Review the work that engineers and clinicians put into creating an amendment to the standard for relative humidity in the operating room and other spaces. The goal was to see whether it was possible to adjust rh thresholds without affecting patient health, but the associated potential equipment savings may make facility budget directors feel the best of all.

On July 10, 2010, ASHRAE, the American Society for Healthcare Engineering (ASHE), and American National Standards Institute (ANSI) completed approving a change (addendum d) in their Standard 170-2008, Ventilation of Healthcare Facilities that addresses the acceptable range of rh in operating rooms (OR) and other spaces in health care facilities.1 In January 2010, the Facility Guideline Institute (FGI) released “Guidelines for Design and Construction of Healthcare Facilities – 2010,” which incorporates Standard 170 as Chapter 6. As a result there is now a single source of standards governing ventilation and filtration for the use of health care engineers and architects and referenced by clinicians and organizations. This change automatically updates the FGI guidelines for ventilation requirements.


ASHRAE received a request from California to consider data supporting the existing lower limit of the 3% to 60% rh range in ORs and similar areas in hospitals. Dropping below 30% rh was uncommon, but meeting this requirement, as enforcement by federal and state regulators, required huge investments in expensive equipment for the rare occasion when rh could dip below 30%. Installing equipment and then turning it off, never to be used, was common practice.

Other states, from Florida to Minnesota, have experienced similar problems. The California group had prevailed on infectious disease experts to assess the health effects of lowering the lower limit and referred their concerns and the data to ASHRAE, the FGI Guideline, and other organizations as well, as standard setting groups used by regulators such as the Centers for Medicare and Medicaid (CMS). This matter was undertaken by ASHRAE’s 170 Standing Standard Project Committee (SSPC), which includes representatives of both ASHRAE and FGI.


If this change could be made without jeopardizing patient outcomes, estimated savings to the health-care industry could be more than $200 million in the next 10 years, which would be attained by reducing the initial cost of installing the ventilation system, eliminating the need to modernize existing systems to maintain 35% rh, and providing energy conservation savings.


ASHRAE’s 170 SSPC’s effort by was extensive and covered almost all aspects of the OR environment, from fire safety to viral or surgical site infections. Building on the California work, ASHRAE asked Farhad Memarzadeh, P.E., Ph.D., director, division of technical resources at the National Institutes of Health (NIH), to help perform a scientific literature search and evaluation of its findings. SSPC 170 also worked with the Association for Professionals in Infection Control and Epidemiology (APIC), the Association of Perioperative Registered Nurses (AORN), and the Centers for Disease Control & Prevention (CDC) to assess whether there would be any patient safety issues with lowering the rh to 20% in the OR.

ASHRAE SSPC 170 process. The basis for lowering the humidity level included a review of scientific literature on the potential impacts of lowering rh from 30% to 20% on outcomes of surgery and other procedures commonly performed in the defined spaces. Sufficient data had already resolved questions related to static electricity or its effects on equipment. Since flammable anesthesia is no longer an issue for operating rooms, the remaining question revolved around the perception that lower rh conditions affect infectious outcomes.

The proposed change addressed the impact of rh on viral infections; that literature was examined first. No studies were identified documenting the effect of rh below 20% on viral agent survival.

The investigation next moved to the examination of infectious disease outcomes, specifically surgical site infections (SSI) in OR settings. It was determined from the literature that SSIs are caused by bacteria and fungi, not environmental viruses. The few studies specifically associating SSI with temperature and rh involve prolonged periods of rh exceeding 60%. Reviews conducted by Dr. Memarzadeh further noted that survival and growth of bacterial agents are less likely at lower temperatures and rh.

Reducing rh to 20% from 35% in NFPA 99 and 30% in ASHRAE 170 and the FGI Guidelines would therefore have no traceable adverse effect on system performance and patient safety in anesthetizing locations. This statement is based on findings from these reviews as well as expert testimony from professionals in infection prevention and control and health care epidemiology, perioperative care, mechanical engineering, and health-care engineering. The upper limit of 60% rh continues to be an important screening level to monitor since excessive humidity can lead to development of mold and mildew in the OR, anesthetizing locations, and other short-term patient spaces.

Conclusion. Members of the ASHRAE 170 SSPC concluded that the issue of lower rh in these settings for patients and staff is related to comfort and not infection, further supporting adoption of addendum D.

Final approvals. Following ANSI consensus protocols, the proposed change in Standard 170-2008 was approved by the ASHRAE Standards Committee on June 26, 2010, by the ASHRAE Board of Directors on June 30, 2010, by the American Society for Healthcare Engineering of the American Hospital Association (ASHE/AHA) on July 9, 2010, and by ANSI on July 10, 2010.

Areas affected. This change reduces the lower limit of the design specifications for HVAC systems that supply short-term patient treatment spaces from the prior 30% rh to 20%. The full text describing these changes in Standard 170-2008 has been published as “Addendum D” and may be downloaded from ASHRAE.2 The affected spaces are listed in the ventilation table and include:

  • Class B and C [minor to major surgery with sedation] ORs

  • Operating/surgical cystoscopic rooms

  • Delivery room (caesarean)

  • Treatment room

  • Trauma room (crisis or shock)

  • Laser eye room

  • Class A [minor surgery without preoperative sedation] operating/procedure room

  • Gastrointestinal endoscopy procedure room

Enforcement issues affecting health-care facilities. One barrier remains to complete implementation in some states. The reference to 35% rh in the OR will be gone in the upcoming edition of NFPA 99, the document referenced by CMS when surveyors enforce the old NFPA Life Safety Code (LSC) 2000 still used by CMS. The 2012 edition of the LSC will probably not be finalized until fall 2011, so efforts are underway to request that CMS anticipate the change and stop enforcing 35%.


A comprehensive overview of the issue titled, “The Role of Humidity in Operating Rooms…Demystifying the Myth,” was presented at an ASHE Workshop held on June 8, 2010, at the Carolinas Medical Center in Concord, NC. The presentation highlighted key issues from stakeholders that included professionals representing the fields of infection prevention and control and health care epidemiology, clinicians who care for surgical patients, perioperative care, mechanical engineering, and health care engineering. Other sponsors of this conference included the AHA and the FGI and endorsements from APIC and AORN. Additional details and faculty are available from ASHE.3 Summaries are highlighted below.


Overview. This change in the lower level of rh facilitates flexibility in HVAC parameters that will have little if any risk of adverse effect on performance and patient safety. Importantly, it broadens the range of humidity that health care engineers work hard to maintain without requiring investment in expensive changes to HVAC systems which up to now have been required to keep rh greater than or equal to 30%.

In addition, rh is intimately tied to outdoor air conditions and local climate conditions. Many facilities in the U.S. are located in more arid climates or experience variable seasons, and maintaining a 30% rh is often impossible to achieve given these ambient, local conditions. It should be emphasized that the range values are design - not operational standards. It is understood that operationally there may be slight variations at the high end at certain weather-related times, as well as at the low end as discussed.

Engineering. Dr. Memarzadeh had conducted critical research on the role of HVAC parameters on outcomes such as SSI. He presented both the findings from his decades of investigation and a comprehensive review of the scientific literature at the ASHE workshop and concluded, “…there is no clinical evidence or research that shows any correlation between minimum levels of relative humidity and hypothermia or wound infections in short-stay patient spaces.”

Dr. Memarzadeh also investigated the impact of minimum levels of rh on survival of viruses in health-care facilities and concluded there is none.

Lastly, any concerns about the discharge of static electricity had been resolved following prior investigations, and no such problems have been reported in the literature nor have any been documented in national databases of adverse events during surgical care assessed by Dr. Memarzadeh. These included databases of adverse events during surgical care maintained by Food and Drug Association (FDA) and the ECRI Institute.

AORN position on rh. AORN has endorsed this change in the lower limit of rh, and Ramona Connor, R.N., manager of standards and recommended practices, also a member of the faculty at the ASHE Workshop, reinforced that AORN will recognize this change as it cites the FGI “Guidelines for Design and Construction of Health Care Facilities, 2010,” as the criterion reference for their Perioperative Standards and Recommended Practices. The 2010 FGI Guidelines incorporate ASHRAE Standard 170-2008, and therefore the approval of this change by ASHRAE means AORN will adopt the most current version of this addendum to the Standard. Similarly, this standard would be referenced by the CDC as appropriate.

Surgical view - clinical outcomes. Lenox Archibald, M.D., Ph.D., reviewed the pathogenesis and epidemiology of SSIs at the workshop and concluded this change in rh will have negligible impact of outcomes of surgical care. He instead reinforced and highlighted the multitude of factors and variables that do have a significant impact on incidence of SSI captured in the CDC Guideline for Prevention of SSI, 1999.4 He emphasized that much of this knowledge has changed little since this CDC Guideline was published and that strategies for prevention need to emphasize processes of care around the surgical site and less so on the environmental HVAC conditions. His assessment of the literature found very few reports of correlation between rh and SSI - actually, those he did identify involved significant elevation of rh over the upper bound of 60%.

The evidence and impact on regulatory and accreditation requirements. I, (Judene Bartley, M.S, M.P.H., vice president, Epidemiology Consulting Services and clinical consultant for Premier’s Safety Institute) rounded out the workshop program by providing an in-depth examination of the scientific evidence, highlighting that evidence for SSIs that occurred over prolonged periods of higher temperatures and rh and are only one set of variables among others such as airflow direction and exchange, temperature, and filtration. I also emphasized that FGI Guidelines parameters pertain to design - and not operations - of health-care facilities. The facility engineer, in concordance with the infection preventionist (IP), requires that all HVAC parameters meet design specifications during commissioning of newly renovated or constructed spaces, especially these short-stay, special pressure spaces. It is the continuing preventive maintenance that is the challenge.

Operationally, requirements from the CMS, NFPA 99, and accreditation agencies such as The Joint Commission only specify that HVAC variables must be in place. No agency specifies frequency or method of documenting ventilation conditions; rather, these are the responsibility of the team at the facility.

Interpretation of temperature and rh variations in the field. The entire panel addressed the realities of daily interpretations of changing values. There was strong agreement that IPs and facility managers working with the OR manager and surgical staff, recommend reinforcing good preventive maintenance and operational practices, such as minimizing traffic in and out of the OR during surgery and thoroughly cleaning the OR in between cases, etc.

If a variable like rh is out of the high- or low-range limit, then the engineer, IP, and perioperative professionals need to assess the risks and enact appropriate responses. Decisions to close ORs over concerns of infection should be based on observable conditions likely to pose SSI risks - as opposed to interpreting temperature and/or humidity readings out of context of OR conditions, even if they are not within design specifications.

If the temperature and rh stay at prolonged highs, they are likely to see sweating surgeons, moisture on the walls, or strike-through of moisture into sterile packs, and appropriate immediate and long-range actions can be taken. As has been determined, there are no such issues at the lower end of rh, yet as recently as the freeze in Florida in 2010, hospitals were questioning whether they needed to shut down ORs with 28% rh.

This consensus development process by ASHRAE is rigorous and well-supported by involvement from professionals representing all stakeholders. This amendment is a positive step in maintaining safe patient care and cost-effective delivery of essential procedures, while opening the door to major savings in the industry, estimated at more than $200 million in the next 10 years. ES