Particle Size Measurement:

"Reviewer Guidance for Nebulizers, Metered Dose Inhalers, Spacers and Actuators".  Federal Drug Administration, Office of Device Evaluation, Division of Cardiovascular and Respiratory Devices, Anesthesiology and Defibrillator Devices Branch.  1 October 1993.

Highlights: Section VII (Particle Size Delivered from Mouthpiece) states, "the most commonly used and the most widely recognized is the cascade impactor method..." with at least one of the two required measurement methods being with the cascade impactor (with at least six stages).  The airflow rate range should be approximately 10-15 liters/minute.