Investigating the influence of ambient humidity on pMDI aerosols downstream of the extrathoracic region using laser diffraction

The size characterization of aerosols emitted from pressurized metered dose inhalers (pMDIs) is complicated by propellant evaporation and potential hygroscopic size changes in the presence of ambient humidity. This study used laser diffraction (LD) for in situ measurements of pMDI particle size distributions downstream of the Alberta Idealized Throat (AIT) and Alberta Idealized Child Throat (AICT) in low and high ambient humidity. In low humidity, agreement between LD-derived measurements and cascade impactor size distributions varied across inhalers. Extrathoracic deposition was higher in the AICT than the AIT. In high humidity, LD-derived particle sizes downstream of the model throats increased for one solution inhaler, but decreased for the remaining (suspension) inhalers. Experiments performed with a blank pMDI containing only propellant recorded non-zero size distributions downstream of the extrathoracic region in high humidity, suggesting that drug-free droplets may contribute to size distributions measured by LD for suspension pMDIs. Further experiments conducted using an extension tube downstream of the extrathoracic models to prolong exposure to high humidity demonstrated that hygroscopic size changes progressed slowly downstream of the extrathoracic airways. Differing drug deposition and particle sizes measured between the AIT and AICT underscore the importance of using age-appropriate extrathoracic models during size characterization of particles entering the lungs of adults and children. The use of LD for the measurement of pMDI aerosols should be considered on a product-specific basis. Size measurements in high humidity require careful interpretation, particularly for suspension pMDIs, as LD estimates could be impacted by the measurement of API-free droplets.Copyright © 2026 American Association for Aerosol Research