The science of droplets on surfaces is well established for pure fluids1, and this includes the key effects of evaporation and droplet dynamics under a variety of environmental conditions (temperature, humidity, etc.). However, as it pertains to SARS-CoV-2, much of the uncertainty derives from the complex biophysical and biochemical characteristics.
Higher level of certainty:
Deposition onto surfaces: Respiratory droplets from individuals, especially large droplets will quickly settle (a 100 µm droplet will sediment from 1 m in 3 seconds), they are also likely to contain higher concentrations of pathogens. Viral particles can be deposited onto surfaces and resuspended via airflow patterns and other sources of natural or man-made turbulence.2,3,4
Stability on different surfaces types: The SARS-CoV-2 virus can remain viable on surfaces in the order of hours, days or weeks, with different inactivation rates depending on the surface type. With viability on some surfaces of 28 days or more for non-porous surface at 20°C and 50% RH# (copper for hours, cotton, paper & cardboard for days; plastic, vinyl, glass and stainless steel > 28 days).2,5,6,7
Environmental effects on droplets: Viruses within saliva droplets have marginally longer survival times than in water droplets at varied relative humidity.8 Coronaviruses survivability is reduced by half when RH is between 40-60%2. SARS-CoV-2 is stable at lower temperatures and becomes increasingly less stable at higher temperatures and relative humidity#2,6,7,9. SARS-CoV-2 half-life at 70°C is ~ 1 h, but is ~ one week at 4°C7. Virus survival times at 50% relative humidity and three temperature levels (20◦C, 30◦C and 40◦C) showing that the half-life of the virus reduces drastically with temperature from 43 – 66 hours at 20◦C to 10 – 33 hours at 30◦C to 1.5 – 3 hours at 40◦C for most surfaces6. Coronaviruses are known to be inactivated by sunlight and UVC10,11
Importance of complex physico-chemical characteristics: The evolution and dynamics of SARS-CoV-2 viral droplets must account for the complex interior structure since it exhibits many differences to pure fluids (e.g. different evaporation rates).12
Lower level of certainty:
Experimental studies are not representative of real-life situations: There is general uncertainty in regards to the representative nature of experimental studies. In particular, concentrations of 104-107 viruses/ml used in fomite studies may not be representative in environments beyond hospital-like situations13. Viral numbers can be an order of magnitude smaller and transmissions through inanimate surfaces are unlikely14. There is evidence of increased viral stability or survivability with increased titre size used in these studies.6
Uncertainties from physico-chemical characteristics: The detailed biophysical and biochemical characteristics of expelled SARS-CoV-2 droplets is not well known, and this has a significant impact on the ability to estimate dynamics. Real respiratory droplets vary widely and are difficult to study, with limited data.12
Drying and inactivation times: Saliva droplets dry to a highly concentrated ‘droplet nuclei’ containing glycoproteins and lipids from the body. This may provide a beneficial microenvironment, protecting virus from inactivation, some evidence is significant enough to account for discrepancy between drying time and inactivation time.8,12
Interactions with complex surfaces and complex environments estimations: It remains unclear how SARS-CoV-2 interacts with complex multi-scale surfaces (porous surfaces, carpets, etc.). The extent of danger of different transmission pathways (air, surface, environmental) is still unknown.4,15
Transfer from surfaces to skin: Viral transfer from surfaces to skin is known to happen but experimental data is not yet available for SARS-CoV-2. For modelling purposes data for other viruses (e.g. influenza, rhinovirus and norovirus) can be used.16
P.-G. de Gennes, F. Brochard-Wyart, and D. Quéré, Capillarity and Wetting Phenomena. Springer New York, 2004.
L. Dietz, P. F. Horve, D. A. Coil, M. Fretz, J. A. Eisen, and K. Van Den Wymelenberg, “2019 Novel Coronavirus (COVID-19) Pandemic: Built Environment Considerations To Reduce Transmission,” mSystems, vol. 5, no. 2, Apr. 2020, doi: 10.1128/msystems.00245-20.
C. Y. H. Chao et al., “Characterization of expiration air jets and droplet size distributions immediately at the mouth opening,” J. Aerosol Sci., vol. 40, no. 2, pp. 122–133, 2009, doi: 10.1016/j.jaerosci.2008.10.003.
S. W. X. Ong et al., “Air, Surface Environmental, and Personal Protective Equipment Contamination by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) From a Symptomatic Patient.,” Jama, pp. 3–5, 2020, doi: 10.1001/jama.2020.3227.
N. Van Doremalen et al., “Aerosol and surface stability of SARS-CoV-2 as compared with SARS-CoV-1,” New England Journal of Medicine, vol. 382, no. 16. Massachussetts Medical Society, pp. 1564–1567, Apr. 16, 2020, doi: 10.1056/NEJMc2004973.
S. Riddell, S. Goldie, A. Hill, D. Eagles, and T. W. Drew, “The effect of temperature on persistence of SARS-CoV-2 on common surfaces,” Virol. J., vol. 17, no. 1, p. 145, Oct. 2020, doi: 10.1186/s12985-020-01418-7.
A. W. H. Chin et al., “Stability of SARS-CoV-2 in different environmental conditions,” The Lancet Microbe, vol. 1, no. 1, p. e10, May 2020, doi: 10.1016/s2666-5247(20)30003-3.
L. Liu, J. Wei, Y. Li, and A. Ooi, “Evaporation and dispersion of respiratory droplets from coughing,” Indoor Air, vol. 27, no. 1, pp. 179–190, Jan. 2017, doi: 10.1111/ina.12297.
J. Biryukov et al., “Increasing temperature and relative humidity accelerates inactivation of SARS-COV-2 on surfaces,” mSphere, vol. 5, no. 4, Aug. 2020, doi: 10.1128/mSphere.00441-20.
M. Buonanno, D. Welch, I. Shuryak, and D. Brenner, “Far-UVC light efficiently and safely inactivates airborne human coronaviruses,” Sci Rep , vol. 10, no. 10285, Apr. 2020, doi: 10.21203/RS.3.RS-25728/V1.
S. Ratnesar-shumate et al., “Simulated Sunlight Rapidly Inactivates SARS-CoV-2 on Surfaces,” J. Infect. Dis., no. 52281, pp. 1–9, 2020, doi: 10.1093/infdis/jiaa274.
E. P. Vejerano and L. C. Marr, “Physico-chemical characteristics of evaporating respiratory fluid droplets,” J. R. Soc. Interface, vol. 15, no. 139, p. 20170939, Feb. 2018, doi: 10.1098/rsif.2017.0939.
E. Goldman, “Exaggerated risk of transmission of COVID-19 by fomites,” The Lancet Infectious Diseases, vol. 20, no. 8. Lancet Publishing Group, pp. 892–893, Aug. 01, 2020, doi: 10.1016/S1473-3099(20)30561-2.
M. U. Mondelli, M. Colaneri, E. M. Seminari, F. Baldanti, and R. Bruno, “Low risk of SARS-CoV-2 transmission by fomites in real-life conditions,” Lancet Infect. Dis., vol. 0, no. 0, Sep. 2020, doi: 10.1016/S1473-3099(20)30678-2.
D. E. Harbourt et al., “Modeling the Stability of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-1 2) on Skin, Currency, and Clothing 2 3,” medRxiv, p. 2020.07.01.20144253, Jul. 2020, doi: 10.1101/2020.07.01.20144253.
A. N. M. Kraay et al., “Fomite-mediated transmission as a sufficient pathway: A comparative analysis across three viral pathogens 11 Medical and Health Sciences 1117 Public Health and Health Services,” BMC Infect. Dis., vol. 18, no. 1, p. 540, Oct. 2018, doi: 10.1186/s12879-018-3425-x.
Previous Contributors: Dr Philippe Trinh, Dr Michael Short, Dr Oleksiy Klymenko, Yyanis Johnson-Llambias, Clint Wong, and Piotr Morawiecki
Please help We would welcome expert contributions in this field of study and are looking for input to Aerosol / Droplet Deposition or Loss. We have more than 30 papers than need reviewing and any contributing input to this would be welcomed.
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