Navigating hospital ventilation design during COVID-19

In early 2020, the novel coronavirus SARS-CoV-2, also known as COVID-19, caught the world by surprise. As a society, the virus will have lasting impacts among most aspects of life, and it will certainly change the landscape of design in perhaps a permanent way.

Not only will the design concepts in most public and commercial spaces be reconsidered, but this pandemic will be on the forefront of building owners’ minds as they progress through hospital designs in the future. Regardless of the length of time that this virus continues to impact our communities, it will not be quickly forgotten and will have long-lasting effects on design considerations in the future.

Standards review, implementation

As engineers across the country started quickly responding to the disaster, it was evident that no singular design standard or code adequately guided the design community. Realizing the deficiency, many organizations such as the Centers for Disease Control and Prevention, the American Society for Healthcare Engineering, and ASHRAE banded together to develop some loose guidance that could be used specifically in the health care setting.

Engineers, hospital owners, and health care experts all over the U.S. quickly formed focus groups to identify design strategies that could both be implemented for not only the influx of the surge of COVID-19 patients, but also permanent concepts that could be executed for pandemic scenarios in the future. As more information has been realized regarding the virus, these recommendations have continued to evolve and will eventually lead to changes in future publications and addenda in current standards.

To help assist with advancing the current standards to include guidance around the design and installation of heating, ventilation and air conditioning systems, ASHRAE created an epidemic task force and position documents to address the challenges of the current pandemic as it relates to the disease transmission in multiple public and private environments, including in health care facilities.

One of the prominent ASHRAE standards that is used for most health care projects, ASHRAE Standard 170: Ventilation of Health Care Facilities, is continuing to be evaluated by the committee for formal guidance and possible revisions to include provisions required for pandemic solutions. As these recommendations were continually being clarified, the ASHRAE website was updated to keep the design community abreast with the most up-to-date information. Other resources such as webinars and local chapter presentations have also proved to be useful for distribution of recommendations.

At this time, it is unknown when permanent standards will be written or enforced. The current effort of the epidemic task force is focused on building readiness to allow for reopening of buildings in general, and not code-required specifics yet for modifications to permanently installed ventilation systems in health care facilities. Many of the schematics demonstrated throughout this article are based on previous recommendations for highly infectious disease units per the CDC and other notable resources, as well as requirements as enforced by the local authorities having jurisdiction.

COVID-19 ventilation concepts 

Although there are several unique design considerations that need to be considered in relation to the COVID-19 crisis, most notably and impactful to the HVAC world is the need for negatively pressurized spaces and careful consideration to airflow relationships with adjoining spaces and departments. This requires special consideration with regards to the ventilation system.

While the term “ventilation air” can suggest many meanings, the ASHRAE Fundamentals 2017 defines it simply as “air used to provide acceptable indoor air quality.” Ventilation is the primary strategy in limiting the spread of infectious diseases through an air system and includes an array of control mechanisms including dilution, filtration, source capture and exhaust components. These functions can occur either at the base air handling unit system or at the room itself and carry a varying weight of effectiveness and cost.

While there are many strategies that can mitigate the transmission of disease, arguably the most effective as it relates to the transfer within the HVAC system is intercepting the return air and diverting it to the exterior to prevent recirculation of the virus back throughout the hospital air system. This captures the contaminated air at the source and eliminates the ability for infected air to find its way back into the spaces and potentially infect other patients or staff.

However, this solution may not be as feasible depending on the system configuration and the ease of routing exhaust ductwork to the exterior. This is especially true in retrofit applications where air systems were designed in a traditional manner that did not include diverting return air paths. Depending on the complexity of the air distribution systems and physical location of the source equipment, this strategy can become a burdensome and costly solution.

Easing the surge

Many of the early concepts that were created at the start of the COVID-19 crisis included simplistic temporary solutions to help assist during the surge, but were not meant for permanent applications. This included creating temporary negative pressure rooms from existing patient rooms using direct exhaust through high-efficiency particulate air filtration or installing temporary ante rooms at each patient room to create a sealed air barrier.

In both applications, the return air grille is sealed to eliminate air transfer back to the main air handling unit and the HEPA filtered negative air machines creates the pressure relationship required to reduce contaminant spread. Pressure monitors, via digital or mechanical means, are provided to ensure continuous monitoring occurs and that the correct pressure relationship is maintained. While these solutions allow for temporary conversion of the spaces during surge applications, they are not suitable or practical for long-term operations of ongoing or recurrent pandemics.

If possible, operating the AHUs that serve the pandemic area in 100% outside air with full exhaust is the most preferred method, although careful consideration should be taken before activating this sequence to ensure that the AHUs, coils, humidifiers and associated outside air ductwork and louver sections have been sized appropriately based on the seasonal duration of use. Potentially lowering the leaving supply water temperature at the chilled water plant can be investigated if capacity at the coils are deficient. Individual spaces would still need to be balanced to ensure that negative pressure relationships are maintained, but this strategy does ensure that contaminated air is not reentrained back into clean spaces.

Additional protection either in the AHU or in the ductwork of HEPA filtration, use of ultraviolet-C lights, and/or ionic purification strategies could all be implemented in any of these strategies; however, the most effective strategy remains removal of the contaminated air from the spaces in conjunction with providing negative pressure spaces.

Permanent pandemic solutions

While these temporary conversion concepts of existing areas into intensive care units has been required to quickly adapt to the surge capacity needs, many hospital owners have seized the moment during existing construction projects to make crucial modifications to the mechanical systems to allow for conversion to fully negative pressure wings or suites. While each design solution is unique to the complexities of the individual project, the most overwhelmingly effective strategy is to divert the airstream to be a fully exhausted system. By carefully sequencing motorized dampers at strategic locations within the system, the dampers can be automated to divert the return airstream to be fully exhausted so no air from the contaminated area returns to the AHU (see Figure 1).

This allows the AHU to operate in a more traditional mode during nonpandemic operation and reduce the additional energy penalties. If the air was designed to be permanently exhausted rather than returned to the AHU, the additional outside air would be required during all hours of the year, resulting in significantly higher energy costs.

Still, in this strategy, careful consideration needs to be taken to ensure that the outside air is properly balanced to make up for the additional exhaust that is removed from the building to maintain an overall positively pressurized building during pandemic mode. This increase of outside air could have impact on the overall size of the cooling coils, preheating coils and humidifiers within the central AHU system and the central energy plant that should be considered and evaluated.

In addition, individual space pressurization needs to be assessed. While the motorized dampers will allow for an entire area or floor to become negative if additional air is removed above the existing return airflow values, the energizing of the exhaust fan and damper control will not alone create negative rooms without another means of rebalance at the space level. This rebalance can be accomplished either through a manual rebalance of each space or through the activation of independent air valves provided on the supply and return systems. The air valves can be programmed with two distinct airflow setpoints: one for normal mode of operation and one for the pandemic mode and would operate in conjunction with the motorized dampers.

Table 1: Typical air distribution requirements for different patient room space types

While changing the pressurization from positive/neutral to negative pressure at the space level can be a challenge, perhaps one of the most debated topics is the quantity of airflow that is required to be delivered and exhausted from the space. To modify a room from an ICU airflow to meet that of an airborne infection isolation room requires an increased airflow and total air quantity required to meet that of an airborne infection isolation room above a medical/surgical patient room is even higher. Refer to Table 1 for typical patient room type comparisons.

Depending on the type of pandemic, a patient room designed as a true airborne infection isolation room may not be required. As with COVID-19, which the main route of transmission is through respiratory droplets, the additional air change rate to turn over the air in the space may not be needed. Per the World Health Organization, these droplets are heavy enough that they cannot travel or linger in the air as long as other airborne viruses, such as the measles.

Therefore, providing protection in a room with 12 air changes per hour of exhaust volume is not the governing factor and instead, the pressure relationship becomes the key driver to minimizing virus transmission in adjacent spaces. Airflow patterns as they relate to the patient and caregiver relationship should be considered to limit the spread within the space as well. To maintain a negative pressure relationship, either a reduction in the supply air or an increase in the return/exhaust airflow is required to achieve negative pressurization.

The approach to simply change the airflow and not necessarily increase total airflow to the requirements of 12 ACH of exhaust was evident in the beginning of the COVID-19 crisis, as most authorities having jurisdiction would accept air balance modifications as necessary to become negative pressure, rather than enforcing a higher air change rate. This greatly reduced the burden on existing infrastructure systems that were already in operation or in construction to become more reasonably modified. However, total ACH is continuing to be evaluated by the ASHRAE 170 committee for guidance on future pandemics.

COVID-19 has forever changed our world and the lens through which we design through will continue to evolve. The ASHRAE epidemic task force and standards committees will continue to evaluate design requirements that will be adopted in formal standards in the forthcoming years to help direct engineers how to best design and prepare health care facilities, nursing homes and outpatient facilities during these difficult, unprecedented times.

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