A number of methods are available for estimating the particle or droplet size of pharmaceutical aerosols.

1. The aerosol output may be sized using inertial techniques. These methods may involve impaction, centrifugation, or sedimentation. The most popular techniques for the assessment of the particle size of pharmaceutical aerosols are techniques that use a filter, impactor, or impinger.

2. The particles can be visualized using optical or electron microscopy, and specific dimensions can be measured with less subjectivity. The advent of computers also allows particles to be sized by image analysis techniques based on optical or electron photomicrographs.

3. Laser-scattering techniques have been used in a variety of operational modes to size pharmaceutical aerosols. These may be listed as forward scattering, Fraunhofer diffraction, and laser Doppler velocimetry, in conjunction with either accelerating nozzles or acoustic excitation.

These techniques use different operating principles and often express the size of the particles according to different definitions. It is important that the investigator understand the size parameter being measured. It is valuable to estimate particle size using different techniques to obtain a broad view of the behavioral characteristics of an aerosol. For this reason, the pharmaceutical aerosol scientist should combine techniques from the previously mentioned groups.

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