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

## TL;DR

This study explores how humidity and ethanol affect aerosol particle sizes from an inhaler, showing that ethanol can help optimize drug delivery to the lungs.

## Contribution

The novel contribution is demonstrating how ethanol content and humidity dynamically influence aerosol size distributions using two complementary measurement techniques.

## Key findings

- Increased relative humidity leads to larger aerosol particle sizes.
- Higher ethanol content consistently reduces both particle size and mass.
- Single droplet data can accurately model aerosol plume behavior.

## Abstract

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® Soft Mist Inhaler (SMI) formulated with and without ethanol content.

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.

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.

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.

The online version contains supplementary material available at 10.1007/s11095-025-03851-1.