Can Air-Conditioning Systems Contribute to the Spread of SARS/MERS/COVID-19 Infection? Insights from a Rapid Review of the Literature

Abstract

The airborne transmission of SARS-CoV-2 is still debated. The aim of this rapid review is to evaluate the COVID-19 risk associated with the presence of air-conditioning systems. Original studies (both observational and experimental researches) written in English and with no limit on time, on the airborne transmission of SARS-CoV, MERS-CoV, and SARS-CoV-2 coronaviruses that were associated with outbreaks, were included. Searches were made on PubMed/MEDLINE, PubMed Central (PMC), Google Scholar databases, and medRxiv. A snowball strategy was adopted to extend the search. Fourteen studies reporting outbreaks of coronavirus infection associated with the air-conditioning systems were included. All studies were carried out in the Far East. In six out the seven studies on SARS, the role of Heating, Ventilation, and Air Conditioning (HVAC) in the outbreak was indirectly proven by the spatial and temporal pattern of cases, or by airflow-dynamics models. In one report on MERS, the contamination of HVAC by viral particles was demonstrated. In four out of the six studies on SARS-CoV-2, the diffusion of viral particles through HVAC was suspected or supported by computer simulation. In conclusion, there is sufficient evidence of the airborne transmission of coronaviruses in previous Asian outbreaks, and this has been taken into account in the guidelines released by organizations and international agencies for controlling the spread of SARS-CoV-2 in indoor environments. However, the technological differences in HVAC systems prevent the generalization of the results on a worldwide basis. The few COVID-19 investigations available do not provide sufficient evidence that the SARS-CoV-2 virus can be transmitted by HVAC systems.

Aerosol Transmission of SARS-CoV-2: Physical Principles and Implications

“…Social distancing (91) and well-fitting masks (121) help to reduce aerosol transmission as well as large droplet transmission, but other precautions specific to aerosols are also needed. These could include operational changes to ventilation systems in public buildings and public transport (98)(99)(100)(101), UV lamps in some indoor locations (112,116,117), and attention to wind (61) and sunlight (7) in outdoor settings.…”

“…The risk then depends on the duration of exposure (16) as well as the fresh air ventilation rate. Conversely, longdistance indoor transport by natural or mechanical ventilation is a potential hazard that does not exist for infection by larger droplets (23,97,98).…”

Could thermodynamics and heat and mass transfer research produce a fundamental step advance toward and significant reduction of SARS-COV-2 spread?

“…Otherwise, it is necessary to understand the mechanisms which may lead to allow the diffusion through the HVAC systems, which surfaces and filters are usually one of the favourite mechanisms of deposition of the virus. [ 156 , 157 ]. Optimal disinfection systems and modes may be studied to reduce HVAC plant-related risks.…”

“…The studies carried out in the Wuhan area (China), Northern Italy, and New York City, experimentally demonstrate that airborne transmission is the dominant route for the spread of the disease and the cause of highest virulence cases [56] . Besides, typical indoor airspeeds, which are generated by mechanical HVAC plants, could allow a 5 μm droplet will travel tens of meters [ 57 , 58 ], which is much greater than the scale of a standard room if emitted from a height of nearly 1.5 m to the floor.…”

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“…Experimental studies in a laboratory−controlled environment demonstrated that aerosolized SARS−CoV−2 particles (<5 µm) remained suspended in the air for at least 3 hours, viable in air for at least 1 hour, and on surfaces for up to days. 1 …”