As neat as the corona virus appears in some images, it is invisible to the human eye. But what you can see is the classic transmission path: droplet infection. Studies on this are spreading faster than the virus. But a sense of proportion is required.
Munich / Stuttgart (dpa) - There is something in the air: the corona virus can spread in small particles. Lively and clearly visible when sneezing or coughing, but also rather timid and hardly visible to the human eye when simply speaking.
US researchers have recently shown this impressively using laser light: while a man says "stay healthy", "green speckles" sparkle against a black background. If the speaker wears a mask, none of it can be seen.
How wet the pronunciation is depends, among other things, on the volume and the sounds, explains environmental physician Claudia Traidl-Hoffmann from the Helmholtz Zentrum München and the Institute for Environmental Medicine at the Technical University of Munich. The th from English as in "thunderstorm" (thunderstorm) or hissing sounds were wonderful for demonstrations. In other words: "If an infected person stands in front of me and says thunderstorm, there is a high risk of catching me."
Like other seasonal corona, flu or rhinoviruses, Sars-CoV-2 is traditionally transmitted by droplet infection. That is why it means: keep your distance. And: mouth-nose protection.
But how far do these droplets fly? How big is the risk of infection? And does it make a difference whether people are in a closed room or in the fresh air - and at what speed are they traveling there?
As new as the new type of corona virus is, so fresh, partly unchecked and based on small samples are tests and models that have been published in the past few days. "At the present time, this is often speculation," emphasizes Traidl-Hoffmann. However, a study from China suggests that the corona concentration inside buildings is usually higher than in public places.
Researchers from the Netherlands and Belgium have recently published calculations from the wind tunnel that the recommended 1.5-meter distance when moving faster is not sufficient to avoid all droplets. Anyone walking at around five kilometers an hour should keep a distance of five meters, joggers at 14.4 km even around ten meters. Scientists at Aalto University in Finland visualized the spread of a cloud of breath when someone coughs unprotected between supermarket shelves, for example.
However, such modeling is often quite theoretical. The makers of the jogging study, for example, admit that tailwind and crosswind should be taken into account. And environmental medicine specialist Traidl-Hoffmann also advises considering overhurried reactions as to what the individual can deduce from this.
Bernhard Weigand, who does research at the Institute for Thermodynamics of Aerospace at the University of Stuttgart on drip dynamics, among other things, says: "Directly behind a runner or cyclist, the flow breaks off, particles stay in the air. But if you don't when you’re driving Tour de France, don’t get that close. "
In addition, models often do not take into account the behavior of drops, as the professor makes clear. "Very small droplets evaporate in a fraction of a second. Large droplets sink very quickly and do not follow the air flow." With a view to possible infections, 30 to 40 micrometer drops are interesting - that's about half the thickness of a human hair. At a temperature of 20 degrees, the 20 to 30 seconds lasted. Models based on a spread of several minutes are far from realistic.
In addition to the temperature, the humidity is decisive for the evaporation. The higher this is, the worse evaporate drops. In general, one can say: the hotter and drier, the faster the evaporation, the lower the risk of infection. Drafts blow the drops away and boost the evaporation.
It is also crucial where the drops and viruses arrive and how infectious they are, says Traidl-Hoffmann. On the nasal mucosa they are quickly blown out. "If you breathe them deep into your lungs, they do the most damage."
The professor also endeavors to apply a well-known wisdom in pharmacy: the dose makes the poison. For example, there could be 1600 pollen in one cubic meter of air, which allergy sufferers are feeling again these days. "How high the concentration of virus particles is around a corona patient is so far unclear," says the environmental doctor. What is certain is that the Sars-CoV-2 virus is 160 nanometers in size - so there might be 100 viruses in smaller particles in the air. "How many of these virus particles are necessary to become infected is unclear and is also crucially dependent on the recipient and his susceptibility."
With the help of a cascade impactor, Traidl-Hoffmann and her team now want to investigate the particle size in the air on which the virus spreads. Sieves with different pore sizes are installed in the device, which filter so-called bioaerosols by size. The scientists want to find out how high the virus concentration in the air is when an infected person lies quietly in bed, for example, or when he is intubated. Medical personnel in particular become infected because they are particularly exposed to the viruses, says Traidl-Hoffmann. But this research project is still at the very beginning.
RKI on transmission routes
Visualization of the spreading of droplets when speaking
Study on Sars-CoV-2 concentration in the air
Model of the corona cloud in the supermarket
Jogging study
Homepage Traidl-Hoffmann
Homepage Weigand
Additional Q&A for the jogging study