Determination of hygroscopic growth of aerosol particles under physiologically relevant conditions

Particle size is a key parameter determining the deposition region in the human airways. Inhaled particles are subject to water-saturated conditions arising from the continuous evaporation of water from the airway epithelia into somewhat colder and drier inhaled air. These conditions result in a net mass transfer of water from airway walls through the surrounding gas phase to the particles, causing them to grow in size and mass. This process is called hygroscopic growth.

We evaluated the hygroscopic growth of liquid particles generated by consumer products while they are transported from ambient conditions to the human respiratory system with high relative humidity. Measuring hygroscopic growth directly in the human airways is impossible due to the invasive character of such measurements. An alternative way is to employ computational methods. However, experimental data to validate the predictions obtained by the models are rather limited. We used a newly developed humidified mouth-trachea mechanistic model. Inner surfaces of the model were kept wet and at body temperature, allowing for efficient heating and humidifying ‘inhaled’ test aerosols. To investigate the translatability of findings on hygroscopic growth made under very simple conditions (low complexity aerosols) to real-world scenarios, we followed a three-step approach: most fundamentally, we used monodisperse aerosols containing propylene glycol (PG) and vegetable glycerol (VG), both the primary constituents of aerosols generated by electronic cigarettes (ECs). Aerosol complexity was then increased by adding nicotine, flavoring ingredients, and organic acids (which are often added to ECs to modify the volatility of nicotine), but maintaining monodispersity. Finally, we used real-world, polydisperse aerosols generated from a (non-commercial) liquid containing PG, VG, nicotine flavors, and organic acids by a commercially available EC.

The results showed that the hygroscopic growth of the particles was mainly influenced by the content of the main liquid components (PG, VG, and nicotine). In contrast, the trace amounts of flavorings and/or acids in the liquids did not change particle growth. Growth factors increased with increasing residence time and relative humidity. The results for the liquid particles generated by the test consumer products indicate that the particle growth in the human respiratory system could increase the deposition of these particles in the lower airways.

This study was conducted as part of a collaboration between the Research Group of Aerosol Chemistry and Physics and Philip Morris Products S.A.

• Mašková L., Trávníčková T., Lee T., Schwarz J., Ondráček J., Paříková A., Havlica J., Steiner S., Białek J., Lucci F., Kuczaj A.K., Ždímal V.: Determination of hygroscopic growth of aerosol particles under physiologically relevant conditions. Results Eng. 2026, 30(June 2026), 110167. doi.org/10.1016/j.rineng.2026.110167