HVAC Systems in Healthcare: What You Don't Know Can Hurt You!

Christopher Roman, CEM


July 26, 2022

Christopher Roman, CEM

According to the Centers for Disease Control and Prevention (CDC), healthcare-associated infections are included in several categories for tracking and data collection. However, the biggest potential area for improvement in today's environment is the surgical-site infection (SSI) category.

PSNet, an official website of the US Department of Health & Human Services, points out that "SSIs occur in 2% to 4% of all patients undergoing inpatient surgical procedures." Although most infections are treatable with antibiotics, SSIs remain a significant cause of morbidity and mortality after surgery. They are the leading cause of readmission to the hospital following surgery, and approximately 3% of patients who contract an SSI will die as a consequence."

With today's technology and available heating, ventilation, and air conditioning (HVAC) equipment, these infection percentages seem abnormally high and provide a source of "low-hanging fruit" for immediate improvement. HVAC technology can assist in reducing the number of hospital-related SSIs by focusing on the three pillars of air treatment:

  • Enhanced air filtration with increased filtration efficiency to remove even submicron–sized pathogens

  • The application of ultraviolet lights, sometimes referred to as ultraviolet germicidal irradiation (UVGI)

  • Precision humidity and temperature control for every hour of the year

HVAC air distribution systems use air-handling units (AHUs). AHUs are designed to deliver cleaned air to a space with the proper temperatures and relative humidity (RH) requirements met before the air is delivered. Before the air can be delivered, it must pass through the air-filtration system. Air-filtration systems have varied requirements on minimum filtration levels, based on the building and occupancy type. For example, some surgical-suite AHUs are equipped with in-AHU filtration, in addition to filtration at each individual operating room.

To achieve our desired level of air filtration, we need to specify two pre-filters that have Minimum Efficiency Reporting Value (MERV) filter ratings of 11 and 16, respectively. Together the combined filters exceed the CDC filtration recommendations of MERV 13. Placing a MERV 11 filter before the MERV 16 filter allows for enhanced filtration on the pre-filter setup. The secondary pre-filter, the MERV 16, removes 95% of COVID-19–sized particles.

More critical spaces, such as surgical suites, need additional filtration enhancement on the after-filtration section of the air-handling unit. Combining a MERV 16 and a high-efficiency particulate air (HEPA) filter traps over 99.99% of viral-sized matter. Having both the pre-filter and after-filter arrangements is a necessary first step in reducing SSIs. Once we have the proper air filtration setup in the AHUs it becomes important to discuss how the air travels through the filtration systems. According to a NASA study, filter efficiency is highly dependent on the velocity of the air in the system. Traditionally, an AHU is designed for a face velocity of air at 450-500 ft/min and will have less filtering ability than an AHU designed for a face velocity of air at 250-300 ft/min. Therefore, specifying a lower face velocity in an AHU with superior pre-filtration and after-filtration is another major step in reducing SSIs nationally.

The second pillar of air treatment inside an AHU is the utilization of UV-C lights. UV-C lights must be strategically positioned in an AHU to pair with an enhanced filtration system. The proper pairing of these two technologies can reach a combined capture-and-kill rate that exceeds 99.999% effective. UV-C lights should be directed at coils inside the AHUs as well as at drain pans and any other location that could be considered ideal for bacterial (or any other) growth inside the unit.

The third pillar of air treatment is enhanced RH control. The goal for any space where SSIs are considered a potential issue is to keep the RH between 40% and 60% for every hour of the year. Bacteria, viruses, fungi, mites, and other pathogens all thrive in environments below 40% RH and above 60% RH. Therefore, RH control becomes synonymous with maximizing patient care. Dew point is the temperature the air needs to be cooled to in order for the HVAC system to condense moisture out of the air. Achieving the required dew point ensures that moisture in the air is removed before the air enters the after-filter system. All water dripping off the coils is ushered into a drain pan where UV-C lights are shining to reduce and minimize any growth.

Given these three pillars, SSIs could be drastically reduced from their current national numbers. HVAC technologies such as high-efficiency dehumidification systems (HEDS) are maximizing these three pillars to ensure that a very high level of air purity is delivered. I hope — knock on wood — that I do not need surgery in the near future. However, you can be sure that if I do, I will be asking about the three pillars of AHUs that serve the operating room I will be in. Because what you don't know can actually hurt you.

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