Aerodynamic Particle Size Distribution (APSD)

Aerodynamic Particle Size Distribution, or APSD, measures the distribution of aerosolised particles according to their aerodynamic diameter. Aerodynamic diameter reflects how particles behave in a moving air stream, taking into account factors such as particle size, density and shape. In inhaler testing, APSD helps laboratories assess where particles emitted from an inhaler or nasal drug product are likely to deposit in the respiratory tract.

APSD stands for Aerodynamic Particle Size Distribution. It is commonly used in orally inhaled and nasal drug product testing to assess the aerodynamic size of particles or droplets emitted from a device.

Aerodynamic particle size is important because it helps determine whether a product delivers the dose to the intended region of the respiratory tract. Particles that are too large may deposit in the mouth, throat or nasal passages, while smaller particles may travel further into the lungs. Understanding and controlling aerodynamic particle size therefore supports targeted drug delivery, product performance, patient safety and therapeutic efficacy.

Aerodynamic particle diameter describes how a particle behaves in a moving airstream, rather than only its geometric or physical size. It takes into account factors such as particle size, shape and density and inertia, all of which can affect how easily a particle is carried in the air or deposits on a surface. As a result, particles with the same geometric size may not have the same aerodynamic diameter, making this measure especially important when assessing inhaled aerosols and likely respiratory deposition.

Fine Particle Dose, or FPD, is the mass of drug contained in the fine particle size range, commonly defined as particles below 5 µm aerodynamic diameter. For APSD assessments, it is used to help assess the portion of the emitted dose that may be capable of reaching the lower respiratory tract.

APSD is typically measured using cascade impactors or impingers, which separate aerosolised particles into different size fractions as the aerosol passes through a series of stages. In a cascade impactor, precision nozzles subject the particles to progressively increasing air velocities, causing particles with different aerodynamic diameters to deposit on different stages. The collected drug is then recovered and analysed to determine the aerodynamic particle size distribution of the emitted dose.

Under controlled test conditions. The collected drug is then analysed to determine the aerodynamic particle size distribution of the emitted dose.

APSD measurement is relevant to all orally inhaled and nasal drug products, including metered dose inhalers, dry powder inhalers, nebulisers, soft mist inhalers, nasal sprays, nasal aerosols and nasal powders.

APSD measurement requires equipment such as a Next Generation Impactor, Andersen Cascade Impactor, Multi-Stage Liquid Impinger, flow controllers, preseparators, induction ports and product-specific accessories. The correct set-up depends on the device type and test method.