Coronavirus: Does it stay in the air? – Usually no

Lately, some academic studies suggest that the virus can stay in the air for long periods of time. The articles’ abstracts reach the press – who summarizes them even further in one headline: The virus can stay in the air for hours. They are wrong and misleading the public.

An asymptomatic person carrying the COVID-19 virus walks in the street. He/she breathes normally and expels the COVID-19 virus attached to small saliva droplets. How long does the virus stay in the air?

We all know (since Galileo’s famous experiments in the tower of Pisa in the 17th century) that all objects – when left free to fall from a given height h – they all reach the ground at the same time t. Most of us remember the simple kinematic formula from school that relates the height h of the object to the time t it takes for the object to reach the ground:

h = (1/2)*g*t*t

Here, g = 9.8 meter/(second*second) is the acceleration of gravity. For a normal breathing person the droplets are expelled from the mouth at a height h of about 1.5 meters, mostly in a horizontal direction, that is, their initial velocity towards the ground is zero. Hence, using the above equation, the time it takes for the droplet to reach the ground is approximately:

t = square root of 2*h/g = square root of 2*1.5/9.8

t = 0.5 seconds

It takes less than one second for a droplet emitted from a normally breathing person to reach the ground: it does not stay ‘floating’ in the air.

But just a moment, the reader will say: Galileo’s experiment was done with massive heavy objects, balls weighting several kilograms. If instead of these massive objects we let a feather fall, experience tells us that instead of 0.5 seconds it might take several seconds to reach the ground: the theory fails for light objects!

Yes, but why is so? The scientific answer is air pressure and friction. Their origin are the millions and millions of molecules in the air (mainly oxygen and nitrogen molecules) constantly colliding with the aerodynamic form of the feather, slowing down its fall towards the ground.

Could these collisions slow down the fall of the droplets carrying the virus to the extent that it could take hours for them to reach the ground? Each droplet (or even an isolated virus) consists of millions and millions of atoms held together. The droplets are extremely massive and compact objects compared to the simple molecules of oxygen or nitrogen in the air. Collisions of the droplets with these tiny molecules in the air will not generate enough pressure and friction to slow down neither the droplets carrying the viruses nor even a few isolated viruses expelled during normal breathing. If not half a second, these collisions will perhaps slow down the fall to a few seconds, but never to long minutes or hours as suggested by the press (or the abstracts of the academic studies).

Hence, the public should not panic: it you are strolling outside in the street – just keep a two meter distance with other people: any droplets expelled by other people during normal breathing will reach the ground and not your mouth. You will not breathe in the viruses left by an asymptomatic person who happened to be in the same place a few moments ago: all the viruses he/she expelled during his/her normal breathing would be already laying on the ground when you reach the point where he/she was standing or walking moments ago. And wear a good mask: when you are in a public space you never know how close another person might unexpectedly come to you.

And avoid dining or drinking at restaurants and bars. As much as people love to be social, these places increase significantly the risk to catch the virus. Here the problem is different: it is virus transmission through shared surfaces. Whenever an asymptomatic person carrying the virus eats or drinks at a table in a restaurant, at some time he/she will touch his mouth with his hand, and then place the hand on the table or any other shared surface. With this simple action he carried the droplets with the virus from his mouth to the surface. Viruses can subsist for hours on surfaces. Even without touching his mouth with his hand, during normal breathing while seating at a table, the droplets expelled from the mouth and carrying the virus will land on the table and the surrounding chairs.  When minutes later the next customer will sit at the same table he will do the opposite movement: at some time his hands will go from resting on the surface of the table to touching his mouth, and the transmission cycle of the virus from one person to the next will be completed. Do not expect the impossible from restaurant owners: they will not disinfect each table and chairs every time after a customer leaves and before the next customer comes.

The situation in the operating room of a hospital is different: the air in this environment is contaminated with many heavy particles (not just the tiny oxygen and nitrogen molecules) and constant air recirculation keeps these particles (and the virus) in the air.  It is in these highly contaminated environments that the virus could “float” in the air for a substantial period of time, making it dangerous for a healthy person to stay there. Most probably the authors of these academic papers had this type of heavy polluted environment in their mind when they carried their studies.

About the Author
Jaime Kardontchik has a PhD in Physics from the Technion, Israel Institute of Technology. He lives in the Silicon Valley, California.
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