Characterizing the Influence of Relative Humidity and Ethanol Content on the Dynamic Size Distributions of Aerosols Generated from a Soft Mist Inhaler

Objective Inhaled drug delivery systems need to ensure that the delivered aerosol effectively reach the lungs while overcoming challenges related to environmental conditions, such as relative humidity (RH). This study investigates the impact of environmental factors on aqueous aerosol behaviour using a Respimat (R) Soft Mist Inhaler (SMI) formulated with and without ethanol content. Methods Comparative Hygroscopic Aerosol Particle Sizing (CHAPS) was used to measure aerosol size distribution under varying RH levels, while single droplet analysis was conducted using Comparative Kinetics-Electrodynamic Balance (CK-EDB) to assess particle behaviour. Results The findings reveal that increased RH results in larger particle sizes, while elevated ethanol content consistently decreases both particle size and mass. The strong agreement between CHAPS measurements and CK-EDB data suggests that aerosol plume behaviour can be accurately modelled from single droplet data. Conclusion The study highlights ethanol's role in optimizing particle size distribution, which is crucial for enhancing the therapeutic efficiency of inhaled medications. These results underscore the importance of tailoring formulation and environmental conditions to improve drug delivery outcomes in pulmonary therapies and the importance of recognising that aerosol particle size distributions are dynamic and highly compositionally dependent.