Relationship between the spread of new coronavirus (COVID-19) and the transport phenomenon of aerosol particles

Transport phenomena that even infectious disease experts may overlook

The new coronavirus (COVID-19) continues to mutate and its infectivity has been reported in news worldwide, but unfortunately, only the infectivity of the virus seems to be foucused as a problem. More than a year has passed since the infection of the coronavirus was reported, and our understanding of the situation increasing high risk has progressed. Discussions on the spread of infection and temperature/humidity have also been taken up early on, however, no reliable correlation with the factors is understood. Infectious disease specialists may be missing simple but important phenomena because they are not familiar with transport phenomena.

Aerosol particles change its size in air

In a cold season, when you exhale, it becomes like a white mist for a moment and it will be invisible in a few seconds. Relatively large particles of water vapor reflect visible light and look like mist, but become smaller in less than a second or less and are difficult to see with the human eye in most people's environments. It changes to small particle size (~ 1 μm or less) in a few seconds. The size of the new coronavirus is as small as about 0.1 μm, and it is difficult to see in front of you how it is floating with some water vapor attached. In addition, the amount of virus contained in the exhaled breath varies depending on the condition of the infected person and the condition of the exhaled breath/cough.
If an aerosol containing 5 viruses is released, for example, there is a high possibility that the 5 viruses will fall apart, due to lower saturated vapor pressure in winter. When the temperature is high (as in summer), the saturated water vapor content is an order of magnitude higher, so it is highly likely that it remains a larger size of aerosol. Therefore, at different temperatures, it can be expected that when an infected person releases the same amount of virus, the number of aerosol particles containing the virus will be higher in cold seasons.

Changes the size and the number of aerosol particles with coronavirus (before and after evaporation)

Another important point is how long the aerosol particles with some water vapor can keep floating and how far it can be carried. Small aerosols, which are difficult to see with our naked eye, are transported along with the flow of air, and especially in an environment where the the temperature in the room is somewhat lower than that of our body, they rise due to buoyancy and are quickly transported in a short time of several tens of seconds, as simulated with the computation（see "The Japan Society of Applied Physics" (in Japanese)）
Because small and light aerosol particles are carried along the flow of air, even if they are moving toward the wall or ceiling, they will change the direction as the air flow direction, as a result, it is difficult to attach to the wall. The actual spread of the coronavirus differs between summer and winter, as shown in the figure below.

Prediction of the new cases of COVID-19 at Tokyo (at January 22, 2021)

Predictions for each prefecuture is shonw in prediction for new cases of COVID-19, while it is currently available in only Japanese.

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