Inhalation product testing: Particle size distribution

Pharmaceutical inhalation products, including metered-dose inhalers (MDIs) and dry powder inhalers (DPIs), are devices that deliver a specific quantity of drug to the lungs. 

The particle size distribution and shape of the delivered dose is critical because these determine the inhaled dose (which can be absorbed and exert a therapeutic effect) and how much is deposited in the lungs. 

The optimum aerodynamic particle size distribution has generally been recognized as being in the range of 1-5 µm. Analysis using cascade impactors is used for aerodynamic particle sizing from inhalers and nebulizers. 

This method is performed by collecting several particle size fractions from an inhalation device using a cascade impactor, and subsequently analyzing their content using an assay, such as HPLC. 

Other common techniques to determine the particle size distribution are dynamic image analysis (DIA), static laser light scattering (SLS, also called laser diffraction), dynamic light scattering (DLS) and sieve analysis.