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Coronavirus spread: what role do aerosols play?


Coronavirus aerosols: Infectious mini droplets in the air

More and more research indicates that aerosols in the air play a greater role in the spread of the novel coronavirus SARS-CoV-2 than previously thought. An expert in aerosols summarizes the current state of knowledge.

Large droplets that arise when you cough, sneeze, speak and breathe quickly sink to the ground. But corona viruses can remain in the air longer even in mini-droplets. These so-called aerosols are increasingly becoming the focus of research.

Aerosols are becoming increasingly important in research

When researching corona infection pathways, scientists are increasingly examining so-called aerosols. This is a mixture of solid or liquid airborne particles that can contain Sars-CoV-2 particles.

Smear infections do not seem to play a special role

"We are pretty sure that aerosols are one of the ways that Covid-19 spreads," said former President of the International Society for Aerosols in Medicine, Gerhard Scheuch, in Gemünden (Wohra) to the German Press Agency ( dpa). Smear infections, for example, played a minor role.

But there are still many questions open, says Scheuch - for example, how the virus spreads when speaking or what role temperature plays. A lot of research has to be done, ”he said. "But there is more and more research going in this direction." It is therefore far from clear how infectiously dried aerosols are.

Assessment is currently difficult

According to the Robert Koch Institute (RKI), the transmission of the novel virus takes place mainly via droplets that arise when coughing and sneezing and are absorbed through the mucous membranes of the person opposite. Aerosols - defined as droplet cores smaller than five micrometers - could also contribute, “even if a final assessment is difficult at the present time”.

Self test with a candle

There are already studies dealing with the spread of drops and aerosols in the air. However, the results are sometimes different. For example, a team led by Christian Kähler from the Institute of Fluid Mechanics and Aerodynamics at the University of the Federal Armed Forces in Munich carried out experiments with a singer and came to the conclusion that when singing, the air is only set in motion up to 0.5 meters in front of the mouth - independently about how loud the sound was. As a tip for self-testing, the researchers advise you to stand in front of a lit candle and see when the flame begins to flicker when you approach it as you speak.

How the wind spreads aerosols

The scientists Talib Dbouk and Dimitris Drikakis, in turn, have calculated how far saliva drops spread when coughing lightly: no more than two meters without wind, but certainly six meters in winds of 4 and 15 kilometers per hour. The concentration and size of the drops decrease, but a distance of two meters may not be sufficient.

Washington researchers analyzed contagion within a choir and suspected that the transmission was less than two meters away. However, Kähler makes it clear that in addition to the distance, it should also be noted whether hygiene rules were observed in each case or whether, for example, hands were shaken and chairs were moved together.

Height and ventilation of rooms seem to play a role

Other aspects that can have an impact on the infection path include the height of the room and ventilation. For example, Kähler advises that “on the one hand, the air exchange rate should be increased significantly in times of the pandemic, on the other hand, with ideal room ventilation, the air should be supplied from below through the floor and sucked off across the ceiling”.

Aerosols are quickly diluted in the fresh air

In Wuhan, China, researchers searched for a study in clinics for Sars CoV-2 genes in aerosols. The amount was very low, for example in ventilated patient rooms, but higher in toilet areas. It was undetectable in the open air, except in two areas that were prone to overcrowding. Kähler also says that there is hardly any danger outdoors. You exhale about half a liter of air, which is quickly diluted. It becomes dangerous if you come closer because of a brass band in the background and speak louder. But this is again a question of distance.

How long do aerosols stay in the air?

Researchers have also investigated how long a potential danger has been: Another team from the USA has used laser light to measure the lifespan of small droplets in the air that arise when speaking. Accordingly, they only disappear in a closed environment with standing air after eight to 14 minutes. In conclusion, it says "that there is a significant probability that normal speaking in restricted environments will cause a virus to be transmitted through the air". According to Scheuch, who runs a bio-inhalation company, aerosols could even last for hours in enclosed spaces and be infectious. One breath contains 1000 particles. "Thinning is strong outside, inside it collects."

What protection do face masks offer?

The mouth-nose protection should remedy this. However, you have to know that the so-called community masks can hardly stop particles with a diameter of up to two micrometers, which Kähler's team has impressively illustrated with video recordings. Nevertheless, the simple masks would have an important effect, the professor emphasizes: “They offer flow resistance. Instead of blowing particles far out, they stick close to your head. ”

Scheuch even goes one step further: because the corona virus is only around 0.1 to 0.14 micrometers in size, not even the so-called FFP masks were enough. “They are for larger bacteria. But such small particles are difficult to filter. ”Particulate matter filters are probably more suitable. But that was still to be researched. (vb; source: Marco Krefting, dpa)

Read also: Indoor coronavirus risk 19 times higher than outside.

Author and source information

This text corresponds to the specifications of the medical literature, medical guidelines and current studies and has been checked by medical doctors.

Graduate editor (FH) Volker Blasek

Swell:

  • Talib Dbouk, Dimitris Drikakis: On coughing and airborne droplet transmission to humans; in: Physics of Fluids, 2020, aip.scitation.org
  • Lea Hamner, Polly Dubbel, Ian Capron, u.a .: High SARS-CoV-2 Attack Rate Following Exposure at a Choir Practice; Centers for Disease Control and Prevention, 2020, cdc.gov
  • Valentyn Stadnytskyi, Christina E. Bax, Adriaan Bax, and others: The airborne lifetime of small speech droplets and their potential importance in SARS-CoV-2 transmission; in: PNAS, 2020, pnas.org
  • RKI: SARS-CoV-2 Fact Sheet for Coronavirus Disease-2019 (COVID-19) (as of May 29, 2020), rki.de
  • Yuan Liu, Zhi Ning, Yu Chen, Ming Guo, among others: Aerodynamic analysis of SARS-CoV-2 in two Wuhan hospitals; in: Nature, 2020, nature.com
  • Christian J. Kähler, Rainer Hain: Making music during the pandemic - what does science advise? University of the Bundeswehr Munich (as of May 8, 2020), unibw.de
  • Christian J. Kähler, Rainer Hain: flow analyzes for the SARS-CoV-2 protective mask debate; University of the Bundeswehr in Munich (as of April 11, 2020), unibw.de


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