A face mask is the best protection

January 11, 2021
In an interview, published by Pro-Physik, the fluid physicist Eberhard Bodenschatz explains how aerosols form and spread and what protection masks offer.

SARS-CoV-2 is transmitted through human aerosols and droplets. At the Max Planck Institute for Dynamics and Self-Organization in Göttingen, Eberhard Bodenschatz and his team have been carrying out measurements on more than 140 volunteers since March 2020. The investigations showed, among other things, that face masks provide very good protection, that playing wind instruments is possible but singing is not a good idea.

What are the factors involved in the transmission of the coronavirus through aerosols?

We exhale droplets that contain the virions, the virus particles that are responsible for infection. Infection happens less through hand transmission because we have now learned to wash our hands regularly and thoroughly. The main route of transmission is via droplets. Larger droplets over 50 micrometers fall to the ground and dry. However, when these are stirred up again, they can become contagious again.

And what about the smaller droplets?

They stay in the air for a very long time. In the quickly drying droplet the cores, i.e. aerosols, are essentially salt, proteins and here and there a virus. The probability of infection by ingested virions is cumulatively probably over eight hours.

What does that mean?

It means you don't pick up your viral load all at once, but rather you pick up more and more virions over a period of hours - while shopping at the supermarket, riding the underground or going for a walk. The higher the viral load you pick up, the more likely you are to become infected.

Is there anything that surprised you in the aerosol measurements of the last few months?

I was massively surprised that humming and singing are practically equally dangerous, as is breathing through the mouth or nose. It also turned out that shouting is worse than coughing, especially since you can shout much longer than you can cough. I also found it surprising that oily lipstick helps.


Saliva doesn't stick to lips with a fatty lip balm, and there are no big droplets.

How dangerous is playing wind instruments?

Without a mask, like singing. We have made many measurements with trumpets and shown that a filter at the sound outlet suppresses the aerosol output excellently.

How do you protect yourself from infection?

A mask is the best protection - it not only protects me, but also those around me. The droplets that come out of your mouth when you speak are moist and thus larger by a factor of 4 to 5, making it more difficult for them to pass through the mask than dry droplets from outside. An FFP2 mask filters 99.9 per cent of virus-laden particles. Even the ones from the discount store! But be careful: the mask must be tight around the face. I use double-sided medical tape to seal leaks at the nose.

In reality, most people wear a fabric mask. What is the situation like with that?

It's still much better than none at all, because the larger droplets all get trapped and it's also very efficient on smaller droplets because they come out of the lungs moist first. The protection is about a factor of 5 better than without a mask.

What do you think about a visor?

We haven't studied that yet, but it's important that the visor is deep enough. Because they exhale at an angle of 30 degrees downwards. Big drops hit the visor and are gone, the small ones go past it and become aerosols. So it's not a good protection.

You have also developed an app to calculate the risk of infection. How does it work?

For the calculation you have to enter the size of the room, the number of people and their ages, the duration of the gathering, and make a statement about what activities you plan to do. The app then calculates the risk of infection for that situation.

Sounds very user-friendly.

The app is very easy to use. If you want to be more specific, an advanced version allows you to provide additional information and take into account the effects of masks, or air exchange.

What role does ventilation play?

Ventilation is extremely important! If I have a family party, it is best to ventilate for ten minutes every 20 minutes - open the window on the upper floor and tilt windows downstairs. Then very efficient displacement ventilation sets in, with aerosol-laden air escaping out of the window.

What hobbies would you advise against?

Singing is actually ten times more dangerous than talking, though you could sing with a decent mask. In addition, we have measured that aerosol emissions increase with physical exertion.

So gyms should stay closed?

With the right air purification device, they could open. I have a device in the institute that filters about 3500 cubic meters of air per hour. With that, I create almost clean room quality in a 100 cubic meter room. If I set up four of these devices and support the mixing of the air with fans, there is no difference any longer compared to outdoor sports. Fitness studios usually also have very good displacement ventilation. By the way, we also measured this in the large lecture hall in Göttingen.

With what result?

The top four rows had to be closed because the air exchange did not work there.

How does optimal ventilation work?

Ideally, the window should be replaced by a transparent panel with a fan. The fan blows the air outwards - either constantly or by shock ventilation. The fan also pushes air away from the building, allowing the flow of clean air back into the building. For classrooms, this is an excellent solution that costs only about a thousand euros. With it, you can excellently regulate the air exchange.

As opposed to normal ventilation with the windows open....

Right. This natural ventilation is extremely dependent on outdoor conditions - wind pressure and also temperature difference. In winter, you achieve air exchange in a flash, but when temperatures rise in spring, there is hardly any air exchange when you ventilate. That is why a technical solution is much better.

Is this the only sensible solution for classrooms?

That's exactly my point: according to our measurements, shock ventilation in classrooms is not enough. Air purification devices also have their disadvantages, because their filters have to be changed regularly. We have tested devices where the coarse filter had to be cleaned after just one week! The facility managers can't change that many filters, and it gets very expensive.

This is probably a growth industry at the moment.

There is a lot of money involved. Even in a medium-sized city, there must be a thousand classrooms, which all would have to be equipped with one or two air purification devices with costs of a few thousand euros each. Even though the situation is not good, we have been lucky so far.

What do you mean?

The corona virus stops its activity with a half-life of about 1.6 hours and is hardly contagious. That means it can be brought under control. If we had an aggressive virus, it would look bad for Germany now. In the beginning, we didn't even have masks. I didn't understand the situation around the cloth masks: There must be more than 500,000 sewing machines in Germany. If everyone who owned one had sewn cloth masks, the whole of Germany could have been supplied with cloth masks within a few days.

Sounds quite simple...

It is. It's not rocket science! Putting two T-shirts on top of each other and seeing how much aerosols pass through is not that complicated. We already did that in March. We've had mandatory masks at the institute since the beginning of April. So far, there has been no contagion.

What are you working on at the moment?

We are continuing to develop our app. Among other things, we want to simulate rooms that are larger than 300 cubic meters. To do this, we simulate for each person surrounded by an aerosol cone what effect these aerosol clouds have on the probabilities of infection. This will allow us to look at how aerosols evolve locally or outdoors. The next thing is to directly determine the viral load of different people.

How do you go about doing that?

We have developed a special mask for this, a converted diving mask. It has the advantage that the inhaled and exhaled air are completely separated from each other, so we can measure exactly which aerosols are emitted. We want to use it to study how the aerosols develop during the course of the disease. When a sick family takes part in the test, they receive four masks and a measuring device. After 14 days we receive the data and the measuring device. The test subjects can keep the masks and continue to use them as diving masks.

What strategy would you advise politicians and the public?

We need to make good masks available cheaply and wear them consistently until the aerosol load has dropped again with low numbers of infected people. For our institute, we bought 30,000 FFP2 masks for less than 40 cents each. It is and will remain essential to ensure efficient air exchange in schools.


Eberhard Bodenschatz has been interviewed by Maike Pfalz

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