Scientists find key reason why loss of smell occurs in long COVID-19

At least for the tiny respiratory droplets that are mostly formed in the lungs, children exhale much less potentially infectious particles than adults. The research contributes to our understanding of how illnesses like influenza and COVID-19 can be controlled in their spread.

The University Gottingen Medical Center and the Max Planck Institutes for Dynamics and Self-Organization (MPI-DS) for Chemistry conducted the study, which has this as one of its main findings (UMG).

Using data from 132 individuals of all ages, the researchers examined the amounts of aerosol particles and droplets released with the breath while breathing, speaking, singing, and yelling.

Infectious diseases are often transmitted via particles exhaled from infected persons. Yet, the size of such aerosol particles varies greatly, depending on their origin in the respiratory tract. In the lung, mostly small particles with less than five microns, that is five-thousandths of a millimetre are produced, also known as PM5. In contrast, bigger particles are produced in the upper respiratory tract.

As the measurements showed, children exhale far less smaller particles than adults. “We found that the concentration of small particles below five microns increases with age and is particularly low in children. As a result, adults are much more likely to trigger spreading if the infection is only in the lower respiratory tract,” says Mohsen Bagheri, research group leader and lead author of the study at the MPI-DS. Notably, larger particles that originate in the throat are spread by children and adults to the same extent, according to the study. The researchers did not find a correlation between the concentration of exhaled particles and the person’s gender, weight, fitness or smoking habits.

In this comprehensive study, the researchers recorded data from 132 healthy volunteers. The study also included children and adolescents between 5 and 18 years about which very little data were available. They used various instruments installed in a clean room to measure the full range of particle sizes exhaled: from a tenth of a micrometer to a quarter of a millimeter. The participants performed different vocalization activities such as singing, speaking and shouting for 20 minutes in total. “Vocalization and age are shown to be independent risk factors for particle production,” reports Prof. Simone Scheithauer from the Department of Infection Control and Infectious diseases at the UMG.

Although human drops and aerosols contain mostly small particles, larger particles constitute the major part of the total volume that can contain pathogens. “If the pathogen mainly resides in the upper respiratory tract, the large particles are by far the main transmitter of the disease,” explains Eberhard Bodenschatz, director at the MPI-DS. “It is thus important to consider the localization of the infectious particle in the respiratory tract to decide on appropriate protection,” he continues. “For example, the current Omicron variant of the Coronavirus seems to be localized more to the upper respiratory tract and this is why even simple filtering face masks are a great protection.”