The Effect of Humidity on the Size of Particles Delivered from Metered-Dose Inhalers

The presence of humidity in air supplied to intubated patients has long been identified as a limiting factor in the delivery of therapeutic aerosols during mechanical ventilation. In this work, the well-known reduction in drug delivery to the lung observed when metered-dose inhalers (MDIs) are actuated into warm, humid airflow was investigated through examination of the evolution of particle size distributions delivered from two commercial hydrofluoroalkane (HFA) MDIs actuated through a standard mechanical ventilation holding chamber. Aerosol deposition in the holding chamber and mass median aerodynamic diameter (MMAD) increased significantly with humidity for both MDIs, regardless of the presence of cosolvent and surfactant in one MDI formulation but not the other. However, in humid air MMAD decreased with distance downstream from the holding chamber, again for both MDIs studied. A modification to the popular assumption of MDI particle growth by condensation is proposed, in which condensed water evaporates back into the supplied airflow subsequent to an initial transient, nucleated condensation. It is anticipated that the present improved understanding of MDI aerosol behavior in humid air may lead to the development of enhanced techniques of drug delivery to mechanically ventilated patients.