Recommendations for OR Ventilation during the SARS COV-2 Pandemic – Staying Positive

Charles E. Cowles, Jr., MD, MBA, FASA; Jeffrey Feldman, MD, MSE; Marshal B. Kaplan, M.D.
This article was previously published on the APSF online portal.
The present version is updated and modified by the author for the present APSF Newsletter.

Dear Rapid Response:

Has APSF developed definitive recommendations regarding negative pressure operating rooms for patients who are known or suspected to have SARS-COV-2 infection? If not, when will it happen?

Thank you and kind regards


Marshal B. Kaplan, M.D.
Clinical Professor
Director of Airway Management
Co-Chair Performance Improvement Committee
Department of Anesthesiology
Cedars Sinai Medical Center

The author has no conflicts of interest.



Dear Dr. Kaplan,

While APSF does not have definitive recommendations as you have requested, the following response from Dr. Charles Cowles, ASA liaison to the National Fire Protection Association (NFPA), details the important considerations for developing a local approach to caring for these patients.

Thank you for your inquiry.


Jeffrey Feldman, MD, MSE is professor of Clinical Anesthesiology, Children’s Hospital of Philadelphia, Perelman School of Medicine, University of Pennsylvania, and Chair of the APSF Committee on Technology.

The author has no conflicts of interest.


  1. Overview of NFPA codes and standards that apply to emergency power systems in healthcare facilities. Accessed December 21, 2020.
  2. Lessons learned from Hurricane Sandy and recommendations for improved healthcare and public health response and recovery for future catastrophic events. American College of Emergency Physicians, Dec 22, 2015. Accessed December 21, 2020.



Positive pressure, where the pressure in the operating room is greater than the adjacent areas, is the typical approach to OR ventilation. This approach is employed to prevent circulation of pathogens that could contaminate an open wound from entering the OR. For all patients undergoing a surgical procedure, positive pressure is an accepted infection prevention strategy. Negative pressure, where the pressure in the room is less than the adjacent areas, can be used to prevent airborne pathogens from leaving the room. While not a standard, negative pressure has been advocated for hospital rooms where a patient is known or suspected to be infected with an airborne pathogen.

What is the best strategy for OR ventilation when a COVID-19 patient, or a person under investigation (PUI), requires a procedure in the operating room? The existing approach of positive pressure ventilation is best to protect the patient coming to the OR, but how can the risk to staff and other patients from any aerosol generating procedures be minimized? The following is intended to provide the information needed to make an informed decision about the approach to OR ventilation best suited to the local conditions.


What are the current recommendations for ventilation in the operating room?

The American Institute of Architects (AIA) recommends 15 air exchanges per hour combined with a minimum of 3 air exchanges of outside (fresh) air for operating rooms.1 In addition, air flow should be designed to create positive pressure in the operating room relative to areas outside the OR to prevent the entry of common pathogens (e.g., Staphylococcus aureus) that could contaminate an open wound. These basic requirements are the standard for all patients receiving care in the operating room.


Should the operating room (OR) ventilation be converted to negative pressure to protect the staff from COVID-19 exposure when caring for known or suspected COVID-19 positive patient?

No, the American Society for Healthcare Engineering (ASHE) recommends the same strategy for COVID patients in the operating room as they do for other airborne diseases such as TB.2 This includes the following, if feasible:

  • Only medically necessary procedures should be scheduled “after hours.”
  • Minimize staff in the room and all staff involved should wear N95 or HEPA respirators.
  • The door to the operating room should be kept closed throughout the procedure.
  • Recovery should be accomplished in an Airborne Infection Isolation Room (AIIR).
  • Terminal Cleaning should be performed after sufficient number of air changes has removed potentially infectious particles.

Pathogens such as staphylococcus can be pulled into the operating room if a negative pressure configuration is chosen.3 Taking all factors in to account, negative pressure should not be instituted in operating rooms. When treating a positive COVID patient or a person under investigation (PUI) for COVID, aerosol-generating procedures (AGPs) such as intubation, should be performed in an Airborn Infection Isolation Room (AIIR), separate from the OR, if feasible.


What exactly is an Airborne Infection Isolation Room and how does this differ from a negative pressure room?

According to the American Institute of Architects (AIA) guidelines, AIIRs must meet several criteria for room ventilation unrelated to the pressure differential:

  • At least 12 air exchanges per hour
  • Inability to inadvertently change the ventilation modes from a negative mode to a positive mode
  • Tightly sealed doors
  • Self-closing doors
  • A permanent indicator of airflow that is visible when the room is occupied and
  • A filtration system with at least 90% efficiency.1

The AIIR can be positive pressure if a negative pressure anteroom (see below) is used. The AIIR should be negative pressure in relation to the corridor in the absence of an anteroom. A negative pressure room can be created by having a return air system rate greater than the supply of air, but it is not an AIIR unless it meets the other criteria. Rooms in the ICU, PACU and repurposed spaces can be configured to meet the AIIR criteria to facilitate caring for COVID-19 patients.


Are there other actions we can take to protect the staff and other patients if the OR does not have negative pressure?

Creating a negative pressure anteroom to the OR can help control the movement of contaminated air and is a fairly simple modification which can either be temporarily or permanently constructed (see Figure 1). This anteroom is a small room built adjacent to the patient entry door to the OR and contains a portable air handler that creates a negative pressure which prevents airborne particles from being pushed out of a positive pressure operating room and into a hallway or other adjacent room. Anterooms should be large enough to maneuver a bed into the OR and also hold a small air handling unit. Locating the anteroom near a return air duct simplifies the routing of the air handling duct work. These rooms can be designed in a hallway with self-closing doors which can allow personnel to walk through the area. If an anteroom is deployed, then other doors to the OR should be sealed to airflow.

Figure 1: Depicts the creation of a temporary wall with a door to create an anteroom from an OR hallway.

Figure 1: Depicts the creation of a temporary wall with a door to create an anteroom from an OR hallway.

Figure 2: Portable air handler inside anteroom to create negative pressure.

Figure 2: Portable air handler inside anteroom to create negative pressure.

Both the Anesthesia Patient Safety Foundation and the American Society of Anesthesiologists offer additional guidance. In the absence of a negative pressure anteroom to the OR, efforts should be made to minimize environmental contamination and staff risk during any aerosol-generating procedure. Intubating, extubating, and recovering the patient in an AIIR separate from the OR is one approach, but it requires transporting an intubated patient and the need to filter any exhaled gases during transport. If the airway is managed in the OR, staff in the room should be the minimum required to secure the airway, all must wear PPE, and other doors to the OR should remain closed. Once the airway is secured, or the patient has been extubated, other staff should not enter the OR until sufficient time has elapsed to clear the room of any airborne pathogens.4,5


How long does it take after an aerosol-generating procedure (AGP) for the air in the room to be completely filtered?

The CDC provides a chart which shows that at 15 air exchanges per hour, 99% of airborne contaminants can be removed in about 14 minutes.6 However, these data are an estimate of fairly complex calculations for which many factors need to be taken into account. The efficiency of 99% assumes that all of the air is cycled by pushing air in a laminar flow pattern. However, large nonaerodynamic objects such as anesthesia machines, OR tables, and other equipment can result in turbulent airflow and create dead air spaces where air is not circulated. This air does not consistently participate in the 15 air exchanges, but also airborne contaminants would likely bypass these dead air spaces as they circulate from the infected source to the exhaust vents. Another factor for determining adequate time is air filtration. A High Efficiency Particulate Air (HEPA) filter is one common type of filter. Air filtration is actually rated using the Minimum Efficiency Reporting Value (MERV) system. The higher the MERV number, the more efficient the filter is at filtering small particles. Hospital ORs should have a filtration system rated 14 or greater.7 A HEPA filter exceeds this MERV threshold.


Where can I find more information on ventilation standards and recommendations?

Every facility will have different constraints that will dictate the procedures to care for COVID-19 patients. The availability and location of AIIRs outside of the operating room will determine where airway management can be performed and where patients should be allowed to recover. Negative pressure anterooms to the OR are useful to prevent spread of airborne pathogens outside of the OR but may not be feasible. The number of air exchanges per hour will also vary and dictate the time required for airborne pathogens to be cleared from the OR environment. Resources for currently accepted standards and recommendations include the following.


Charles E. Cowles, Jr., MD, MBA, FASA, was associate professor and chief safety officer at the University of Texas MD Anderson Cancer Center, Houston, TX.

The author has no conflicts of interest.


  1. American Institute of Architects. 2006 American Institute of Architects (AIA) Guidelines for Design and Construction of Hospital and Health Care Facilities. 2016.
  2. American Society for Healthcare Engineering. Accessed August 6, 2020.
  3. Chow TT and Yang XY. Ventilation performance in operating theatres against airborne infection: review of research activities and practical guidance. J Hosp Infect. 2004;56:85–92.
  4. Anesthesia Patient Safety Foundation. Accessed August 10, 2020.
  5. American Society of Anesthesiologists. Accessed August 10, 2020.
  6. US Health and Human Services Center for Disease Control (CDC) Accessed August 6, 2020.
  7. Barrick JR, Holdaway RG. Mechanical Systems Handbook for Health Care Facilities. American Society for Healthcare Engineering. 2014.

The information provided is for safety-related educational purposes only, and does not constitute medical or legal advice. Individual or group responses are only commentary, provided for purposes of education or discussion, and are neither statements of advice nor the opinions of APSF. It is not the intention of APSF to provide specific medical or legal advice or to endorse any specific views or recommendations in response to the inquiries posted. In no event shall APSF be responsible or liable, directly or indirectly, for any damage or loss caused or alleged to be caused by or in connection with the reliance on any such information.