APSF Newsletter: Online-Only

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

September 5, 2020

Charles E. Cowles, Jr., MD, MBA, FASA


Disclaimer: Viewers of this material should review the information contained within it with appropriate medical and legal counsel and make their own determinations as to relevance to their particular practice setting and compliance with state and federal laws and regulations. The APSF has used its best efforts to provide accurate information. However, this material is provided only for informational purposes and does not constitute medical or legal advice. This response also should not be construed as representing APSF endorsement or policy (unless otherwise stated), making clinical recommendations, or substituting for the judgment of a physician and consultation with independent legal counsel.

Q Has the APSF come up with definitive recommendations regarding negative pressure operating rooms for patient who are known or suspected to have SARS-COV-2 infection?

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


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

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 (eg., 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?

The American Society for Healthcare Engineering (ASHE) recommends the same strategy for COVID-19 patients in the operating room as they do for other airborne diseases such as TB2. This includes the following if feasible:

  • Only medically necessary procedures should be scheduled “after hours.”
  • Minimize staff, and all staff involved to wear N95 or HEPA respirators.
  • Door to 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-19 patient or a person under investigation (PUI) for COVID, aerosol generating procedures (AGPs) such as intubation, should be performed in an AIIR, separate from the OR, if feasible.


What exactly is an Airborne Infection Isolation Room (AIIR) 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:

1) at least 12 air exchanges per hour

2) inability to inadvertently change the ventilation modes from a negative mode to a positive mode

3) tightly sealed

4) self-closing doors

5) a permanent indicator of airflow that is visible when room is occupied and

6) a filtration system with at least 90% efficiency.1

The AIIR can be positive pressure if a negative pressure ante room (see below) is used. The AIIR should be negative pressure in relation to corridor in the absence of an ante room. 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 ante room 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. This ante room 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. Ante rooms should be large enough to maneuver a bed into the OR and also hold a small air handling unit. Locating the ante room near a return air duct simplifies the routing of the air handing 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 ante room is deployed, then other doors to the OR should be sealed to airflow.

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 mins.6 However, these data are an estimate for 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 non-aerodynamic 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 not likely circulate in these dead air spaces. Another factor for determining adequate time is air filtration, while many are familiar with High Efficiency Particulate Air (HEPA) filters, that is just one 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 of 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
Associate Professor / Assistant Clinical Director
Department of Anesthesiology and Perioperative Medicine
Univ. of Texas MD Anderson Cancer Center – Houston

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. https://www.ashe.org/covid-19-frequently-asked-questions [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 Feb;56(2):85-92.
  4. Anesthesia Patient Safety Foundation. https://www.apsf.org/covid-19-and-anesthesia-faq/#clinicalcare [Accessed August 10, 2020]
  5. American Society of Anesthesiologists. https://www.asahq.org/about-asa/governance-and-committees/asa-committees/committee-on-occupational-health/coronavirus. [Accessed August 10, 2020]
  6. US Health and Human Services Center for Disease Control (CDC) https://www.cdc.gov/infectioncontrol/guidelines/environmental/appendix/air.html#tableb1 [Accessed August 6, 2020]
  7. Barrick JR, Holdaway RG. Mechanical Systems Handbook for Health Care Facilities. American Society for Healthcare Engineering. 2014.