The role of heating, ventilation, and air-conditioning (HVAC) systems in the transmission of SARS-CoV-2 is unclear. To address this gap, we simulated the release of SARS-CoV-2 in a multistory office building and three social gathering settings (bar/restaurant, nightclub, wedding venue) using a well-mixed, multi-zone building model similar to those used by Wells, Riley, and others. We varied key factors of HVAC systems, such as the Air Changes Per Hour rate (ACH), Fraction of Outside Air (FOA), and Minimum Efficiency Reporting Values (MERV) to examine their effect on viral transmission, and additionally simulated the protective effects of in-unit ultraviolet light decontamination (UVC) and separate in-room air filtration. In all building types, increasing the ACH reduced simulated infections, and the effects were seen even with low aerosol emission rates. However, the benefits of increasing the fraction of outside air and filter efficiency rating were greatest when the aerosol emission rate was high. UVC filtration improved the performance of typical HVAC systems. In-room filtration in an office setting similarly reduced overall infections but worked better when placed in every room. Overall, we found little evidence that HVAC systems facilitate SARS-CoV-2 transmission; most infections in the simulated office occurred near the emission source, with some infections in individuals temporarily visiting the release zone. HVAC systems only increased infections in one scenario involving a marginal increase in airflow in a poorly ventilated space, which slightly increased the likelihood of transmission outside the release zone. We found that improving air circulation rates, increasing filter MERV rating, increasing the fraction of outside air, and applying UVC radiation and in-room filtration may reduce SARS-CoV-2 transmission indoors. However, these mitigation measures are unlikely to provide a protective benefit unless SARS-CoV-2 aerosol emission rates are high (>1,000 Plaque-forming units (PFU) / min). Author summary Evidence of heating, ventilation, and air-conditioning (HVAC) systems facilitating SARS-CoV-2 transmission is limited, and we do not know the protective effects of recommended HVAC modifications. To address these gaps, we simulated HVAC systems in an office building and three social settings (bar/restaurant, nightclub, and wedding venue), varying key parameters to identify those associated with SARS-CoV-2 transmission. Ultimately, we found that HVAC systems are unlikely to facilitate spread, and that most common mitigation strategies, such as increasing air circulation rates, filtration efficiency, or the fraction of outside air, are able to reduce transmission rates. However, the importance of these mitigation measures is negligible in commercial settings when few SARS-CoV-2 particles are emitted via aerosol; only at very high emission rates did we see substantial reductions in transmission. Our results also indicated that HVAC systems cannot reduce transmission risk to zero. Validation efforts suggested that protective HVAC measures (e.g., increasing air circulation rates, using UVC (ultraviolet light in the 100–289 nm range) filters) may be more beneficial in residential settings, though this was not studied in detail. Identifying the emission rate of SARS-CoV-2 from infected individuals, as well as the dose necessary to infect humans, remains a key data gap.
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