Microdroplet Transmission  

What is microdroplet (aerosol) transmission anyway?   
If water droplets are very small , the droplet may not fall to a surface quickly, and if very tiny, microscopic droplets may linger, invisibly floating in the air.  Covid-19 virus particles are small enough to potentially be clinging to such mini droplets otherwise known as  aerosols.

How does an microdroplet cloud move and dissipate?

It may perhaps be more useful to think of the air around any individual very much like we would the cloud of smoke around a smoker as it lingers it is denser near the person emitting it. If you are several feet away from that individual you probably cannot smell or see the cloud of smoke.That is if you’re outside. But it is certainly spread throughout an enclosed room.   
Unlike the familiar odour and blueish tinge visible from a microdroplet cloud emitted from a person who is smoking a cigarette with cigarette smoke,  one challenge with microdroplet clouds breathed out by Covid-19 infected individuals is that we can not see or smell that cloud. 
 So we might not have any notion of this microdroplet cloud if we enter a room which has such an invisible cloud of floating particles which can linger for hours unless. 

Inside vs Outside - What's the difference?

What is so special about outside air?   Let's consider some familiar settings:


A cloud of cigarette smoke outside
Observing our local environment can provide us a number of clues as to how airflow outside disperses a cloud of smoke. 
We would also, I believe, generally agree that the disbursement would be much faster on a windy mountain top compared to a calm and narrow pathway in the forest valley.


A cloud of cigarette smoke inside
When the covid containing cloud of air goes out the window -> it becomes so diluted that there is no longer enough concentration of covid virus particle to cause an infection.
We are quite familiar with this in avoiding smoke from those smoking in a room. Open the window and the smoke goes out the window and diluted to the comfort of the people in the room.
The airflow through the room and out the window certainly has something to do with diluting and thus dispersing the cloud of smoke to the outside .  

Outside is the Current Buzz Word

The now familiar comments by Dr. Fauci And other public health officials about social distancing and using a mask to avoid virus transmission has started to evolve. Since mid July "Open the window to get some fresh air" has become a common message from public health and others.

So we are now hearing how opening the window is helpful to improve safety against Covid transmission. It seems like a simple message and quite a logical one

That short phrase “open the window to air out the place” is actually describing the value of ventilation (ie air movement) through the building. 

This ventilation moves clean air in while moving air out of the building which may contain floating virus particles.

Once outside, the virus particle concentration becomes so dilute so as to be negligible and therefore no longer a concern for the virus transmission to other humans breathing the air .

The following clips from Japanese researchers helps us understand the way airborne particles become trapped indoors and the impact of ventilation.

The following graphics show six people gathering in an enclosed space for multiple hours.

After four hours together without wearing masks, five others will be infected by patient 0, even if safe distances were maintained.

Even when face mask are worn, four people are infected from the prolonged exposure in a poorly ventilated area.

When the area is ventilated, everyone wears face masks, and the duration of the gathering is cut in half, then the risk of infection drops to below one.

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 The "open the window" message is really saying:
                          - COVID-19 can be in an invisible microdroplet  cloud not just droplets
                          - The cloud may linger for hrs
                          - Encouraging the cloud to move outside is a simple way to make a covid cloud harmless [outside = infinitely diluted]
Learn More about Indoor Air Quality