The aerodynamic particle size distribution (APSD) of dry powder inhalers (DPIs) is typically measured via cascade impaction techniques.

• These methodologies rely on the fact that an average adult generates approximately 4kPa pressure drop across a given forced inhalation and inhales a total volume of 4L of air.
• These parameters are used to define a constant test flow and duration and give origin to a square wave profile that is applied during dose uniformity and cascade impactor testing. In addition, the actual acceleration of the flow from the inhalation manoeuvre is lower than the acceleration of the flow provided by a vacuum pump; hence, most patients may not be able to achieve the inhalation volume of 4L.