Why aerodynamic size reflects airway deposition
- Aerodynamic particle size distribution (APSD)
- Aerodynamic particle size distribution is the way the mass of an aerosolized active substance is spread across aerodynamic diameters. Aerodynamic diameter is the diameter of a unit-density sphere that settles at the same velocity as the particle, so it captures how inertia and gravity carry a particle through the airways rather than how the particle looks or scatters light.1,6
Because deposition in the mouth, throat, and lung is driven by inertial impaction and sedimentation, the aerodynamic distribution is the size basis that connects a measured aerosol to its likely deposition pattern. A metered dose inhaler, dry powder inhaler, nebulizer, or nasal product can share a target diameter range yet behave differently once flow, device resistance, and formulation are accounted for.3,6
This page focuses on the aerodynamic, inhalation-specific view of particle size. For the wider set of sizing bases, including optical and geometric diameters, see the particle size explainer, and for spray pattern and plume geometry, see the spray and plume dynamics explainer. Aerodynamic assessment answers a narrower question: how inhaled mass separates by settling behavior.6
MMAD and GSD summarize the distribution
- Mass median aerodynamic diameter (MMAD)
- Mass median aerodynamic diameter is the aerodynamic diameter that divides the aerosolized active substance mass into two equal halves, so half of the mass is carried by larger particles and half by smaller ones. It is read from the cumulative mass-weighted aerodynamic distribution and is most meaningful when that distribution is close to log-normal.1,6
Geometric standard deviation reports how widely the distribution spreads around the median. A near-log-normal aerosol can be described by these two values, while a distribution with more than one mode, or with mass at the extremes, needs the full staged data rather than a single median. For that reason, MMAD and geometric standard deviation are reported alongside the underlying stage masses, not in place of them.1,4,6
| Metric | What it represents | Typical basis |
|---|---|---|
| Mass median aerodynamic diameter (MMAD) | The aerodynamic diameter that splits the active substance mass into equal halves | Median of the cumulative mass-weighted aerodynamic distribution |
| Geometric standard deviation (GSD) | The spread of the distribution around the MMAD | Ratio measure that applies when the distribution is close to log-normal |
| Fine particle fraction (FPF) | The portion of the dose in particles below a defined aerodynamic cut-off, commonly near 5 µm | Impactor mass below the cut-off relative to the delivered or metered dose |
| Fine particle dose (FPD) | The mass of active substance below the defined cut-off diameter | Sum of stage masses on impactor stages below the cut-off |
| Delivered (emitted) dose | The mass of active substance leaving the mouthpiece per actuation | Collected on a dose unit sampling apparatus, separate from the size split |
Fine particle fraction and fine particle dose describe the part of the dose small enough to reach the lower airways. Fine particle fraction is the portion of the dose below a defined aerodynamic cut-off, commonly near 5 µm, and fine particle dose is the corresponding mass of active substance. The exact cut-off, and whether the fraction is referenced to the metered or the delivered dose, is set by the method and the product.1,4
Cascade impaction is the reference method
A cascade impactor draws the aerosol through a stack of stages, each with progressively finer nozzles. Particles with enough inertia impact and are collected on a given stage, while smaller particles follow the airflow to the next stage. Recovering and assaying the active substance on each stage gives the mass collected in each aerodynamic size band, which is the aerodynamic distribution itself.5,6
Two designs dominate inhaler testing. The NGI is a seven-stage impactor with a micro-orifice collector and a pre-separator, applied across metered dose inhalers, dry powder inhalers, nebulizers, and nasal aerosols. The Andersen cascade impactor is an earlier multi-stage design still referenced in compendial methods. Both separate particles by aerodynamic behavior, and both report stage masses that are converted into the size distribution.2,5,6
| Impactor stage | Aerodynamic cut-off diameter |
|---|---|
| Stage 1 | 8.06 µm |
| Stage 2 | 4.46 µm |
| Stage 3 | 2.82 µm |
| Stage 4 | 1.66 µm |
| Stage 5 | 0.94 µm |
| Stage 6 | 0.55 µm |
| Stage 7 | 0.34 µm |
Stage cut-off diameters depend on the flow rate through the impactor, so the operating flow is fixed to the product and method. The finest particles that pass every stage are captured by the micro-orifice collector, which closes the mass balance at the small end of the distribution.2,5
Compendial frameworks for aerodynamic assessment
USP <601> sets performance quality tests for inhalation and nasal aerosols, sprays, and powders, including aerodynamic size assessment reported through MMAD and geometric standard deviation when the distribution is log-normal. USP <1601> adds characterization tests for nebulized products, where droplet size is assessed by cascade impaction at 15 L/min. In Europe, Ph. Eur. 2.9.18 covers aerodynamic assessment of fine particles, with the Andersen impactor calibrated at 28.3 L/min and MMAD and geometric standard deviation derived from a log-probability plot.1,2,4
- The measured distribution depends on the operating flow rate, which is matched to the product and to the compendial method.2,5
- MMAD and geometric standard deviation assume an approximately log-normal distribution, so multimodal aerosols are better described by the full stage data.4,6
- Fine particle fraction and fine particle dose depend on the chosen cut-off diameter and on the dose reference used.1,4
- Aerodynamic results describe the emitted aerosol under the test conditions and are a characterization measurement, not a clinical deposition outcome.3,6
Abbreviated impactor measurement in routine work
Full cascade impaction is data-rich but slow, so routine and quality-control programs sometimes use an abbreviated impactor measurement, which retains only the stages needed to split the aerosol into a coarse and a fine fraction. Paired with efficient data analysis, this reduced approach can track changes in the fine fraction and the impactor-sized mass, while full APSD stays available for characterization and comparison work.6
How ARE Labs measures APSD
ARE Labs measures aerodynamic particle size distribution by cascade impaction, choosing the impactor, flow rate, and staging to match the product and the decision behind the study. Metered dose inhalers, dry powder inhalers, nebulizers, and nasal products each set different actuation and flow conditions, and the method plan records those conditions with the stage masses.1,2,5
The reported output can be the full staged distribution, MMAD and geometric standard deviation, or fine particle fraction and fine particle dose, alongside delivered-dose and breathing-simulation data when the program needs them. As an aerosol characterization service, ARE Labs measures the emitted distribution under defined conditions and states what those measurements can and cannot support.1,4,6