Operating Room Positive Pressure Requirements: What ASHRAE 170 Actually Requires

Key Takeaways

• Operating rooms are classified as positive pressure spaces under the Facility Guidelines Institute (FGI) Guidelines and ANSI/ASHRAE/ASHE Standard 170, so the room has to sit at higher pressure than the corridors and support spaces around it
• Pressure differential, air change rate, filtration, temperature, and humidity figures are edition specific, so confirm each one against the Standard 170 and FGI edition your authority having jurisdiction (AHJ) enforces before it enters a policy document
• Rooms fall out of spec for a short list of recurring reasons: supply and exhaust imbalance, a failing fan, blocked diffusers or return grilles, an adjacent fume hood or biosafety cabinet, and renovation HVAC work that was never rebalanced
• Surveyors want the excursion record itself, when it started, how long it ran, who was notified, and what closed it out, so a building automation system alarm nobody logged does not answer the question
• Hexmodal's Smart Room Pressure Monitor closes the detection and documentation gap through continuous logging and exportable excursion history, while testing and balancing the mechanical system stays with a qualified engineer

Operating rooms are classified as positive pressure spaces under the FGI Guidelines and ANSI/ASHRAE/ASHE Standard 170, meaning the OR must be maintained at a higher air pressure than the corridors and support spaces around it. This keeps air flowing out of the room and stops it from being pulled in, which is what helps keep contaminants from adjacent areas from reaching the sterile field. A room that passed this test during commissioning does not stay compliant automatically. It has to keep meeting the standard every day the room is in use.

What ASHRAE 170 Requires for Operating Room Positive Pressure

Which Spaces Require Positive Pressure

Per ASHE's Mechanical Systems Handbook for Health Care Facilities, the 2014 FGI Guidelines and Standard 170-2013 list operating rooms alongside a specific group of other spaces that must be positively pressurized relative to surrounding areas: delivery rooms, trauma rooms, the NICU, laser eye rooms, protective environment (immunocompromised patient) rooms, pharmacy, and central sterile clean and sterile storage areas. These rooms sit at the opposite end of the pressure spectrum from spaces like airborne infection isolation rooms (AIIRs), which are deliberately held at negative pressure so they contain airborne contaminants inside the room.

What Positive Pressure Means Mechanically

Positive pressure is achieved by supplying more air to the room than is exhausted or returned from it, so the pressure differential pushes air out through door gaps and other openings rather than allowing corridor or adjacent-room air to migrate in. In an OR, this mechanism exists for one reason: protecting the patient and the sterile field from airborne pathogens that may be present in adjacent, less-controlled spaces. When positive pressure is lost, even temporarily, that protective airflow direction reverses or collapses, and the aseptic environment is compromised for as long as the condition persists.

Core Requirement Categories

ASHRAE 170 sets minimum requirements across several categories for OR spaces, including the required pressure differential, total and outdoor air change rates, filtration efficiency, and acceptable temperature and humidity ranges. These figures are edition specific. Standard 170 has been revised multiple times, and the FGI Guidelines edition your facility operates under depends on when the space was built or last renovated and which edition your AHJ has adopted. Do not rely on a number quoted in a vendor blog post, an AI-generated summary, or a prior facility's documentation. Before you cite a specific pressure differential, ACH figure, or filtration rating in your own policies, confirm it directly against the current ANSI/ASHRAE/ASHE Standard 170 and the FGI Guidelines edition your AHJ enforces, and cross-check against the current Joint Commission Edition for any related EC or PE standard language.

The Sterile Storage Pressure Relationship Question

ASHE has separately flagged that the pressure relationship between operating rooms and adjacent areas, including sterile storage, is a recurring point of confusion. Because sterile storage is also classified as a positive pressure space, facility teams sometimes assume a default hierarchy between the OR and adjacent storage that isn't clearly specified, particularly where storage opens directly onto restricted areas of the surgical suite. This is not a detail to guess at. Resolve the intended pressure relationship with your AHJ and document the agreed configuration rather than assuming one space should always dominate the other.

How Compliance Is Verified Today

Positive pressure in an OR is produced by the mechanical system, not created or repaired by any monitoring device. Getting a room into spec, and keeping it there, still depends on qualified mechanical professionals performing testing and balancing (TAB) work on the supply and exhaust systems. That work is the one part of this requirement that no software replaces.

Day to day, facilities lose positive pressure for a short list of recurring, well-documented reasons:

• Supply and exhaust airflow rates fall out of balance
• A supply or exhaust fan begins to malfunction
• Diffusers or return grilles get partially blocked, sometimes by staff adjusting airflow for personal comfort
• A fume hood or biosafety cabinet in an adjacent space pulls harder than the design anticipated
• An HVAC change made during a recent renovation was never rebalanced afterward

None of these causes are exotic. They are the same handful of mechanical and behavioral issues showing up again and again in ASHE's guidance.

Historically, facility teams have relied on a mix of building automation system (BAS) alarms, periodic manual spot checks, and, where required by code, visual verification methods such as smoke trail testing, ball-in-tube gauges, or flutter strips to confirm airflow direction. Verification cadence and method requirements are also edition specific and should be confirmed against your adopted standard rather than assumed. What matters most to an AHJ, CMS surveyor, or Joint Commission tracer team is not just whether the room was in spec at some point. It is whether you can produce a record: when did an excursion start, how long did it last, who was notified, and what corrective action closed it out. A BAS alarm that nobody logged, or a spot check that wasn't documented, does not answer that question during a survey.

How Hexmodal Automates the Verifiable Parts

Hexmodal's Smart Room Pressure Monitor addresses the documentation and detection gap described above, not the mechanical one. It continuously tracks pressure differential, temperature, and humidity in the room, and automatically logs every reading, every excursion, and every corrective action with a timestamp, so there is no dependency on someone remembering to write it down. When a room drifts out of range, alerts route to the right people by room, unit, or facility, rather than sitting unnoticed in a BAS alarm log.

Trend graphs, alarm history, and time-stamped excursion records export in formats built for CMS, AHJ, and internal survey use, showing when an excursion started, how long it lasted, who was notified, and what correction was made. Installation involves no wiring or IT department time, and one gateway can cover up to 250,000 square feet and more than 1,000 devices across a facility.

What this system does not do is fix the room. It does not perform HVAC testing and balancing, duct rebalancing, or mechanical correction of an out-of-range space. A qualified engineer or contractor still has to do that work. It also does not perform initial ASHRAE 170 commissioning or design verification, and it is not a substitute for any code-required manual visual verification method your standard may call for.

The Smart Room Pressure Monitor includes NIST-traceable probes. Hexmodal keeps the sensor's NIST certificate continuously valid through a two-year sensor module swap rather than a manual annual recalibration visit, but that process covers the sensor itself, not recalibration or rebalancing of the building's HVAC system.

Schedule a Demo

Speak with us to see how Hexmodal's Smart Room Pressure Monitor supports continuous excursion logging for OR pressure compliance documentation.

FAQ

Do operating rooms have to stay positive to every adjacent space, including sterile storage?

‍Both ORs and sterile storage are classified as positive pressure spaces under the FGI Guidelines and ASHRAE 170, but the intended pressure relationship between the two, especially where storage opens directly onto restricted OR suite areas, is not always clearly specified. Confirm the intended hierarchy directly with your AHJ.

What actually causes an OR to lose positive pressure after it already passed inspection at construction?

‍The most common causes are supply/exhaust fan imbalance, blocked diffusers or return grilles (sometimes blocked by staff for comfort), fume hood or biosafety cabinet operation in an adjacent space, and HVAC changes made during a renovation that were never rebalanced afterward.

How often are we supposed to verify operating room pressure, and who is responsible for that testing?

‍Verification cadence is specified by the edition of ASHRAE 170 and the FGI Guidelines your AHJ has adopted, so confirm the exact interval against your current standard. Ongoing mechanical testing and balancing is performed by qualified engineers or mechanical contractors, not by monitoring software.

What is the difference between a positive pressure room and a negative pressure (isolation) room in a hospital?

‍A positive pressure room, like an OR, pushes air outward to keep contaminants from adjacent spaces out of the room. A negative pressure room, like an AIIR, pulls air inward to contain airborne contaminants generated inside the room and prevent them from escaping into corridors.

What documentation does a surveyor expect to see for OR pressure compliance during a Joint Commission or CMS visit?

‍Surveyors using tracer methodology generally expect a defensible record showing when a pressure excursion occurred, how long it lasted, who was notified, and what corrective action was taken, not just proof that the room passed a test at some point in the past.

Our building automation system already has a pressure alarm. Do we still need a separate monitoring or documentation system to be survey-ready?

‍A BAS alarm can flag an excursion in real time, but it does not automatically create the time-stamped, exportable record of the event, notification, and correction that a surveyor expects. Facilities that cannot produce that record on demand often add Hexmodal's Smart Room Pressure Monitor to generate that log automatically.