Airborne Viral Transformation
While large droplets generated in the upper respiratory tract and oral cavity sediment rapidly and travel only a short distance (typically <2 m), the smaller aerosol particles (typically < 5mm diameter) generated in the lower respiratory tract and larynx can remain suspended and airborne for many minutes or hours.1,2 During airborne transport, pathogens may lose viability and infectivity, and their survival decay dynamics must be understood.3,4
High level of certainty:
- Although some pathogens are only transmitted by the airborne aerosol route, referred to as obligate airborne transmission (e.g. Tuberculosis), some may transmit preferentially (e.g. measles and smallpox), and others may transmit only opportunistically by the airborne route, largely transmitting by other routes but with airborne transmission possible under certain situations.5 Other coronaviruses (e.g. SARS-CoV-1 and MERS) are considered to be opportunistically spread by the airborne route.5,6
- While suspended in small respirable aerosol particles, the airborne survival of pathogens (including SARS-CoV-2) is dependent on the environmental conditions, specifically relative humidity (RH) and temperature. At intermediate RHs and room temperature, the survival decay times for SARS-CoV-1 and SARS-CoV-2 are similar, with half-lives of 1-2 hours in tissue culture media or artificial saliva.7–9
- Both steady equilibrium solute concentrations in aerosol particles at a particular RH can lead to steady osmotic stresses on pathogens, rates of change of solute concentrations may also be influential in determining pathogen survival.10–12
- Exposure of airborne SARS-CoV-2 to light (particularly UV-A, 315-400 nm, and UV-B light, 280-315 nm) has shown that survival is significantly reduced, with decay half-times reduced to under 10 minutes in the strongest lighting conditions.7
Low level of certainty:
- The dependence of airborne survival on relative humidity and aerosol particle composition remains uncertain. In one study, SARS-CoV-2 survived for longer at intermediate RH than high RH in tissue culture medium, with the reverse trend in artificial saliva.9
- Although viral RNA has been identified in air samples in a number of studies, RNA genome copy numbers per litre of sampled air have spanned a large range from undetectable or close to the limit of detection (0.001-0.042 copies/L)13 to >3 copies/L of air.14,15 By comparison, air samples have reported up to 10,000 copies / L of air for influenza virus.16
- Although the airborne transmission of SARS-CoV-2 has been established between ferrets17 and the airborne transmission in humans has been inferred (e.g. during a choral society rehearsal),18 the relationship between RNA genome copy numbers and infectious virus remains uncertain.
- As of 26th August 2020, the authors are unaware of any study to have identified infectious virus from air samples. Other human respiratory viruses have been isolated and identified from airborne samples in a COVID clinic, but infectious SARS-CoV-2 has not been isolated.15,19 Low virus concentrations in airborne samples and the challenges of ensuring the integrity of viruses are not compromised during sampling could be possible reasons.
* The statements above are intended to be reviewed regularly as more information and new references emerge. If you have queries about the content of the above and wish to discuss these please contact the editors.
- Johnson, G. R. et al. Modality of human expired aerosol size distributions. J. Aerosol Sci. 42, 839–851 (2011).
- Nicas, M., Nazaroff, W. W. & Hubbard, A. Toward understanding the risk of secondary airborne infection: Emission of respirable pathogens. J. Occup. Environ. Hyg. 2, 143–154 (2005).
- Drossinos, Y. & Stilianakis, N. I. What aerosol physics tells us about airborne pathogen transmission. Aerosol Sci. Technol. 0, 1–5 (2020).
- Asadi, S., Bouvier, N., Wexler, A. S. & Ristenpart, W. D. The coronavirus pandemic and aerosols : Does COVID-19 transmit via expiratory particles ? Aerosol Sci. Technol. 54, 635–638 (2020).
- Roy, C. J. & Milton, D. K. Airborne Transmission of Communicable Infection – The Elusive Pathway. N. Engl. J. Med. 350, 1710–1712 (2004).
- Tellier, R., Li, Y., Cowling, B. J. & Tang, J. W. Recognition of aerosol transmission of infectious agents: A commentary. BMC Infect. Dis. 19, 1–9 (2019).
- Schuit, M. et al. Airborne SARS-CoV-2 is Rapidly Inactivated by Simulated Sunlight. J. Infect. Dis. 222, 564–571 (2020).
- van Doremalen, N. et al. Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1. N. Engl. J. Med. 382, 1564–1567 (2020).
- Smither, S. J., Eastaugh, L. S., Findlay, J. S. & Lever, M. S. Experimental Aerosol Survival of SARS-CoV-2 in Artificial Saliva and Tissue Culture Media at Medium and High Humidity. Emerg. Microbes Infect. 9, 1415–1417 (2020).
- Lin, K. & Marr, L. C. Humidity-Dependent Decay of Viruses, but Not Bacteria, in Aerosols and Droplets Follows Disinfection Kinetics. Environ. Sci. Technol. 54, 1024–1032 (2020).
- Marr, L. C., Tang, J. W., Van Mullekom, J. & Lakdawala, S. S. Mechanistic insights into the effect of humidity on airborne influenza virus survival, transmission and incidence. J. R. Soc. Interface 16, 20180298 (2019).
- Yang, W., Elankumaran, S. & Marr, L. C. Relationship between Humidity and Influenza A Viability in Droplets and Implications for Influenza’s Seasonality. PLoS One 7, 1–8 (2012).
- Liu, Y. et al. Aerodynamic analysis of SARS-CoV-2 in two Wuhan hospitals. Nature 582, 557–560 (2020).
- Guo, Z.-D. et al. Aerosol and Surface Distribution of Severe Acute Respiratory Syndrome Coronavirus 2 in Hospital Wards, Wuhan, China, 2020. Emerg. Infect. Dis. 26, (2020).
- Chia, P. Y. et al. Detection of air and surface contamination by SARS-CoV-2 in hospital rooms of infected patients. Nat. Commun. 11, 2800 (2020).
- Thompson, K. A. et al. Influenza Aerosols in UK Hospitals during the H1N1 (2009) Pandemic – The Risk of Aerosol Generation during Medical Procedures. PLoS One 8, (2013).
- Richard, M. et al. SARS-CoV-2 is transmitted via contact and via the air between ferrets. Nat. Commun. 11, 1–6 (2020).
- Miller, S. L. et al. Transmission of SARS-CoV-2 by inhalation of respiratory aerosol in the Skagit Valley Chorale superspreading event. MedRxiv doi:10.1101/2020.06.15.20132027
- Lednicky, J. A. et al. Collection of SARS-CoV-2 Virus from the Air of a Clinic within a University Student Health Care Center and Analyses of the Viral Genomic Sequence. Aerosol Air Qual. Res. 20, 1167–1171 (2020).