The pandemic of SARS-CoV-2 asked for airborne transmission risk assessments in occupational and daily life situations. Some physical activities (singing, wind instrument playing, sport) seem to have higher infection risk than speaking or breathing.

We conducted an exploratory human study (n = 30; n_female = 13, n_male = 17) that assessed the aerosol emission per breath depending on breathing volume and inspiratory duration. The PExA instrument, "Particles in Exhaled Air 2.1" (PExA AB, Gothenburg, Sweden), was customized to measure inhalation flow rate. The instrument allows to simultaneously quantify breathing volumes and emitted aerosol particle concentrations using a Grimm 11-D aerosol spectrometer with size range 0.31 to >3.42 μm.

The results revealed a non-linear correlation of emitted aerosol particles on both breathing volume and inspiratory duration (R2V = 0.55). The logarithmised particle counts exhibit a sigmoidal dependence on breathing volume and a decay-like dependence on inspiratory duration. Error-weighted data fitting was used to determine the parameters of our model function and enables to predict the aerosol emission per breath.

The interpretation of the aerosol emission data strengthens the theory of bronchiole fluid film burst. It suggests that fluid film burst starts not in respiratory bronchioles as is generally assumed, but in upper bronchioles. The increase of intrathoracic pressure for higher breathing volume leads to dynamic compression of upper and terminal bronchioles, and thus to an increase in particle emission. Its exponential increase that became evident in our data can be explained by the bifurcative branching along the respiratory tree. This interpretation further refines the theory of particle formation in human respiratory tract. Our model provides a functional parametrization as a tool to predict human aerosol emission for a wide range of breathing conditions. It is considered a valuable tool for airborne risk assessments.

This article is published in the "Journal of Aerosol Science" (2025).