A University of Oregon study has found viral RNA from SARS-CoV-2 in the air handling units (AHUs) of a healthcare facility, thus raising new questions about the possible role of HVAC systems in spreading the novel coronavirus.

The study collected 56 samples from three different air handling units at the Oregon Health and Science University hospital in Portland, Ore., on four days in May and June 2020. Three areas along the path of airflow were sampled, including pre-filters, finals filters, and supply air dampers. About 35% of prefilter samples, 17% of final filter samples, and 21% of air damper samples contained viral RNA at detectable levels.

Viral genetic material

Modern filtration at the highest level of purification in a healthcare environment is still not sufficient

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes the illness known as COVID-19. RNA (ribonucleic acid) is present in all living cells, and in this case was identified as originating from SARS-CoV-2. The infectious potential of viral genetic material is unknown.

The HVAC system at the buildings studied exceeded ASHRAE Standard 170-2017 Ventilation for Healthcare Facilities guidelines. The pre-filters are rated MERV-10, and the final filters are rated MERV-15. Based on engineering calculations and equipment documentation, the HVAC system is capable of cycling air from the ward, to the AHU, and back to the ward in a time between 90 seconds and five minutes, depending on travel distance to each room location.

The study, which has not been peer-reviewed, suggests that modern filtration at the highest level of purification in a healthcare environment is still not sufficient to rule out the passage of viral RNA and possible viral particles through an HVAC system. The researchers did not assess the infectivity of the samples.

Transmission via HVAC systems

More work is needed to further evaluate the risk of SARS-CoV-2 transmission via HVAC systems"

The presence of viral RNA in air handlers raises the possibility that viral particles can enter and travel within the air handling system of a hospital, from room return air through high-efficiency MERV-15 filters and into supply air ducts,” reports the study, which was published on the preprint server “medRxiv.”

More work is needed to further evaluate the risk of SARS-CoV-2 transmission via HVAC systems and to verify effectiveness of building operations mitigation strategies for the protection of building occupants,” according to the study.

“Droplet spread,” from one person to another, has been identified as the most common mechanism of transmission of the novel coronavirus, although some studies have suggested that air movement patterns indoors induced through HVAC systems may contribute to transmission through “small particle aerosols.”

Negative pressure rooms

Negative pressure rooms are used in hospitals and medical centers to prevent cross-contamination

Patients with known or suspected COVID-19 should be placed in negative pressure rooms when available, but most hospitals and outpatient clinics do not have enough negative pressure rooms to accommodate all patients. Negative pressure rooms are used in hospitals and medical centers to prevent cross-contamination from room to room.

The study results are particularly timely as more indoor spaces begin to reopen and increase in occupant density; more individuals will occupy shared spaces serviced by HVAC units for extended periods of time. As knowledge regarding ventilation during the pandemic continues to expand, it is likely building operations best practices will continue to be updated.

Hospitals have higher levels of mechanical filtration and room air exchange than almost any other building. Therefore, the study suggests that even the most extreme filtration practices may not eliminate the passage of SARS-CoV-2 RNA, and potentially viral particles, through HVAC systems and potentially back into the supply air.

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