Understanding the factors that influence the airborne survival of viruses such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in aerosols is important for identifying routes of transmission and the value of various mitigation strategies for preventing transmission. We present measurements of the stability of SARS-CoV-2 in aerosol droplets (∼5 to 10 µm equilibrated radius) over timescales spanning 5 s to 20 min using an instrument to probe survival in a small population of droplets (typically 5 to 10) containing ∼1 virus/droplet. Measurements of airborne infectivity change are coupled with a detailed physicochemical analysis of the airborne droplets containing the virus. A decrease in infectivity to ∼10% of the starting value was observable for SARS-CoV-2 over 20 min, with a large proportion of the loss occurring within the first 5 min after aerosolization. The initial rate of infectivity loss was found to correlate with physical transformation of the equilibrating droplet; salts within the droplets crystallize at relative humidities (RHs) below 50%, leading to a near-instant loss of infectivity in 50 to 60% of the virus. However, at 90% RH, the droplet remains homogenous and aqueous, and the viral stability is sustained for the first 2 min, beyond which it decays to only 10% remaining infectious after 10 min. The loss of infectivity at high RH is consistent with an elevation in the pH of the droplets, caused by volatilization of CO2 from bicarbonate buffer within the droplet. Four different variants of SARS-CoV-2 were compared and found to have a similar degree of airborne stability at both high and low RH.
Original languageEnglish
Article numbere2200109119
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number27
Early online date28 Jun 2022
Publication statusE-pub ahead of print - 28 Jun 2022

Bibliographical note

Funding Information:
ACKNOWLEDGMENTS. This work was funded by the National Institute for Health Research-UK Research and Innovation (UKRI) rapid COVID-19 call, the Elizabeth Blackwell Institute for Health Research, the University of Bristol, and the Medical Research Council. Additionally, this work was supported by funding from the PROTECT COVID-19 National Core Study on transmission and environment, managed by the Health and Safety Executive on behalf of Her Majesty’s Government. A.E.H. and M.O.-F. received funding from the Biotechnology and Biological Sciences Research Council, Project BB/T011688/1. A.D.D. is a member of the G2P-UK National Virology consortium funded by the Medical Research Council/UKRI (Grant MR/W005611/1) that supplied SARS-CoV-2 variants. H.P.O. is supported by funding from the Defence Science and Technology Laboratory and the Engineering and Physical Sciences Research Council. We also thank Robert Alexander for his contributions to discussions on aerosol pH.

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Copyright © 2022 the Author(s).


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