The impact of heating, ventilation and air conditioning (HVAC) design features on the transmission of viruses, including SARS-CoV-2: an overview of reviews
Authors: Thornton Gail M. , Kroeker Emily , Fleck Brian A. , Zhong Lexuan , Hartling Lisa 
Published: September 2021

AbstractBackgroundThe 2019 novel coronavirus or severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) outbreak was declared a pandemic by the World Health Organization (WHO) in March 2020. Almost two years later (early-February 2022), the WHO reported over 383 million cases of the disease caused by the virus with over 5.6 million deaths worldwide. Debate regarding routes of transmission was substantial early in the pandemic; however, airborne transmission emerged as an important consideration. Infectious airborne agents can spread within the built environment through heating, ventilation, and air conditioning (HVAC) systems. Multiple features of HVAC systems can influence transmission (e.g., ventilation, filtration, ultraviolet radiation, humidity). Understanding how HVAC features influence airborne transmission is critical to mitigate the spread of infectious agents.ObjectiveGiven airborne transmission of SARS-CoV-2, an overview of reviews was conducted to understand what is already known from the scientific literature about how virus transmission may be affected by HVAC design features in the built environment.MethodsOvid MEDLINE and Compendex were searched from inception to January 2021. Two reviewers independently screened titles and abstracts and full text of potentially relevant reviews, using a priori inclusion criteria. Inclusion criteria were systematic reviews examining effects of HVAC design features on virus transmission. Two reviewers independently assessed methodological quality using AMSTAR2.ResultsSearching identified 361 citations, 45 were potentially relevant, and 7 were included. Reviews were published between 2007 and 2021, and included 47 virus studies. Two earlier reviews (2007, 2016) of 21 studies found sufficient evidence that mechanical ventilation (airflow patterns, ventilation rates) plays a role in airborne transmission; however, both found insufficient evidence to quantify minimum mechanical ventilation requirements. One review (2017) of 9 studies examining humidity and indoor air quality found that influenza virus survival was lowest between 40% and 80% relative humidity; authors noted that ventilation rates were a confounding variable. Two reviews (2021) examined mitigation strategies for coronavirus transmission, finding that transmission decreased with increasing temperature and relative humidity. One review (2020) identified 14 studies examining coronavirus transmission in air conditioning systems, finding HVAC systems played a role in virus spread during previous coronavirus outbreaks. One review (2020) examined virus transmission interventions on public ground transportation, finding ventilation and filtration to be effective.ConclusionsSeven reviews synthesizing 47 studies demonstrate a role for HVAC in mitigating airborne virus transmission. Ventilation, humidity, temperature, and filtration can play a role in viability and transmission of viruses, including coronaviruses. Recommendations for minimum standards were not possible due to few studies investigating a given HVAC parameter. This overview examining HVAC design features and their effects on airborne transmission of viruses serves as a starting point for future systematic reviews and identifying priorities for primary research.