Air Time

The primary means of controlling airborne infection in the hospital environment. During the COVID-19 pandemic, many healthcare organizations have updated or retrofitted their HVAC systems to minimize the recirculation of contaminated air in hospital air distribution systems to keep patients, staff, and visitors safe.

While the industry continues to learn more about the means of transmission of the
COVID-19 virus, this topic is sure to stay top of mind for months to come. This means design and facilities engineers will need to work in concert to ensure that modified hospital building systems are able to perform effectively to minimize risk.

Filtration basics
Design parameters for mechanical ventilation systems, such as air change rate, outside air quality, outside air quantity, and filtration effectiveness, are important in minimizing airborne infection. The goals of these systems are to replace contaminated air with clean air, minimize the mixing of dirty and clean air, and inactivate or kill microorganisms.

The ANSI/ASHRAE/ASHE Standard 170—2017 defines filtration requirements by a Minimum Efficiency Reporting Value (MERV) from 1 to 16: the higher the rating, the better the filter is at trapping specific types of particles. For example, ANSI/ASHRAE/ASHE Standard 170—2017, Ventilation of Health Care Facilities, mandates a minimum filtration requirement of MERV 13 or MERV 14 for critical spaces such as operating rooms, inpatient and ambulatory diagnostic and therapeutic radiology, inpatient delivery and recovery spaces, and airborne infection isolation (AII) rooms. A MERV 14 unit will filter particle sizes between 3 to 10 microns, such as mold spores, legionella, and cement dust. However, these filters aren’t effective in removing viruses, which can vary in size from 0.02 to 0.25 micron, from the air stream of a hospital HVAC system.

A High Efficiency Particulate Air (HEPA) filter, a type of pleated mechanical air filter, is better at capturing very small particles such as virus and bacteria. For a long time, it’s been a best practice to install HEPA filtration in air handling units serving surgical suites to protect patients who undergo invasive procedures, because HEPA filters provide a more enhanced filtration level than MERV 14 filters. HEPA filtration is also mandated by Standard 170 for protective environment rooms, which are usually set aside for patients with compromised immune systems. However, it’s important to note that this outlines only minimum standards. That means that facilities are free to be more stringent in their filter selections, with some hospitals opting to install HEPA filters in their air handling units, inline ductwork in operating rooms, and AII rooms.

Retrofitting with HEPA filtration
While there’s still much debate on whether COVID-19 is spread via droplets or aerosol, basic HEPA filtration is recommended as a best practice for intercepting and inactivating the virus before it spreads to the air system. Since the onset of the pandemic, many healthcare facilities have retrofitted their air distribution systems with HEPA filtration. Here are several solutions worth considering:

HEPA on exhaust: Installation of a HEPA filter on an air exhaust system ensures that air isn’t dumped to the external atmosphere without being properly filtered. This is also important to ensure that contaminated exhaust air isn’t entrained in supply air intake louvers. Utilizing a bag-in/bag-out HEPA filter access system for removing contaminated particulate collection will protect facility engineers from having direct contact with filter media during replacement.

HEPA on return: During the first surge of COVID-19 response in hospitals, mechanical engineers worked with facilities to modify their air handling units to operate with 100 percent outside air where possible. This is done by modifying the controls system to exhaust all the return air instead of the normal method where it’s mixed, filtered, and returned to the hospital space. This solution replaces all the air within the hospital that’s being possibly exposed to a virus with outside air.

HEPA fan filter units: These standalone fan units have a HEPA filter and control system and are usually used in clean rooms and laboratories to remove harmful airborne particles from the air stream. During the first surge, many healthcare facilities installed HEPA filter fan units in patient rooms to create negative isolation rooms, either by using the fan as a standalone unit that exhausts air directly to the outside or by connecting the units to the return ductwork in the ceiling space and closing off the return air diffusers.

AII rooms: These negative pressure rooms, which exhaust more air than what is being supplied to the room, are usually designed to have an anteroom and a separate exhaust system, which allows the unidirectional flow of air from outside the adjacent corridors to inside the room. AII rooms can be sealed and gasketed to prevent infiltration of air out of the room while the exhaust system is usually fitted with a HEPA filtration system to ensure that the contaminated air doesn’t impact the surrounding areas where it’s discharged.

During the initial COVID-19 surge, many healthcare facilities modified existing medical/surgical patient rooms into temporary isolation rooms by fitting them with HEPA filtration in different configurations such as increasing the exhaust from the bathrooms, bringing the exhausting ductwork through windows into the patient rooms, and installing HEPA fan filter units. ASHRAE and ASHE also released several recommendations on how to convert these spaces into temporary negative isolation rooms. However, it’s important to note that these modified patient rooms are not true AII rooms, because they have less air change rates than true AII patient rooms. (ASHRAE 170 recommends a minimum air change rate of 12 for an AII patient room and 6 for medical/surgical patient rooms.) To ensure these spaces remain negative pressure to protect staff and patients, the rooms should be fitted with a visual pressure monitor or other mechanical pressure device so staff can check if the pressure in the room is safe.

Lessons learned
Easy implementation of an enhanced filtration system can tackle some of the challenges associated with an airborne pandemic. Looking ahead, healthcare facilities should consider selecting new air handling units with additional filter sections so that HEPA filtration can be installed, if needed. Additionally, engineers should pay close attention to the air handling unit design to ensure that fans are selected in a manner that ensures efficient operation under a normal and pandemic mode. By taking these steps, facilities can prepare themselves for the next COVID-19 surge or future pandemic.

Gary Hamilton PE, LEED AP, is senior vice president at WSP (Arlington, Va.). He can be reached at

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