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HVAC Systems in Healthcare: What You Don’t Know Can Hurt You! By: Christopher Roman C.E.M.

According to the Centers for Disease Control and Prevention (CDC), healthcare-associated infections have 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 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. Where HVAC technology can assist in reducing the number of hospital-related SSIs is by focusing on the three pillars of air treatment.

The first pillar is enhanced air filtration with increased filtration efficiency to remove even sub-micron-sized pathogens from the air being circulated through the facility. The second pillar is the proper application of ultraviolet lights, sometimes referred to as UV-C or Ultraviolet Germicidal Irradiation (UVGI). The third pillar is precision humidity and temperature control for every hour of the year to ensure that the surgical team and patient needs are met.

HVAC air distribution systems use air-handling units (AHUs). AHUs are designed to deliver cleaned, and tempered air to a specific space with the proper temperatures and relative humidity (RH) requirements met before the air is delivered. Before the air can be delivered to a space it must pass through the air-filtration systems. The air filtration systems are typically located inside the AHU, and there are 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.

With today’s airborne threats, code-mandated minimum levels of air filtration are not enough! AHU technologies like the High Efficiency Dehumidification System (HEDS) that was originally developed for use in Department of Defense facilities are already maximizing pre-filter and after-filter air purification systems to achieve 95% to 99.997% viral-sized particulate capture rates inside the AHU. For reference, many AHUs are equipped with MERV-8 filters which are not even rated to capture any submicron sized particles, they are like trying to capture mosquitos with a chain link fence.

To achieve our desired level of filtration, the pre-filter systems consist of Minimum Efficiency Reporting Value (MERV) filter ratings of 11 and 16, which exceed the CDC filtration recommendations of MERV-13 (higher numbers provide reflect better filtration). Placing the MERV 11 before the MERV-16 allows for the reduction of the filter loading on the MERV-16 filter bank, extending its useful life and reducing maintenance needs. The MERV-16 filter removes 95% of COVID-19 sized particles, Legionella, influenza, bacteria, and airborne salt particles.

More critical spaces such as surgical suites and surgical prep are further enhanced by combining MERV 16 and High Efficiency Particulate Air (HEPA) MERV 18 after filters to trap over 99.99% of viral-sized matter. The location of these filters is intended to trap pathogens that may make it past the up-front filters and biologicals that can grow and prosper within the AHU itself, where moisture and dust can create active breeding grounds for many forms of pathogens. Having this filter arrangement in AHU’s is a necessary first step in reducing SSIs. According to a NASA study (©R. Vijayakumar), 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 feet-per-minute and will therefore have less filtering ability than an AHU designed for a face velocity of air at 250-300 feet-per-minute. Therefore, specifying a lower face velocity in an AHU with superior pre and after-filtration is the first 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 need to 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 the coil(s) inside the AHUs as well as the drain pain(s) and any other location that could be considered ideal to growth inside the unit.

The third pillar of air treatment is the most crucial and challenging to control. That pillar is precision relative humidity (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/or above 60% RH. Therefore, in relation to reducing SSIs, RH control becomes paramount. To achieve RH numbers, typically a very low dew point temperature is necessary. Dew point is the temperature the air needs to be to be cooled to in order for the AHU system cooling coils to condense moisture out of the air (think about condensation on the bathroom mirror when you are taking a shower or condensation on your glasses when you open up a hot dishwasher). Achieving the required dew point temperature inside the AHU ensures that moisture in the air is removed before the air enters the after-filter system. All water dripping off of the coils is ushered into a drain pain where UV-C lights are shining to reduce and minimize any growth.

To demonstrate an HVAC system that meets all three pillars, the California Energy Commission is co-funding a decarbonization-related demonstration project to serve seven operating suites and their support spaces at the University of California San Diego Thornton Hospital complex. The Electric Power Research Institute is the project manager on the project, which is currently in design.

With the proper facility training and a combination of available HVAC technologies, SSIs could be drastically reduced from their current national numbers. HVAC technologies, such as HEDS, are utilizing and maximizing these three pillars to ensure a very high level of air purity that is delivered at the proper temperatures and relative humidity levels and minimizing maintenance needs. 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…

Editor’s Note: Christopher Roman currently leads the strategic business development for Conservant Systems. Christopher is a dedicated energy industry professional who has previously held positions at A.O. Reed & Co., San Diego Gas & Electric, and UC San Diego Health. Throughout Christopher’s career, he is known as a creative problem solver who brings unique solutions to complex problems. Currently, Christopher is focused on solving the challenging industry aspect of decarbonizing facilities in a cost-effective manner. While doing so, Christopher is also passionate about improving the indoor air quality (IAQ) of facilities so that 99.99% of pathogens are removed from the air we breathe. 

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