than other clinical zones because surgical staff in scrubs + lights = high local heat).
OT relative humidity: 30-60% (lower for orthopaedic to prevent fogging; higher for general).
Isolation rooms
AII (Airborne Infection Isolation)
For TB, measles, VHF, COVID-class pathogens. Negative pressure relative to corridor (-20 Pa or more). 12 ACH minimum. Anteroom or air-lock between AII room and corridor — sometimes considered for added isolation.
Exhaust must vent to atmosphere through HEPA filtration, ≥ 8 m horizontal from any operable window/door/HVAC intake.
PE (Protective Environment)
For immune-compromised patients (post-transplant, leukemia chemo). Positive pressure (+10 to +20 Pa). HEPA-filtered supply. 12 ACH minimum.
Combination AII + PE rooms
For patients who are simultaneously infectious + immune-compromised (e.g. HIV with active TB). Anteroom required; corridor pressure between AII negative and PE positive.
Worked example: 50-bed multi-speciality hospital
Building program:
- 1 OT (general surgery)
- 8 ICU beds
- 4 AII isolation rooms
- 30 ward beds
- Public spaces, kitchen, ancillary
OT design:
- 60 m² floor, 3.5 m height = 210 m³ volume
- 25 ACH × 210 / 60 = 87.5 m³/min = 5,250 m³/h = 3,090 cfm
- HEPA H13 at terminal
- 4 ACH outdoor air for makeup (total = 25 ACH)
- Pressure cascade: OT +20 Pa, sub-sterile +10 Pa, corridor 0 Pa, dirty utility -10 Pa
- Temperature 19 ± 1 °C, RH 35-50%
ICU design (8 beds in suite):
- 200 m² × 3.0 m = 600 m³ volume
- 6 ACH × 600 / 60 = 60 m³/min = 3,600 m³/h
- Pressure +5 Pa relative to corridor
- 2 ACH outdoor air
AII isolation rooms (4 rooms × 18 m²):
- 54 m³ volume each; 12 ACH = 10.8 m³/min = 650 m³/h per room
- Negative -20 Pa per room with anteroom at -10 Pa
- HEPA exhaust to atmosphere
- 2 ACH minimum outside air
Ward (30 beds in 6 rooms × 5 beds):
- 80 m² each; 6 ACH; +5 Pa positive
System architecture:
- Twin chiller + chilled water plant (N+1 redundancy)
- Twin AHU per major zone (concurrent maintainability)
- Dedicated AHU for OT with HEPA at terminal
- Dedicated AHU for AII isolation rooms with HEPA on exhaust
- Standard AHU for ward + general areas
Common hospital HVAC mistakes
1. Mixing OT and ward HVAC on same AHU. Cross-contamination risk; must be separate.
2. Insufficient redundancy on critical zones. OT failure during surgery = patient harm; redundant cooling is ethical not just technical.
3. AII exhaust to nearby HVAC intake. Re-entrainment of pathogens; exhaust must be ≥8 m horizontal from any opening.
4. No anteroom on AII or PE rooms. Door-opening events compromise pressure.
5. HEPA replacement schedule ignored. Filter integrity drifts; annual DOP test mandatory.
Quick checklist
- [ ] Space classification per NABH/FGI/ASHRAE 170
- [ ] Pressure relationship matrix completed (every space referenced to corridor)
- [ ] Outdoor air ACH minimums met for each space
- [ ] Total ACH minimums met
- [ ] OT regime chosen (mixing / laminar / hybrid) per surgical specialty
- [ ] AII isolation rooms with anteroom + HEPA exhaust
- [ ] PE rooms with HEPA supply
- [ ] Twin redundancy on critical AHUs and chiller
- [ ] HEPA test schedule (annual + post-installation)
- [ ] Pressure monitoring at each critical zone (BAS-integrated)
References: NABH 5th Edition Standards for Hospitals; FGI Guidelines for Design and Construction of Hospitals 2022; ASHRAE 170-2021 Ventilation of Health Care Facilities; ISHRAE Healthcare HVAC Handbook (latest edition); ASHRAE Handbook HVAC Apps 2023 Ch 9 (Health Care Facilities).
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