Comparison of Operating Room Relative Humidity Levels Using the EQI Method of Dynamic Simulated Surgical Procedures

Author(s):
Jennifer Wagner; Damon Greeley

Background:

Operating room (OR) Heating, Ventilation, and Air Conditioning (HVAC) design and operation aim to provide comfortable and safe working environments for staff and patients. One aspect of safety is protection from airborne contaminants with the potential to cause surgical site infections. These contaminants can come from the environment or the people in the space and can land on surgical instruments, the patient or staff resulting in increased potential to enter open surgical sites and lead to infection. Properly controlled temperature, pressure, relative humidity and airflow distribution are multifaceted challenges of OR design, engineering, and operation. When the HVAC maintains proper conditions, it assists with asepsis and can create multiple sterile zones in which the patient and instrumentation reside. Specifically, studies have shown that minimum RH levels can prevent the propagation of infectious microbes within close proximity through airborne transmission. Regulated RH levels in ORs historically fluctuate. For example, prior to 2013, OR RH minimum was 35% referencing NPFA 99, in 2013, ASHRAE 170 and the Joint Commission accepted minimum of 20%. In 2015, the minimum was raised back to 30% after concerns for potential failure of medical instrumentation, raised by AAMI. The debate within ASHRAE 170 continues today even though studies have shown RH ranges between 40% and 60% are optimum for control of indoor microbial growth rates.

Hypothesis:

The optimal range of relative humidity will allow the OR air delivery system to be more effective at clearing microbial and particle contamination from the sterile field and other deemed critical zones within the OR, thereby preventing cross contamination of microbes and particles into the sterile field.

Methods:

The Environmental Quality Indicator (EQI) method (Gromley et al 2017) was used to assess the airborne environment in an operating room capable of changing RH levels during dynamic, simulated, scripted surgical procedures. The RH levels were adjusted after each day over three days of testing at 20%, 35%, and 50% relative humidity. The EQIs measured included particle counts, microbial counts, surgical table height air velocities, and humidity at a consistent room temperature of 68 degrees and an air change rate of twenty-one (21) per hour.

Results:

The OR with RH levels of 35% and 50% had significantly fewer bacterial microbes in the sterile field compared to the OR with RH level of 20% (p​<.0167). Significantly fewer 0.5 and 1.0 um particles at 50% RH than 20% (p<.0167).

Conclusions:

Increased RH levels provided less microbial load within the sterile field. Establishing operating room parameters that include higher ranges of room RH can potentially reduce microbial load, thereby potentially reducing surgical site infection risk.