ASHRAE 170 Ventilation Requirements for Healthcare: The Room-by-Room Table

ASHRAE Standard 170, Ventilation of Health Care Facilities, is the mechanical-design standard that sets minimum air-change rates, pressure relationships, filtration and temperature/humidity ranges for every functional space in a hospital or outpatient surgical facility. Infection preventionists, patient-safety officers, quality directors and facilities-safety teams rarely need the whole standard — what they need in practice is one thing: the room-by-room table, because that table is what a construction-risk assessment, a commissioning report, and a Joint Commission or CMS environment-of-care (EC) tracer all get checked against. This page is that table, organized by the room types that actually generate survey findings — operating rooms, isolation rooms, and sterile processing — not a general introduction to hospital HVAC.

What ASHRAE 170 Actually Governs

ASHRAE 170 is a mechanical-engineering standard, but it reaches hospital operations because the FGI Guidelines incorporate it by reference, and most states adopt an edition of the FGI Guidelines into their hospital licensing and construction codes. CMS Conditions of Participation and Joint Commission’s Environment of Care (EC) and Infection Prevention and Control (IC) chapters don’t restate ASHRAE 170’s numbers directly — they require the facility to meet the ventilation requirements applicable at the time of construction or renovation, which sends a surveyor straight back to whichever ASHRAE 170 edition the facility’s building permit was issued under. That’s why a hospital can have two different operating rooms, built a decade apart, governed by two different editions of the same standard — and why “what does ASHRAE 170 require” is really “what did ASHRAE 170 require when this specific room was built or last renovated.”

The standard’s Ventilation Table (numbered Table 7-1 in recent editions) covers, for each listed space: the required pressure relationship to adjacent spaces, the minimum total air changes per hour (ACH), the minimum outdoor-air ACH, whether air may be recirculated within the room or must be exhausted directly outdoors, and design temperature/humidity ranges. The rest of this page walks that table by the room categories most relevant to patient-safety and infection-prevention work.

How to Read the Table

• Pressure relationship — positive, negative, or neutral/no requirement, relative to the corridor and adjacent spaces. This is the single figure surveyors check first, because it’s continuously monitorable and its failure is binary: air is flowing the wrong direction or it isn’t.
• Minimum total ACH — total supply air changes per hour, combining outdoor and recirculated air where recirculation is permitted.
• Minimum outdoor ACH — the portion of total ACH that must be fresh outdoor air, not recirculated.
• Recirculation — whether room air may be recirculated (typically through a HEPA filter for higher-risk spaces) or must be exhausted directly to the outdoors with no recirculation.
• All-outdoor exhaust — for the highest-risk spaces, whether exhaust air must bypass the general return system entirely.

Operating Rooms and Other Class B/C Procedure Spaces

Operating rooms sit at the opposite end of the pressure spectrum from isolation rooms, and mixing the two up on a construction-risk assessment is a recurring finding.

• Pressure relationship: positive to the corridor and adjacent spaces — protects the surgical field from contaminated corridor air, not the corridor from the OR.
• Minimum total ACH: commonly cited at 20 for Class B/C operating rooms (higher than nearly every other listed space in the table).
• Minimum outdoor ACH: commonly cited at 4.
• Design relative humidity: a 20-60% band is the widely cited range for recent editions (older editions specified a narrower, higher band — this is one of the figures most likely to have shifted between the edition your OR was built under and the current one).
• Design temperature: roughly 68-75°F (20-24°C), with anesthesia-provider input often driving the operating point within that band.

The practical failure mode isn’t usually the ACH number — it’s supply/exhaust grille placement and door management undermining a pressure relationship that reads correctly on the building automation system. A positive OR with a door propped for equipment transport is not, in that moment, a positive OR.

Airborne Infection Isolation Rooms (Negative Pressure)

Airborne Infection Isolation Rooms (AIIRs) — used for tuberculosis, measles, varicella and similar airborne pathogens — sit at the negative-pressure end of the table: air flows into the room, not out of it, and is exhausted directly outdoors or HEPA-filtered before recirculation. Commonly cited figures are a minimum of 12 total ACH for new construction (6 ACH for existing, pre-renovation rooms) and roughly 2.5 Pa negative relative to the corridor. This page covers AIIRs only as one row in the broader table; for the full commissioning, anteroom, and continuous-monitoring detail, see Airborne Infection Isolation Room (AIIR): Specifications and Monitoring.

Protective Environment (PE) Rooms

Protective Environment rooms exist for the opposite reason an AIIR does: they protect an immunocompromised patient (a hematopoietic stem cell transplant recipient, for example) from environmental pathogens like Aspergillus, rather than containing a pathogen the patient is shedding. Commonly cited figures are a minimum of 12 total ACH and positive pressure relative to the corridor, with HEPA-filtered supply air. Confusing an AIIR and a PE room on a construction drawing or a room-assignment decision reverses the protective intent entirely — it’s a distinct enough risk that some facilities color-code room signage specifically to prevent it.

Sterile Processing Department Airflow

Sterile Processing (Central Supply/CSSD) is one continuous process that ASHRAE 170 splits into pressure zones matching the “dirty to clean” workflow — this is the section most often missed in a construction-risk assessment because it’s easy to focus on the OR and isolation rooms and treat sterile processing as a back-of-house mechanical room rather than a space the table governs in detail.

• Decontamination/soiled workroom: negative pressure relative to the adjacent clean assembly area and corridor — commonly cited at a minimum of 6 total ACH — so aerosols and bioburden generated during instrument decontamination don’t migrate toward clean areas.
• Clean workroom / preparation and packaging: positive pressure relative to the decontamination room — commonly cited at a minimum of 4 total ACH.
• Sterile storage: positive pressure, commonly cited at a minimum of 4 total ACH, maintained within the temperature/humidity band the sterilization method’s manufacturer instructions for use (IFU) require for maintaining package sterility during storage.

Because these three zones sit next to each other with a required pressure cascade between them, a single failed door seal or an HVAC balancing error at handover can flatten the pressure differential between decon and clean without tripping an obvious alarm — this is a genuine, recurring survey and self-assessment finding, not a hypothetical.

Other Frequently-Cited Spaces

• Endoscopy/bronchoscopy procedure rooms: negative pressure is commonly specified given aerosol-generating and airway-adjacent procedures, at ACH levels closer to a standard procedure room than a full AIIR.
• Emergency department triage/exam and trauma rooms: typically neutral pressure with standard procedure-room ACH, though a facility may designate specific ED rooms as negative-pressure-capable for surge use.
• Pharmacy sterile compounding areas: governed jointly by ASHRAE 170 and USP 797/795 — see USP 797: Sterile Compounding Requirements Explained for the compounding-specific airflow and ISO-classification requirements layered on top of the base ASHRAE table.
• General patient rooms: neutral pressure, standard ACH — the table’s default case, and the baseline every “does this room need to be different” question gets compared against.

How Ventilation Deficiencies Show Up as Survey Findings

For the audience this page is written for, the table only matters insofar as it becomes something a surveyor or an internal quality team can find deficient. In practice that happens through a handful of recurring patterns:

• Missing or unreviewed continuous pressure-monitoring logs for AIIRs, PE rooms, and ORs — the requirement isn’t just that the room is negative or positive, it’s that the facility can show it stayed that way.
• Construction-risk assessments that don’t reference the applicable ventilation requirement for the specific space being renovated — an Infection Control Risk Assessment (ICRA) that addresses dust and debarment controls but never states the target pressure relationship or ACH for the finished space is an incomplete ICRA. See Infection Control Risk Assessment (ICRA) for Construction for how the two documents are supposed to connect.
• Sterile processing pressure cascades that drift out of spec after balancing, filter changes, or a door/seal failure, caught (or missed) on routine facilities rounds rather than a formal commissioning event.
• No documented mapping between “as-built” ventilation and the edition of ASHRAE 170/FGI in force at the time of construction — surveyors and internal auditors both need to know which requirement applies to which room, and an undocumented answer reads as a gap even when the room itself is performing correctly.

Findings against any of these typically land as EC or IC chapter Requirements for Improvement rather than an immediate threat-to-life citation, but they compound: a facility with several small, unaddressed pressure-monitoring gaps across its OR suite reads very differently to a surveyor than one with a single isolated finding.

Verifying and Documenting Compliance

Two verification methods do most of the work, and facilities teams typically use both:

• Continuous monitoring via the building automation system or a dedicated room-pressure monitor, for spaces where ASHRAE 170 and accreditation standards expect ongoing verification (AIIRs, PE rooms, ORs) — this is what produces the log a surveyor will ask to review, and what lets a unit know within minutes, not at the next periodic check, that a room has drifted out of spec.
• Periodic physical verification — smoke-tube or similar visual airflow-direction testing, and airflow-velocity/ACH measurement — for spaces without continuous monitoring, and as a periodic cross-check even where continuous monitoring exists, since a monitor reading correctly doesn’t guarantee its sensor is placed or calibrated correctly.

Document both the result and the applicable requirement it was checked against (edition, table row, target value) — a log that only says “pass/fail” without recording what standard and value it was measured against is difficult for a surveyor, or a future facilities team, to independently confirm.

Frequently Asked Questions

Does ASHRAE 170 apply to existing hospitals, or only new construction?

Both, but differently. New construction and major renovation must meet the current adopted edition’s requirements. Existing, unrenovated space is generally held to the edition in force when it was originally built or last renovated — which is why the applicable ACH or pressure figure for a specific room depends on when that room was actually built, not on today’s calendar date.

What’s the difference between ASHRAE 170 and the FGI Guidelines?

ASHRAE 170 is the mechanical-ventilation standard itself — the room-by-room table. The FGI Guidelines for Design and Construction of Hospitals is the broader healthcare-facility design document that incorporates ASHRAE 170 by reference for ventilation, alongside space-planning, structural, and other requirements. States generally adopt an edition of the FGI Guidelines (which pulls in an ASHRAE 170 edition) into their hospital licensing code, rather than adopting ASHRAE 170 on its own.

Who is responsible for confirming a room meets its ASHRAE 170 requirement — facilities, or infection prevention?

Both, at different points. Facilities/engineering owns the mechanical design, commissioning, and ongoing monitoring; infection prevention owns confirming the correct room type and requirement was specified in the first place (via the ICRA and design review) and flagging when a room’s clinical use has changed in a way that changes which table row applies to it — a general patient room converted to isolation use without a corresponding ventilation review is a common gap between the two functions.

Does a positive-pressure operating room need an anteroom the way a negative-pressure isolation room does?

Not for the same reason. An AIIR’s anteroom exists primarily to maintain the pressure cascade and give staff a controlled space to don/doff PPE without breaching the isolation room’s negative pressure. Operating rooms are more commonly designed with a scrub/sub-sterile transition space serving a workflow and contamination-control function rather than the same pressure-cascade requirement — check the specific table row and any local FGI amendments rather than assuming AIIR anteroom logic transfers directly.

Related reading: the Patient Safety cluster hub, High-Level Disinfection and the Spaulding Classification, Sterile Processing Department (CSSD) Equipment Purchasing, and NFPA 101 Life Safety Code in the Hospital Survey for the adjacent facilities-compliance requirement most EC tracers check alongside ventilation.