Updated September 30, 2026.
Direct answer: Operating room HVAC under ANSI/ASHRAE/ASHE Standard 170-2025 (aligned with FGI 2026) combines a HEPA-filtered primary ceiling array with unidirectional downward flow, at least 20 total air changes per hour (including 4 outdoor ACH), positive pressure to adjacent spaces, and 68–75°F with 20–60% relative humidity. Terminal HEPA at the supply ceiling, low sidewall returns, and documented commissioning are what keep particulate load down around the surgical field.
Why OR air matters
Airborne skin cells and room dust still show up in wound cultures. HVAC cannot replace sterile technique, but it limits what reaches the incision. Healthcare HVAC design under ASHRAE 170 sets minimum ACH, filtration, pressurization, and diffuser rules; your state adoption, FGI 2026, NFPA 99, and survey bodies translate those minimums into what must be proven on the floor.
Surveyors under Joint Commission Accreditation 360 Physical Environment (2026) and CMS Conditions of Participation expect OR ventilation to match the approved design and maintenance records—not a narrative about “clean air.”
Laminar-style arrays and coverage
Standard 170-2025 requires a primary supply diffuser array over the table: unidirectional downward flow, average 25–35 cfm/ft² (127–178 L/s/m²), coverage at least 12 in. beyond the table footprint on each side, and no more than 30% of that primary zone consumed by lights, booms, or penetrations. Group E non-aspirating diffusers are the usual hardware choice.
Returns and flow pattern
At least two low sidewall return or exhaust grilles, opposite corners when possible, with bottoms about 8 in. above the floor. High returns are allowed in addition, not instead. The table sits in the best part of the downward wash; booms and imaging carts are obstruction risks you validate during commissioning, not on paper.
ISO 14644-1 particle classes
Teams still say “Class 100” in conversation. Under ISO 14644-1:2015, ISO Class 5 at ≥0.5 µm means 3,520 particles per cubic meter (cumulative). Certification sampling follows ISO methods; many hospitals run particle counts at turnover and annually, tied to the healthcare HVAC commissioning and compliance verification package.
Temperature, humidity, and control
| Parameter | Standard 170-2025 (operating room) | Practical notes |
|---|---|---|
| Temperature | 68–75°F (20–24°C) | Individual room control; cardiac and pediatric teams often ask for the low end |
| Relative humidity | 20–60% RH | 2021 and later editions allow 20% minimum; many sites hold 40–55% RH for comfort and static control |
| Total air changes | ≥20 ACH (≥15 ACH existing where permitted) | Outdoor air ≥4 ACH of the total |
| Pressure | Positive to adjacent spaces | Minimum about 0.01 in. w.g. (2.5 Pa); tune so doors are manageable |
Lights and electrosurgical units dump heat into the same room you are trying to keep at 68°F. Oversized cooling without rebalancing laminar arrays creates drafts; undersized coils buy surgeon complaints and failed recovery-room handoffs. Humidifiers need the same filtration discipline as supply air—wet heat with dirty water is an infection control problem, not a comfort tweak.
Filtration: MERV stages and HEPA
Operating rooms require at least two filter stages at the air handler, now MERV 7 plus MERV 16 minimum in Standard 170-2025. For orthopedics, transplants, neurosurgery, burn, and similar high-risk suites—and for Class 3 imaging rooms used for those cases—HEPA filters belong in the air terminal device over the field.
Terminal vs central HEPA
Terminal HEPA modules in the ceiling plenum are the default for new laminar ORs because duct downstream of HEPA is where dust collects after a shutdown. Central HEPA at the AHU can work only if the distribution path is sealed, monitored, and maintained like a clean duct system.
Monitoring
- Differential pressure gauges on pre-filters and HEPA banks, with alarms tied to work orders
- Filter change at manufacturer final ΔP, not an arbitrary calendar
- Particle counts when reopening after construction—coordinate with ICRA Class I–V controls before the first case
Outdoor air, recirculation, and pressure
With 20 total ACH and 4 outdoor ACH, roughly 80% of supply is recirculated through HEPA and coils. That ratio saves energy but means filter maintenance is non-negotiable. Outdoor intakes need separation from exhaust and cooling-tower plumes per Standard 170; pretreat outdoor air before it mixes with return.
Positive offset is supply minus exhaust/return. Example: 400 ft² × 14 ft = 5,600 ft³; at 20 ACH you need 1,867 cfm supply. Exhaust might be ~1,760 cfm so roughly 100 cfm net outflow holds pressure without pinning doors shut.
Construction, turnover, and adjacent cleaning
Section 10 ventilation-during-construction requirements in 170-2025 matter when you renovate adjacent suites. HVAC performance at case start also depends on terminal cleaning after the last case—filters do not remove settled bioburden on flat surfaces.
Broader infectious-aerosol planning in facilities may reference ANSI/ASHRAE Standard 241; OR design still anchors on Standard 170 and your adopted FGI edition. See the healthcare HVAC systems professional guide for how OR rules fit the rest of the hospital ventilation map.
Commissioning checklist (field items)
- Airflow visualization (smoke) across the primary array and around booms
- Array average flux within 25–35 cfm/ft² and ACH at each operating volume
- Pressure relationships to corridor and sterile core
- Temperature and humidity trend logs under simulated load
- Particle counts if ISO class is part of the owner’s spec
Frequently asked questions
ANSI/ASHRAE/ASHE Standard 170-2025 requires the primary supply diffuser array to deliver unidirectional downward airflow with an average of 25–35 cfm/ft² (127–178 L/s/m²) over the array. Informative guidance often cites roughly 0.25–0.5 ft/s (0.08–0.15 m/s) in the breathing zone; stay within the standard’s stated array average when commissioning.
Change interval is pressure-driven, not calendar-only. Monitor differential pressure across each HEPA stage and replace when the manufacturer’s final limit is reached—commonly in the 0.5–1.0 in. w.g. range on many modules. Pre-filters upstream protect HEPA media; with MERV 13–16 pre-filtration, many hospitals see 12–24 month HEPA life, but particle counts at commissioning and annual verification should confirm performance.
For operating rooms, ANSI/ASHRAE/ASHE Standard 170-2025 specifies 68–75°F (20–24°C) and 20–60% relative humidity, with each room having individual temperature control. Operating toward the middle of the humidity band (roughly 40–55% RH) limits static discharge and condensation risk while remaining inside code.
Partial upgrades—terminal HEPA arrays, revised diffuser layouts, dedicated return grilles, and corrected pressure control—can close many gaps. Achieving the primary array coverage, low sidewall returns, and documented 20 total ACH with 4 outdoor ACH may still require duct and AHU work. Treat any retrofit as a design submittal with rebalance, visual airflow mapping, and particle testing before case use.
Positive pressure (minimum about 0.01 in. w.g. or 2.5 Pa to adjacent spaces per Standard 170) forces air out when doors open so corridor air does not wash into the sterile field. Pair pressurization with terminal HEPA supply and laminar-style downward flow; pressure alone does not clean air.
Related resources
Standards referenced: ANSI/ASHRAE/ASHE Standard 170-2025; FGI Guidelines (2026); ISO 14644-1:2015; NFPA 99 (2024 edition as adopted locally); NFPA 101 Life Safety Code (as adopted); CMS Conditions of Participation; The Joint Commission Accreditation 360 Physical Environment (effective January 1, 2026).