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Reference work for aerosol research and infectious medicine

An international research team led by Mira and Christoph Pöhlker has summarized the knowledge about infectious diseases that are spread through the air we breathe and published it in a review article in the journal “Reviews of Modern Physics”.

Infectious diseases that are spread through the air we breathe play a major role worldwide. Nevertheless, the essential physical and chemical properties of the particles are still largely unknown. An international research team led by Mira and Christoph Pöhlker has therefore summarized the knowledge and published it in a review article in the journal “Reviews of Modern Physics”. Researchers from the Max Planck Institute for Chemistry in Mainz, the Max Planck Institute for Dynamics and Self-Organization in Göttingen, the Leibniz Institute for Tropospheric Research (TROPOS) in Leipzig, the University of Leipzig, the University of Denver and the Georg-August-University Goettingen.

There are more sniffles and coughs around us again - evidence that the cold season is just around the corner. Many contagious diseases such as colds, flu, Covid-19 but also measles and tuberculosis are transmitted from person to person when speaking, sneezing or coughing. The pathogens - viruses or bacteria - travel as stowaways in aerosols and droplets that an infected person releases through their mouth and nose and thus spreads.

Although infectious diseases play an enormous role worldwide, key physicochemical properties of the particles that spread them remain largely unknown. This also includes the question of where and during what activity they arise in the respiratory tract, how they are transported and which pathogens they take with them.

Now Mira and Christopher Pöhlker present a comprehensive inventory and a scheme that divides respiratory aerosols and droplets into different groups based on their size and where they originate in the respiratory tract. The study, which was carried out with colleagues from the Max Planck Institute for Chemistry, the Max Planck Institute for Dynamics and Self-Organization and other institutes in Leipzig, Denver and Göttingen, also provides initial information about which activities of our respiratory tract cause the spread of which can promote pathogens. “Our classification will help to locate and track the sources of infectious particles,” says Dr. Pöhlker. The aerosol researcher is convinced that the new scheme will help to better understand and contain infectious diseases.

Relationship between respiratory activities and the spread of pathogens

For their study, the couple combed through more than 400 publications on the subject of aerosols and respiratory-related infectious diseases. This also included the numerous theoretical and experimental studies that emerged due to the Covid-19 pandemic. We also carried out our own experiments, for example using high-resolution size spectrometers and holographic recordings to investigate how respiratory aerosols and droplets spread during different activities such as breathing, speaking or singing. The studies on the question of what type of protective measures such as distance, masks and ventilation protect against coronaviruses were also taken into account. “We examined aerosol formation in a total of 132 people during various activities,” says Eberhard Bodenschatz, director at the Max Planck Institute for Dynamics and Self-Organization. “It turned out that the risk of infection from aerosols and the protective measures to be taken depend largely on which part of the respiratory tract the virus is in,” explains Bodenschatz.

The Covid-19 pandemic was also the trigger for the review article now published in the journal “Reviews of Modern Physics”. Christopher Pöhlker normally studies aerosol particles in forest ecosystems such as the Brazilian rainforest; his wife Mira is a cloud researcher at the University of Leipzig and at the Leibniz Institute for Tropospheric Research (TROPOS). Since both were stuck at home like many others during the first lockdown in early 2020, they turned to Covid-19 and other infectious diseases in which aerosols play a crucial role. “We had assumed that the physical and chemical principles of how and where respiratory particles arise and what size they are had long been thoroughly clarified. After all, the mechanisms are highly relevant not only for COVID-19, but also for influenza, tuberculosis, measles and many other diseases,” says Pöhlker. “But that wasn’t the case. To our great surprise, we discovered that the data was very thin and that many studies were only comparable to a limited extent due to a lack of standards. We have now changed this. “Our publication will hopefully be a helpful reference for aerosol researchers as well as infectious disease specialists,” adds Prof. Dr. Mira Pöhlker.

Reference book for aerosol research and infectious medicine

The researchers classify the respiratory particles into a new scheme based on where they originate. The smallest drops with diameters of less than 0,2 micrometers and between about 0,2 and one micrometer belong to two bronchiolar groups because they arise in the bronchioles, i.e. the fine branches of the lungs. The cause is the thin film of liquid on the surface of the bronchioles, which tears when you breathe in.

At the other end of the size spectrum are oral aerosols and droplets. They mostly come from the mouth, tongue and lips and are between 8 and 130 micrometers in diameter. They arise, for example, when we speak or laugh, as saliva threads form in the mouth and break again. When you open your mouth, these droplets find their way out and can hit people in close proximity. “The oral group also includes the droplets that form in the nose and throat when sneezing and which are often so large that you can see them,” says Pöhlker, whose respiratory droplets were also measured in experiments.

In between and as a further category, the aerosol researchers define the larynx-trachea group, since the droplets come from the vocal folds, which lie between the larynx and the trachea. These particles are created when laughing, speaking, singing and coughing. “To protect yourself from small droplets from the lungs, it is therefore particularly important to use face masks with a high filtering effect and a tight fit. For larger droplets from the upper respiratory tract, the filtering effect of the mask material and the fit are less important,” explains Mohsen Bagheri, group leader in Bodenschatz’s department.

“Our hypothesis was that the size of a particle is related to its place of origin, as this is known from atmospheric aerosols,” says Christopher Pöhlker. “The analyzes confirm this and we can use it to explain all the size distributions measured so far.”

The place where a particle is formed determines its size

With regard to the question of which droplets carry which pathogens, the evidence for tuberculosis is clearest. “The lung disease is transmitted by bacteria and they are probably too large for the bronchiolar aerosols. Our results indicate that the pathogens need the medium-sized larynx-trachea particles, i.e. those that arise around the vocal folds,” says Mira Pöhlker. “That matches the cough that is typical of tuberculosis.” Her husband adds “The pathogens are obviously traveling in an optimal transport mode. They travel on particles large enough for them, but small enough to penetrate deep into the lungs of other people.”

For viral pathogens such as influenza and SARS-CoV-2, the assignment of particle size, place of origin and infection potential is still quite unclear. “Whether a drop is involved in the spread of a respiratory pathogen probably depends on the size of the drop as well as the severity and location of the infection,” says Pöhlker. “We don’t have this important information yet. There is very little data from which we can assess how the pathogens move. To clarify this, studies with infected people and collaboration between aerosol experts and clinicians are necessary. This makes it possible to determine more precisely which measures are most effective in preventing the transmission of respiratory diseases,” summarizes the Max Planck researcher.

Original title of the publication in “Reviews of Modern Physics”:

“Respiratory aerosols and droplets in the transmission of infectious diseases,”

News from the “University of Leipzig” from November 09.11.2023th, XNUMX

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