Reference work for aerosol research and infection medicine
Infectious diseases that are spread via the air we breathe play a major role worldwide. Despite this, key physico-chemical properties of the particles are still largely unknown. An international research team led by Mira and Christoph Pöhlker has therefore summarised the available 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-Organisation in Göttingen, the Leibniz Institute for Tropospheric Research (TROPOS) in Leipzig, the University of Leipzig, the University of Denver and the Georg August University of Göttingen were involved in the study.
The sniffles and coughs around us are increasing again - proof that the cold season is just around the corner. Many infectious diseases such as colds, flu, Covid-19 as well as 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 the mouth and nose and thus spreads.Although infectious diseases play an enormous role worldwide, the key physico-chemical properties of the particles that spread them are still largely unknown. This includes the question of where and during which activity they are formed in the respiratory tract, how they are transported and which pathogens they carry with them.
Mira and Christopher Pöhlker now present a comprehensive inventory and a scheme that categorises respiratory aerosols and droplets into different groups based on their size and place of origin 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-Organisation and other institutes in Leipzig, Denver and Göttingen, also provides the first indications of which activities in our airways can promote the spread of which pathogens. „Our classification will help to localise 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 airway activity and the spread of pathogens
For their study, the couple combed through more than 400 publications on the topic of aerosols and respiratory-related infectious diseases. These included the numerous theoretical and experimental studies that emerged as a result of the Covid-19 pandemic. He also conducted his own experiments in which, for example, high-resolution green spectrometers and holographic images were used to investigate how respiratory aerosols and droplets spread during various activities such as breathing, speaking or singing. The investigations into the question of what kind of protective measures such as distance, masks and fans protect against coronavirus were also taken into account. We investigated aerosol formation in a total of 132 people during various activities," says Eberhard Bodenschatz, Director at the Max Planck Institute for Dynamics and Self-Organisation. „It turned out that the risk of infection by aerosols and the protective measures to be taken depend to a large extent on the part of the respiratory tract in which the virus is located,
explains Bodenschatz.The Covid-19 pandemic was also the starting point for the review article now published in the journal „Reviews of Modern Physics“. Christopher Pöhlker normally studies aerosol particles in forest systems such as the Brazilian rainforest, while his wife Mira is a cloud researcher at the University of Leipzig and the Leibniz Institute for Tropospheric Research (TROPOS). As both were stuck at home during the first lockdown in early 2020, like many others, they turned their attention to Covid-19 and other infectious diseases in which aerosols play a crucial role. „We had assumed that the physico-chemical principles of how and where respiratory particles are formed and what size they are had long since 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 was not the case. To our great surprise, we realised that the data situation was very sparse and 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,
Reference book for aerosol research and infection medicine
The researchers categorise the respiratory particles into a new scheme based on their point of origin. The smallest droplets with diameters of less than 0.2 micrometres and between around 0.2 and one micrometre belong to two bronchiolar groups, as they are formed in the bronchioles, i.e. the fine lung tubules. The cause is the thin film of fluid on the surface of the bronchioles, which tears when inhaled.
At the other end of the green spectrum are the oral aerosols and droplets. They mostly originate from the mouth, tongue and lips and are between 8 and 130 micrometres in diameter. They are produced, for example, when we speak or laugh, as saliva foils are produced in the oral cavity and break up again. When we open our mouths, these droplets find their way out and can hit people in the immediate vicinity.
We also include the droplets that are produced in the nose and throat when sneezing and which are often so large that you can see them,
says Pöhlker, whose airway droplets have also been measured in experiments.
In between and as a further category, the aerosol researchers define the laryngeal trachea group, as the droplets originate from the vocal folds, which lie between the larynx and trachea. These particles are produced 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 airways, the filtering effect of the mask material and the fit are less important," explains Mohsen Bagheri, group leader in the Bodenschatz department.
„Our hypothesis was that the size of a particle is related to its place of origin, as is known from atmospheric aerosols“, says Christopher Pöhlker. The analyses confirm this and we can use it to explain all the size distributions measured so far.
The place of origin of a particle determines its size
When it comes to the question of which particles carry which pathogens, the evidence is clearest for tuberculosis. The lung disease is transmitted by bacteria and these are probably too large for the bronchiolar aerosols. Our results indicate that the pathogens need the medium-sized laryngeal trachea particles, i.e. those that develop around the vocal folds," says Mira Pöhlker. „This fits in with the cough typical of tuberculosis.“ Her husband adds „The pathogens are obviously travelling in a transport optimum. They travel on particles that are large enough for them, but small enough to be able 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 droplet is involved in the spread of a respiratory pathogen probably depends on the size of the droplet and the severity and location of the infection," says Pöhlker. „We do not yet have this important information. There is very little data on which we can judge how the pathogens travel. To clarify this, studies with infected people and co-operation between aerosol experts and clinicians are necessary. This will make it possible to determine more precisely which measures are most effective in preventing the transmission of respiratory diseases, the Max Planck researcher concludes.
Original title of the publication in "Reviews of Modern Physics":
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