Corona aerosols: airborne contamination?

Christiane Fux studied journalism and psychology in Hamburg. The experienced medical editor has been writing magazine articles, news and factual texts on all conceivable health topics since 2001. In addition to her work for, Christiane Fux is also active in prose. Her first crime novel was published in 2012, and she also writes, designs and publishes her own crime plays.

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Washing hands and keeping your distance - at least in closed rooms, that may not be enough. It is becoming increasingly clear that so-called aerosols could also contribute significantly to the infection process.

These are the finest microdroplets, usually less than five micrometers in diameter. (For comparison: a human head hair is about 50 micrometers thick). They are expelled during normal speaking or even breathing.

While larger and therefore heavier droplets fall comparatively quickly to the ground, the smaller aerosols waft through the air for a longer period of time. This is why the current distance rule of 1.5 meters does not apply to these floating particles: They can spread over longer distances. They also accumulate in closed rooms.

Minutes to hours in the air

This is illustrated, among other things, by an experiment carried out by Philip Anfinrud and Adriaan Bax from the National Institutes of Health in Bethesda (USA). To do this, the researchers had a test person shout the words “stay healthy!” At different volumes in a darkened box.

Using a laser, they made the ejected aerosol droplets visible - at least the larger of them. These stayed in the air for an average of twelve minutes. The researchers calculated that when you speak out loud, around 1,000 aerosol droplets are produced per minute. In addition, there are the small aerosols that remain invisible and are therefore not counted.

In other experiments, viruses capable of replicating could be detected in artificially generated aerosols three hours later. However, these were laboratory situations - under natural conditions it could look very different.

The final proof is still pending

But we know: some infectious diseases such as measles or chickenpox can be transmitted very easily via aerosols. Experts also speak of the so-called airborne path of infection - i.e. via the air.

Since Sars-CoV-2 multiplies in the throat and lungs, it is not surprising that this pathogen was also found in aerosols. However, it is still unclear whether the amount of viruses it contains is actually sufficient to become infected - and how long the pathogens trapped in the aerosols remain contagious.

Singing is a high risk factor

But there is some evidence that aerosols may have played a central role in the global spread of the disease.

An impressive example of the airborne transmission of viruses is provided by the evaluation of an outbreak during a choir rehearsal. It was held on March 10th in a small church in Washington state. One of the singers had previously been infected with Sars-CoV-2. During this one rehearsal with 61 participants, 52 singers of the infected, 32 of them proven to contract Covid-19. In three of them the disease was severe and two of them died.

The distance rules had not been followed exactly here - some singers were close to each other and a few even handed their neighbors fruit they had brought with them - but the number of people infected remains strikingly high and can hardly be explained by transmission routes beyond the air.

There was a comparable case in the Berlin Kantorei. 60 out of 80 choir members became infected. All have since recovered. Other similar cases are reported in the UK.

It should be noted, however, that singing in particular releases a particularly large number of aerosols. Particles are thrown out of the depths of the respiratory tract with great force. Under the same conditions, but without singing, fewer people present would probably have been infected.

Conversely, the singing could have contributed to why so many participants were infected at après-ski in Ischgl, at the carnival session in the German Corona hotspot in the Heinsberg district, but also at football events. These are all occasions when those present sing or bellow fervently.

Do almost half of all infections occur through the air?

The contagion force can be further graded: Shouting produces more aerosols than loud speaking, which in turn produces more than quiet ones. Least of all, but still measurable, arise when breathing calmly.

Some experts, including the Charité virologist Prof. Christian Dorsten, are already convinced that a large part - possibly even more than 40 percent - of infections occurs via this route. That could explain why the virus was able to spread relatively quickly around the world. The risk of infection would be particularly high in small, closed rooms.

Virus load is crucial

Whether you get infected from an infected person depends crucially on how high the dose of virus hits you. Larger droplets contain more viruses - and are therefore particularly contagious. When it comes to airborne transmission, airborne virus density is critical. And that depends on numerous factors.

Room size: Viruses can spread more easily in larger rooms. The virus density decreases.

Length of stay: The longer you stay together in a closed room, the greater the risk of inhaling more viruses. This could be particularly problematic on longer air or train journeys.

Type of activity: How the infected person behaves is a decisive factor in the virus output. For example, as described above, singing produces more aerosols than breathing calmly.

This is also indicated by a fitness studio study from South Korea: During an intensive dance workout, many participants became infected. It wasn't in the yoga class. More viruses seem to be expelled from intense physical activity than from lighter ones.

Stage and severity of the disease: the stage of the disease with whom you share a room is also likely to be decisive. According to the current state of knowledge, people infected with corona are particularly contagious one day before and one to two days after the symptoms appear.

Five days after the onset of symptoms, the risk of infecting others is already very low for many. There is also evidence that people who are slightly ill or who have no symptoms are often less contagious than those who are seriously ill.

Number of infected people in the room: The more infected people are in a room, the more coronaviruses get into the air. Accordingly, the greater the danger for others.

Conversely, this means: in the open air, where the viruses evaporate quickly, the risk of infection is much lower. Anyone who walks past an infected person is unlikely to get infected - unless someone coughs them up.

Ventilation helps!

Action measures can be derived from these findings: Regular vigorous ventilation can significantly reduce the viral load in the room air. When the outside temperature is warm, it is advisable to work with the windows wide open.

Correctly set up, pedestal fans could support ventilation if they expel room air through the window. Charité virologist Christian Drosten also suggests this for school classes in his podcast. Lessons in the open air are even better.

Do masks reduce aerosol spread?

It is still unclear whether masks will be effective in reducing the spread of Covid-19 via aerosols. In the laser experiment described, a simple washcloth held in front of the mouth prevented the spread of larger aerosols.

However, previous studies show that simple cotton masks in particular could be too wide-meshed. They can catch drops and larger droplets, but small aerosols either penetrate through the mask when speaking or can escape to the side particularly easily.

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