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Pathology lab HVAC should generally move air inward from adjacent spaces, but there is no single table value that can safely be treated as a universal design specification. CDC guidance lists 6 total air changes per hour (ACH), direct outdoor exhaust, no room-unit recirculation and 75°F (24°C); a public ASHRAE/ASHE 170-2017 addendum lists negative pressure, 2 outdoor ACH, 6 total ACH, direct outdoor exhaust and a 70–75°F (21–24°C) range. Those figures belong to their named sources and editions—not automatically to every room in a pathology department or every current project.
For construction and renovation decisions, classify each room by its actual work and hazards, then confirm the complete standard edition adopted by the local authority and the facility’s applicable codes. A routine pathology work area, histology room, autopsy room and infectious-disease lab are not interchangeable HVAC spaces.
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Should a pathology lab be negative pressure?
For the pathology work area addressed in the cited references, airflow should be directed inward from adjacent spaces; the public ASHRAE/ASHE 170-2017 addendum labels the relationship negative. CDC’s ventilation table describes the direction as “in” from the adjacent area. In practical terms, the room’s supply and exhaust arrangement should support air moving into the room rather than uncontrolled air leaving it.
Negative pressure is a relationship between spaces, not a number that can be specified without context. ASHRAE notes that pressure relationships in healthcare areas vary with the medical processes and procedures performed. The cited technical FAQ does not establish a universal pressure-differential value for pathology rooms. Project documents should therefore identify the spaces that form each pressure boundary, the intended direction of airflow, and how the relationship will be verified; do not infer a required pascal or inches-of-water value from the table entries alone.
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These references are design guidance, not a substitute for checking the locally adopted requirements. ASHRAE identifies Standard 170-2025 as the current published edition, superseding the 2021 edition, but a construction team should confirm the complete applicable pathology requirements and the edition adopted by its authority having jurisdiction before design is finalized. The historical 2017 addendum remains useful as a clearly labeled reference point, not as proof of the current legal requirement.
How many air changes per hour does a pathology lab need?
The two cited tables both list 6 total ACH for pathology. ASHRAE/ASHE’s 170-2017 Addendum a also lists 2 outdoor ACH; the CDC table does not show an outdoor-ACH value for pathology. These are source- and edition-specific minimum table figures, not a complete airflow design or a statement that every room in the department requires the same rate.
| Reference | Pressure / airflow direction | Outdoor ACH | Total ACH | Exhaust / room-unit recirculation | Temperature |
|---|---|---|---|---|---|
| CDC, Environmental Infection Control in Health-Care Facilities ventilation table | In from adjacent area | Not shown for pathology | 6 | Direct outdoor exhaust: yes; room-unit recirculation: no | 75°F (24°C) |
| ASHRAE/ASHE Standard 170-2017, Addendum a (2020) | Negative | 2 | 6 | All room air exhausted directly outdoors: yes; room-unit recirculation: NR (no requirement shown) | 70–75°F (21–24°C) |
The CDC values come from its Appendix B air table; the other figures come from ASHRAE/ASHE 170-2017 Addendum a. The CDC web presentation’s publication year is not stated in the accessed table. In both sources, the listed 6 ACH is a design-table figure, not a measured result for a particular room. “NR” should not be read as permission to recirculate: it means that the cited ASHRAE column shows no requirement for that entry, while CDC explicitly says “No” for room-unit recirculation.
ACH describes ventilation flow relative to the room’s volume over time. A table minimum alone does not determine the required supply airflow, exhaust airflow, system capacity, or how the room will hold its pressure relationship under operating conditions. The HVAC design also has to account for the exhaust of containment devices, cooling needed for internal heat gains, minimum ventilation, and airflow needed to maintain pressure relationships. ASHRAE describes these as factors that can drive laboratory airflow.
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The 2017 addendum material also describes a possible route to lower total ACH after a hazard assessment within an effective Laboratory Ventilation Management Plan, subject to stated conditions that include acceptable exposure concentrations or active contaminant sensing. This is not a basis for informally turning down ventilation, and it does not confirm what the 2025 edition allows. Any proposed reduction requires review against the currently adopted standard, a documented hazard assessment, and the facility’s safety and operations requirements.
Does pathology lab air need to exhaust outdoors?
Both cited pathology references indicate direct outdoor exhaust: CDC marks it “Yes,” and ASHRAE/ASHE’s 2017 addendum says all room air is exhausted directly outdoors. CDC also lists no room-unit recirculation for pathology. Because the entries are not identical across every column, preserve the distinction: the CDC table explicitly addresses room-unit recirculation, while the cited 2017 ASHRAE entry is “NR,” or no requirement shown in that column.
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Outdoor exhaust affects more than the exhaust fan. The design team must coordinate exhaust and makeup air, the pressure relationship to adjacent areas, the exhaust from any hoods or other containment equipment, and the building’s wider HVAC operation. OSHA’s laboratory chemical-hygiene recommendations say a negative pressure differential should exist between exhausted and supplied air to prevent uncontrolled chemical vapors leaving a laboratory; that language appears in Appendix A to 29 CFR 1910.1450, which OSHA identifies as non-mandatory recommendations. It is not a numeric code requirement for all pathology rooms.
Do not assume that a room’s exhaust-stack location, discharge velocity, filtration, or exhaust treatment can be determined from the pathology table row. Those questions depend on the complete adopted standard, applicable codes, materials and procedures, hazard analysis, and the facility’s overall design. The sources cited here do not supply universal values for those features.
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What temperature should a pathology lab be?
The CDC table lists 75°F (24°C) for pathology. The public ASHRAE/ASHE 170-2017 addendum lists 70–75°F (21–24°C). Report the range with its edition rather than combining the entries into an uncited universal setpoint.
Temperature targets must also be coordinated with the actual equipment and work program. ASHRAE’s healthcare guidance identifies internal heat gains as one factor that can drive laboratory airflow and cooling. The project team should inventory heat-producing equipment and expected use when sizing and balancing the HVAC system; a room that meets a nominal temperature entry on paper may still need a system designed for its real thermal load. The cited pathology table values do not specify humidity requirements or resolve every comfort, equipment, or process-control need.
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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsWhich rooms in a pathology department need different treatment?
Room function and hazard profile matter. A department name does not establish that every enclosed space should use one pressure direction or the same ventilation assumptions.
- Routine pathology work area: The CDC table describes inward airflow, 6 total ACH, direct outdoor exhaust, no room-unit recirculation, and 75°F (24°C). The 2017 ASHRAE/ASHE addendum calls the room negative and lists 2 outdoor ACH, 6 total ACH, direct outdoor exhaust, and 70–75°F (21–24°C).
- Histology: CDC lists histology separately, with 6 total ACH, inward airflow, direct outdoor exhaust, no room-unit recirculation, and 75°F (24°C). That separate row is a reason to check the room’s precise function rather than silently treating every pathology space as one category.
- Autopsy room: ASHRAE’s healthcare chapter describes autopsy rooms as needing negative pressure relative to adjoining rooms or corridors because of contamination and odor concerns. That discussion applies to autopsy work; it should not be copied as a blanket specification for all pathology rooms.
- Infectious-disease or virus lab: ASHRAE describes special treatment for these labs, including a recommended minimum of 6 ACH or makeup approximately equal to hood exhaust, and negative pressure relative to adjacent areas. These notes are not a substitute for the applicable biosafety and facility-specific design requirements.
- Media-transfer room: The ASHRAE/ASHE 170-2017 addendum lists media transfer as positive. This illustrates why a pressure direction suitable for one room cannot automatically be imposed on another room with a different function.
ASHRAE’s healthcare guidance says pressure relationships vary according to the medical processes and procedures that will occur. For a renovation, that means the room-by-room schedule should be based on how each space will actually be used, not simply its legacy label on an old plan. If a room’s use changes, the airflow classification and supporting HVAC design may need to be reassessed.
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How should a project team specify and verify pathology HVAC?
Start with the adopted requirements and room program, then coordinate the airflow design with containment, thermal loads, and commissioning. A useful sequence for a new build or renovation is:
- Confirm the governing basis. Identify the applicable jurisdiction, healthcare and mechanical codes, facility licensing requirements, and the edition of ASHRAE/ASHE Standard 170 adopted by the authority having jurisdiction. ASHRAE lists Standard 170-2025 as the current published edition, but the locally adopted edition—not publication date alone—controls a project. Obtain and check its complete relevant table and notes.
- Classify every room by use. Document whether the room is routine pathology, histology, autopsy, infectious-disease work, media transfer, or another function. Record procedures, materials, hazards, occupancy, and adjacent spaces so the pressure direction is designed against a defined room boundary.
- Inventory source containment and exhaust. List fume hoods and other primary containment devices, their exhaust needs, and the work they protect. Coordinate these exhaust flows with the supply system and safety staff. General room ventilation dilutes contaminants; it does not replace capture or containment at the source.
- Set the airflow and thermal design basis. Resolve the applicable outdoor and total ventilation requirements, exhaust destination, recirculation constraints, intended pressure relationships, and equipment heat gains. Determine how the design will accommodate hood exhaust, makeup air, cooling, and pressure control together rather than treating ACH as the sole sizing input.
- Coordinate facility stakeholders early. Bring the HVAC engineer, laboratory staff, safety officers, facility operations, and maintenance personnel into the design process. They can identify operating modes, equipment loads, maintenance constraints, and safety needs that are not apparent from a room name or a table entry.
- Document commissioning and verification. Put the intended pressure relationship and airflow behavior into the design and verification plan. ASHRAE notes that a recognized testing-and-balancing professional may be appropriate for critical pressure relationships. The cited FAQ does not specify one universal differential pressure value, so the team should establish acceptance criteria from the applicable standard, design, and authority requirements.
- Reassess before changing HVAC. Before altering an existing system, evaluate the effect on laboratory ventilation, hood exhaust, room pressure, and the wider building system. A local adjustment can change the balance elsewhere; verify the resulting operation rather than assuming the former design intent remains satisfied.
ASHRAE’s laboratory guidance makes the source-containment distinction explicit: “The dilution ventilation provided by this airflow is no substitute for the containment performance of a laboratory fume hood or other primary containment device, regardless of the room ventilation rate.” The ASHRAE laboratory chapter is therefore relevant alongside a healthcare room table when the pathology work uses containment devices or generates hazards that need source capture.
Likewise, OSHA’s cited chemical-hygiene appendix is a recommendation, not a substitute for an enforceable code determination. Its discussion is useful in planning chemical-vapor control, but project teams should distinguish advisory guidance from legal obligations in their jurisdiction. Neither a consumer pressure meter nor a single spot reading can establish compliance by itself; documented design criteria, qualified balancing, and verification against the adopted requirements are needed for a defensible facility decision.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What should not be inferred from a ventilation table?
The pathology rows cited here do not establish every technical requirement a contractor or engineer may need. Do not infer from the row alone a filter efficiency, humidity range, pressure alarm, display location, door arrangement, exhaust-stack detail, or commissioning tolerance. Check the full currently adopted standard, other applicable codes, facility risk controls, and the room’s hazard analysis for those requirements.
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It is also important not to convert a published guidance value into a universal law. The CDC table and the 2017 addendum are distinct references with differences in their entries, and the current published Standard 170-2025 supersedes the 2021 edition. A design decision should state which requirement source and edition it follows and should be confirmed with the authority having jurisdiction. This is especially important when an existing space is being renovated, since the facility’s current room use, existing exhaust connections, and adopted code basis may not match an old drawing.
Frequently Asked Questions
What does 6 ACH mean for a pathology room?
ACH means air changes per hour: the ventilation flow is expressed in relation to the room volume over an hour. It is a design rate, not a promise that every airborne contaminant is removed within a fixed time or that the room is safe without appropriate source containment.
Can a pathology room use a recirculating fan coil?
The CDC pathology table says room-unit recirculation is not allowed in its listed guidance. The cited 2017 ASHRAE addendum shows “NR” for that column, so confirm the complete currently adopted standard and local requirements rather than treating that entry as approval.
Does OSHA set a numeric negative-pressure value for pathology labs?
The cited OSHA Appendix A sentence recommends a negative pressure differential for laboratories but gives no numeric pressure threshold and is labeled non-mandatory. A project’s acceptance criteria must come from the applicable adopted standards, codes, and design basis.
Who should verify a lab’s pressure relationship after construction?
The project should use a documented balancing and commissioning process, with qualified testing-and-balancing support where the relationship is critical. Coordinate the verification plan with laboratory safety and facility operations so the intended room-to-room airflow is checked under relevant operating conditions.
