How is the coronavirus really transmitted in the air?

23 September 2021

Part 2: Airborne Transmission Explained

A recent paper published in the journal Health Science Reports in May 2021, discusses SARS-CoV-2 coronavirus transmission and contamination in the air, and asserts that the typical distinction between droplets larger than 5 microns and airborne contamination is purely academic. Instead, the authors argue that the infectivity of the virus as a function of droplet size is a continuum, meaning that the droplet infectivity is related to droplet size but that there is no “magical” cut-off number as previously believed.

Therefore, we should be talking about one unique airborne transmission mode which varies with the size of the airborne droplets, rather than the so-called “droplet” contamination vs. aerosol contamination. The main reason why such a distinction was made in the first place is because droplets above 5 microns settle relatively quickly by gravitational forces and typically will stay airborne over a lateral distance of a few meters only (depending on the actual droplet size and atmospheric conditions). Calculations showed that, in still air (without wind or convection) a relatively large 100 micron droplet will reach the ground in about 5 seconds, and will travel 2 to 3 meters doing so. This is illustrated is the following picture:

On the other hand, smaller droplets can remain airborne for much longer times, and travel large distances. This is because droplets smaller than 5 microns are much more sensitive to airflow and convection, which can offset the settling force of gravity, and help keep these droplets and particles (which together form an aerosol) airborne for long times. Here is an illustration of an aerosol hanging in the air around someone walking (note that the droplets are not represented at scale, and they are in reality much too small to be visible with the naked eye):

What makes pathogen containing aerosols especially dangerous is that they can spread in an environment without being seen. This is because the droplets are too small to be detected by eye, and they can stay airborne for long times. For example, in still air a 5 microns droplet settles by gravitation in approximately half an hour1, andit was calculated than an aerosol containing 1 micron particles could stay airborne for over 12 hours (refs). Practically this means that an aerosol produced by coughing or simply talking can contain pathogens (e.g. a virus), and it can stay airborne for minutes to hours and spread over relatively long distances (much more than 3 m). Contamination occurs when these small droplets are inhaled and deposit inside the respiratory tract.

Therefore, it is important to realize that the transmission of a virus does not occur only by direct face to face (short distance) interactions, and that contamination from airborne pathogens in the form of small droplets is a very real, and effective, mode of transmission. Of course, the exact airflow and atmospheric conditions will greatly influence the time the contaminated droplets can stay in the air, and the distance that they can reach. The horizontal distance that droplets can cross is a sensitive function of the droplets’ diameter, relative humidity, the velocity at which the droplets are ejected, and the airflow conditions, and it can easily reach distances much longer than 3 m in many circumstances. The physics of how aerosols spread is therefore critical to our understanding of infections in general, and especially today in the case of COVID-19.

Another important question is that of virus viability and inactivation. How long does the virus stay active and infectious while in traveling in the air? How do local atmospheric conditions (e.g. humidity and temperature) influence the viability of viruses? A number of studies have shown that several viruses, including the influenza A and the SARS-CoV-2 viruses can stay viable in an aerosol for several hours.2-5 This clearly indicates that contamination by airborne transmission might be favoured in crowded and indoor spaces, especially when the environment is poorly ventilated.

Next time, we’ll discuss which measures we can take to protect ourselves from contaminated aerosols.

What to remember from this article:

– large droplets are not the only source of contamination

– small droplets and aerosols are one of the main sources of contamination, and virus spreading in the air

– aerosols can spread over long distances (>> 3meters) and stay airborne for hours.

References

1) Drossinos Y., Weber T. P., Stilianikis N. I., Droplets and aerosols: An artificial dichotomy in respiratory virus transmission, Health Science Reports; First published: 07 May 2021.

2) Dabisch P, Schuit M, Herzog A, et al., The influence of temperature, humidity and simulated sunlight on the infectivity of SARS-CoV-2 in aerosols. Aerosol Sci Tech. 2021; 55(2): 142- 153.

3) Kormuth KA, Lin K, Prussin AJ II, et al. Influenza virus infectivity is retained in aerosols and droplets independent of relative humidity. J Infect Dis. 2018; 218(5): 739- 747.

4) Van Doremalen N, Bushmaker T, Morris D, et al. Aerosol and surface stability of SARS-CoV-2 compared to SARS-CoV-1. N Engl J Med. 2020; 382: 1564- 1567.

5) Shaman J, Kohn M. Absolute humidity modulates influenza survival, transmission, and seasonality. Proc Natl. Acad. Sci. USA. 2009; 106(9): 3243- 3248.

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