Factors influencing the delivery efficiency fromHFA-134aMDIs were examined theoreticallyandexperimentally.Thetime required forevaporation of HFA-134a and ethanol droplets were theoreti- cally calculated. HFA-134a droplets were shown to evaporate ap- proximately seven times faster than ethanol droplets of the same size, even though HFA-134a droplets cool to approximately 78 de- grees below ambient temperature during evaporation. MDI deliv- ery efficiency was experimentally shown to decrease with increas- ing ethanol concentration, however, the corresponding decrease in vapor pressure was not the primary variable responsible for the decreased efficiency. Rather, this was shown to be primarily due to the increased time required for the droplets to evaporate as ethanol concentration increased. Droplets that evaporate slowly remain for a longer period in the size range that ismore likely to deposit via tur- bulent deposition in the actuator mouthpiece or USP Inlet during cascade impaction tests. Tests with experimental MDIs using alter- native cosolvents confirmed that MDI delivery efficiency is much more sensitive to the time required for evaporation of the droplets than on the formulation vapor pressure or the size of the atom- ized droplets. This indicates that factors affecting the evaporation of an MDI aerosol play a larger role in determining MDI delivery efficiency than do atomization effects.
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