ASHRAE 170 Requirements and Which Edition Applies to You

Search for the air change rate an operating room has to hold and you will find two different answers, both from sources that look authoritative. One says 15 air changes per hour. The other says 20. Neither is a misprint, and working out which one applies to a particular room takes longer than looking up the number did.

This article covers the ASHRAE 170 requirements from the outside in: what the standard sets and what it leaves alone, how it becomes enforceable, which edition governs a given building, what the 2025 revision changed, and how each value gets verified. For the monitoring side of the question, which parameters to measure in each clinical space and how to choose a device, see our article on hospital environmental monitoring and air quality standards by space.

Quick answer: ANSI/ASHRAE/ASHE Standard 170 sets ventilation design conditions for health care spaces: pressure relationship, minimum outdoor and total air changes per hour, design temperature, relative humidity, and filtration, organized by space type. It sets no indoor concentration limits for carbon dioxide, particulates, or VOCs. The standard is revised every four years to correlate with the FGI documents and becomes enforceable through state adoption of FGI, NFPA 99, and the local authority having jurisdiction. Standard 170-2025 is the current edition, and the 2026 FGI Codes incorporate it along with all issued addenda. FGI lists those codes as scheduled for release in fall 2026, and states adopt a new FGI edition on their own timelines after that, so most facilities are held to an earlier edition today.

Last reviewed: September 4, 2026. This article describes what the named standards cover and cites publicly available sources for each value. It does not reproduce ASHRAE Standard 170 or the USP General Chapters, which are copyrighted and available from their publishers. Confirm the edition in force in your jurisdiction before relying on any figure.

What ASHRAE 170 Sets, and What It Leaves Alone

Standard 170 is a ventilation standard. It tells a designer how air should move through a health care space and what condition that air should be in when it arrives. For each space type it sets a pressure relationship to adjacent areas, a minimum number of outdoor air changes per hour and a minimum total, a design temperature range, a relative humidity limit or range, and the filtration the supply air has to pass through.

What it does not set is a concentration limit for anything in the air.

There is no Standard 170 threshold for carbon dioxide, for PM2.5, or for total VOCs. That surprises people, because those are exactly the parameters an indoor air quality monitor puts on a screen, and it is easy to assume a standard about health care ventilation must govern them.

Two claims are worth separating here, because they get conflated constantly. Standard 170 sets no concentration limits, and neither does ASHRAE’s ventilation standard for other occupancies. ASHRAE’s Position Document on Indoor Carbon Dioxide, reaffirmed in February 2025, puts it plainly: “Despite many statements to the contrary, ANSI/ASHRAE Standard 62.1 does not provide a limit value for indoor CO2.” The document adds that misreadings of earlier editions “continue to lead many to incorrectly attribute a 1000 ppmv limit to ASHRAE.”

NIST researcher Andrew Persily traced that figure in a 2020 conference paper titled “Quit blaming ASHRAE Standard 62.1 for 1000 ppm CO2”, and found it had been out of the standard for close to 30 years, removed because of the confusion it caused and because it is a poor indicator of either ventilation or air quality. ASHRAE does accept CO2 as a diagnostic: readings well above the expected range can indicate a system is not performing as designed, which is a narrower and more useful claim than a limit.

Enforceable concentration limits come from outside ASHRAE entirely. EPA National Ambient Air Quality Standards apply to outdoor air, and OSHA permissible exposure limits govern what staff are exposed to at work. Indoor action levels for CO2 and particulates are matters of internal policy informed by guidance from bodies such as the World Health Organization. They are worth setting, and they are not requirements a surveyor cites against a room.

Which leaves a result worth sitting with: a facility can hold every parameter Standard 170 requires and still have an air quality problem, and it can post excellent CO2 and particulate readings while a protective environment room sits at the wrong pressure. The two questions overlap without being the same question.

How the Standard Becomes Enforceable

Standard 170 is not itself a law. It becomes binding by reference. The FGI documents incorporate it, states adopt or reference FGI for health care construction, and NFPA 99 and the International Mechanical Code have both adopted it as well. Accrediting bodies then survey against the result: the Joint Commission’s environment of care standard EC.02.05.01 addresses ventilation in critical spaces, and CMS and DNV apply comparable expectations. The local authority having jurisdiction has the final say on which edition governs a given project.

Which Edition of ASHRAE 170 Applies to Your Facility

Answer this one first. Every number downstream depends on it.

Standard 170 is revised on a four-year cycle timed to the FGI publication schedule. The 2008 edition was incorporated into the 2010 FGI Guidelines, and the 2021 edition, which folded in 17 addenda, aligned with the 2018 FGI Guidelines. The current edition is ANSI/ASHRAE/ASHE Standard 170-2025, which revises 170-2021.

That cycle is turning over now. FGI is restructuring what used to be a single Guidelines for Design and Construction into enforceable FGI Codes for Planning and Design plus a separate series of FGI Handbooks. As Health Facilities Management describes the split, the Codes “present the minimum requirements intended for adoption and enforcement,” while the Handbooks carry the context and rationale behind them, and the 2026 Codes incorporate Standard 170-2025 including all issued addenda. Requirements were relocated into the new structure rather than rewritten.

Timing matters here, so it is worth being precise about it. As of this article’s review date, FGI lists the 2026 FGI Codes and Handbooks as scheduled for release in fall 2026, with a notification list for the Codes, while the 2026 Handbooks show as available on FGI’s OneSource platform. Some trade coverage in early September described the Codes as released. Treat FGI as the authority on its own publication dates, and check the editions page for current status.

Which brings up the distinction that matters most here: publication is not adoption.

A document written for adoption still has to be adopted, and the 2026 Codes are not there yet on either count. States take up a new FGI edition on their own timelines once it does publish, and some lag by several cycles, so the edition that governs a space is usually not the newest one. Existing buildings add a second layer, because a space is commonly held to the requirements in force when it was built or last substantially renovated. That is how a hospital with a 1990s wing, a 2012 renovation, and a new surgical addition ends up operating three different sets of numbers under one roof.

So the sequence is: what edition has your state adopted, what does that edition reference, and what was in force when this particular space was last renovated. Your authority having jurisdiction can answer all three. A search result cannot.

What Changed in the 2025 Edition

By ASHE’s own account the 2025 revision is a maintenance update, not a rewrite, though several changes matter for anyone specifying or monitoring space conditions. It adds requirements for the optional use of natural ventilation. It changes how total outdoor air is calculated at the system level where one system serves spaces governed by both Standard 170 and Standard 62.1. It clarifies unoccupied turndown in outpatient spaces, updates requirements for Class 2 and Class 3 imaging rooms and their associated non-imaging spaces, clarifies nuclear medicine, and clarifies bronchoscopy requirements. New function categories cover behavioral health spaces, other new space types coordinate with the 2026 FGI space types, and similar space types are brought into line with each other across the different tables.

On the systems side it revises heating and cooling reserve capacity and on-site fuel requirements, clarifies stack discharge heights and separation distances for complex situations, reorganizes Section 10 covering ventilation during construction, and notes acceptable alternative filtration testing to make the standard easier to use internationally.

The 2021 edition is the more useful comparison, since it is the one most facilities are working to. It raised supply filtration in operating rooms and Class 3 imaging rooms from MERV 14 to MERV 16, added a HEPA filtration requirement at the air terminal for higher-acuity procedures including orthopedic and transplant work, introduced explicit unoccupied turndown provisions, and split the outpatient section into specialized and general categories. A facility still operating to those requirements is not out of compliance by default. It is complying with the edition it was held to.

Why Published ASHRAE 170 Room Requirements Disagree

The operating room discrepancy is the clearest case, and the same pattern affects most published room tables.

The CDC’s Appendix B ventilation table is one of the most cited references for hospital air change rates: a US government work, freely available, widely reproduced. It also states plainly that its values come from the 2001 AIA Guidelines for Design and Construction of Hospitals and Health-Care Facilities, and it lists 15 total air changes per hour for surgery with 3 outdoor air changes. Standard 170-2021 sets 20 total and 4 outdoor for an operating room, at positive pressure of at least 0.01 inches water gauge. A page that reproduces the CDC table without naming its basis is not wrong about what CDC says. It is quietly 25 years out of date on what the current standard requires.

Pressure relationships have held steady across editions. Air change rates for surgical spaces have not.

CDC column quoted from the CDC Appendix B air table, a US government work, which states its values derive from the 2001 AIA guidelines. CDC additionally requires isolation and bronchoscopy room air to be exhausted directly outdoors unless HEPA filtered, and specifies HEPA at 99.97 percent efficiency for protective environment rooms. Standard 170-2021 values are attributed to that edition and are not reproduced from it; the 2025 edition clarifies bronchoscopy requirements. Verify against the edition adopted in your jurisdiction.

Two things follow. The first is that “not required” is a real answer and not a gap in the data: most general patient care rooms, including PACU, intensive care and general inpatient rooms, carry no mandated pressure relationship and are commonly designed to 6 total air changes per hour. A facility that installs differential pressure monitoring on every med-surg room has bought something no standard asked for.

The second is that pressure direction explains itself once you look at it. Surgical and protective spaces run positive so filtered air moves outward and protects what is inside the room. Isolation and bronchoscopy spaces run negative so contaminated air stays contained. A reversal in either type defeats the entire purpose of the space, which is why those rooms are the ones expected to hold their differential continuously instead of getting checked on a round.

Sterile Compounding Runs on a Separate Set of Standards

Pharmacy spaces confuse people, because they sit inside a hospital governed by Standard 170 while their own requirements come from somewhere else. Sterile compounding follows USP General Chapter 797, whose revised version became official on November 1, 2023. Hazardous drug handling follows USP General Chapter 800, which became official on December 1, 2019 and compendially applicable on November 1, 2023. Cleanliness classes come from ISO 14644-1.

The USP chapters are more prescriptive than Standard 170 in two specific ways.

They give a pressure differential as a number, where Standard 170 gives a direction. USP’s own FAQ for Chapter 800 states that the containment secondary engineering control has to hold negative pressure of 0.01 to 0.03 inches of water column relative to all adjacent areas at all times, and that a pressure gauge is required to monitor it.

Chapter 797 likewise expresses its differential as a specific figure, 0.020 inches water column, carried to three decimal places, per the American Society of Health-System Pharmacists summary of the revised chapter. Relative humidity in compounding areas is held at 60 percent or below, and an ISO Class 8 room requires more than 20 air changes per hour.

They also expect a second kind of evidence. Alongside the continuous environmental record, sterile compounding carries periodic ISO particle counts performed to a certification schedule, a documentation obligation most clinical spaces do not have. Pharmacy storage adds a temperature question of its own under USP General Chapter 659, which defines controlled room temperature as 20 to 25 degrees Celsius, or 68 to 77 degrees Fahrenheit, with limited excursions calculated as mean kinetic temperature. That one governs drug stability, a different concern from air quality.

Which Values Are Enforceable, and How Each One Gets Verified

Getting this distinction right is what keeps a compliance record defensible. Some of the numbers a facility tracks are requirements a surveyor can cite against. Others are internal action levels that are operationally useful and carry no regulatory weight. Presenting the second group as though it belonged to the first misstates what the standards require, and it tends to unravel under questioning.

Classification reflects whether a value originates in an adopted, enforceable standard or in published guidance. OSHA and EPA values are US government works. ASHRAE, USP, and ISO requirements are attributed to their source standards, not reproduced here. Adoption varies by jurisdiction.

Notice how much of that left column gets verified periodically rather than continuously. Air change rates and filtration are commissioning-and-testing parameters; nobody measures ACH in real time. What is measured continuously is a shorter list: differential pressure where a room’s function depends on it, temperature, and relative humidity. The record a facility builds day by day from those three is what has to exist when someone asks what conditions a room held last March.

How Aethair Supports ASHRAE 170 Documentation

Holding these conditions is a mechanical systems problem. Proving they held is a data problem, and that is the part continuous monitoring addresses. Aethair IAQ measures differential pressure, temperature, and relative humidity alongside particulates, carbon dioxide, and VOCs, so the code-required parameters and the operational indicators land in one record; Aethair PRO extends the gas measurement set where sterile processing or a specific chemical concern calls for it. Every unit is independently 4G connected, so a deployment does not depend on hospital IT, guest WiFi, or a shared gateway that takes a wing offline when it fails.

Environet holds the resulting record: live conditions across monitored rooms, history by space and date range, and alerts set against the thresholds each room has to hold, so a pressure reversal in an isolation room reaches the responsible person while it is still happening. Aethair Reports compiles that data into structured documentation on a schedule or on demand. Noesis, the AI analysis tool in the platform, lets a team ask about the record in plain language and pull the excursions from a given quarter for review. It supports the people accountable for the answer; it does not replace their judgment.

For the monitoring program itself, which parameters matter in each clinical space and how to select a device, see our article on hospital environmental monitoring and air quality standards by space. For how continuous monitoring is treated across other frameworks, see our article on building certifications and air quality, and for the occupational exposure side of the same building, our article on OSHA air quality standards.

ASHRAE 170 Requirements: Frequently Asked Questions

What does ASHRAE 170 actually require?

ANSI/ASHRAE/ASHE Standard 170 sets design and operating conditions for ventilation in health care facilities: pressure relationships between spaces, minimum outdoor and total air changes per hour, design temperature ranges, relative humidity limits, and supply air filtration. It is organized by space type, so the values that apply to an operating room differ from those for a patient room or a soiled workroom. It does not set indoor concentration limits for carbon dioxide, particulate matter, or volatile organic compounds.

Which edition of ASHRAE 170 applies to my facility?

Usually not the newest one. Standard 170 is revised on a four-year cycle to correlate with the FGI Guidelines, and it becomes enforceable through whichever FGI edition a state has adopted, along with NFPA 99 and the local authority having jurisdiction. Standard 170-2025 is the current edition, and the 2026 FGI Codes incorporate it along with all issued addenda. FGI lists those codes as scheduled for release in fall 2026. Adoption is then a separate step: states take up a new FGI edition on their own timelines, and some lag by several cycles. Existing facilities are also commonly held to the edition in force when the space was built or last renovated. Confirm the adopted edition with your authority having jurisdiction; do not assume it is the current one.

What changed in ASHRAE 170-2025?

The 2025 edition adds requirements for the optional use of natural ventilation, changes how total outdoor air is calculated at the system level for spaces served by both Standard 170 and Standard 62.1, and clarifies unoccupied turndown in outpatient spaces. It updates requirements for Class 2 and Class 3 imaging rooms and their associated non-imaging spaces, clarifies bronchoscopy requirements, adds function categories for behavioral health spaces, and introduces space types that coordinate with the 2026 FGI guidelines. It also updates heating and cooling reserve capacity and on-site fuel provisions, clarifies stack discharge heights and separation distances, reorganizes the section covering ventilation during construction, and adds equivalent filtration testing alternatives.

Why do published ASHRAE 170 air change rates disagree with each other?

Because many widely used references cite older editions. The CDC’s Appendix B ventilation table, one of the most frequently quoted sources for hospital air change rates, states that its values come from the 2001 AIA guidelines, and it lists 15 total air changes per hour for surgery with 3 outdoor air changes. The 2021 edition of Standard 170 sets 20 total and 4 outdoor for an operating room. Both documents are accurate about what they describe. They describe different editions, which is why the edition matters more than the number.

Does ASHRAE set a limit for CO2, PM2.5, or VOCs indoors?

No, and this is a common misreading. Standard 170 governs temperature, humidity, pressure relationships, filtration, and air change rates, not indoor concentrations. ASHRAE’s Position Document on Indoor Carbon Dioxide states that Standard 62.1 “does not provide a limit value for indoor CO2,” and that misreadings of earlier editions continue to lead people to attribute a 1,000 ppmv limit to ASHRAE incorrectly. NIST researcher Andrew Persily has documented that the figure was removed from Standard 62.1 close to 30 years ago because of the confusion it caused. ASHRAE does accept CO2 as a way to verify whether a ventilation system is performing as designed. Concentration limits that are enforceable come from elsewhere: EPA National Ambient Air Quality Standards apply to outdoor air, and OSHA permissible exposure limits govern occupational exposure for staff.

How is each ASHRAE 170 value verified during a survey?

Verification differs by parameter. Air change rates and filtration are established at commissioning and confirmed by periodic airflow and filter testing. Pressure relationships in airborne infection isolation rooms, protective environment rooms, and hazardous drug compounding rooms are expected to hold continuously, since the protection those spaces provide fails the moment the differential reverses. Temperature and relative humidity are recorded over time. Surveyors generally ask for evidence that conditions held, which means retained trend data, alert records showing deviations were caught and addressed, and current sensor calibration records.

This article is an independent summary and is not affiliated with or endorsed by ASHRAE, ASHE, FGI, NFPA, USP, AAMI, or ISO. The named standards are the property of their respective organizations and are available from their publishers.

Aethair provides monitoring and documentation tools designed to support regulatory compliance. Aethair does not certify compliance with CMS, Joint Commission, DNV, or other regulatory requirements.