Particle size is a measurement choice
- Particle size distribution
- A particle size distribution describes how many particles, or how much particle mass or volume, falls into each size interval. For aerosols, this distribution is more useful than a single average. Different size fractions can affect inhalation, deposition, filtration, or sensor response.5,6,7
What a particle-size result means depends on the measurement principle. Laser diffraction reports a distribution based on light-scattering behavior and an optical model. Health-related air sampling often uses inhalable, thoracic, and respirable particle-size fractions. Regulatory inhalation studies may focus on aerodynamic particle size because particle motion in air affects how emitted material separates in an impactor.3,5,6
Two instruments can therefore report different values for the same aerosol without either result being wrong. Each method answers a specific question: how the particle scatters light, how it moves in an air stream, how it behaves in an electrical mobility field, or where collected mass lands in a staged impactor.5,8
Diameter basis changes the answer
- Aerodynamic diameter
- Aerodynamic diameter is the diameter of a spherical particle with a density of 1 g/cm³ that has the same terminal settling velocity in air as the particle being measured. It describes settling behavior rather than a directly measured geometric width. Particles with different shapes or densities can therefore have the same aerodynamic diameter.1
| Basis | What it represents | Useful when |
|---|---|---|
| Aerodynamic diameter | Diameter of a unit-density sphere with the same terminal settling velocity in air | Inhalation products, cascade impactors, and health-related sampling fractions |
| Optical-equivalent diameter | Light-scattering response translated through an optical model | Sprays, powders, aerosols, suspensions, and rapid formulation screens |
| Mobility diameter | Particle motion in an electric field after charge conditioning | Fine and ultrafine aerosol measurements where number concentration changes quickly |
| Geometric or image-based diameter | A visible dimension from microscopy or image analysis | Morphology checks, agglomerate review, and particle-shape context |
Laser diffraction can cover many two-phase systems, including powders, sprays, aerosols, suspensions, emulsions, and gas bubbles in liquids. ISO 13320 also describes a limitation: for non-spherical particles, the reported distribution is model-based and can differ from distributions produced by sedimentation, sieving, or other physical principles.5
Read the distribution basis before comparing medians
| Reported result | What the distribution weights | How to read it |
|---|---|---|
| Number distribution | Each measured particle contributes equally to the count | Many small particles can dominate the count even when larger particles account for most of the volume |
| Volume distribution: Dv10, Dv50, Dv90 | Particle volume in each size interval | 10%, 50%, or 90% of the measured particle volume lies below the corresponding diameter; Dv50 is a median, not an arithmetic mean |
| Mass distribution | Particle mass in each size interval | A volume distribution is equivalent to a mass distribution only when particle density is uniform |
| Mass median aerodynamic diameter: MMAD | Mass distributed by aerodynamic diameter | Half the measured mass lies below the MMAD and half above it; this is a different diameter basis from an optical volume median |
Why aerosol programs care about distribution
For orally inhaled and nasal products, aerodynamic particle-size distribution can be part of the quality and performance evidence. FDA inhalation guidance discusses MDI and DPI product quality considerations. FDA nasal aerosol guidance discusses cascade impactor particle or droplet size distribution and, for certain suspension products, drug particle-size distribution by microscopy.3,4,10
- Inhalation and nasal drug-product work may need staged mass, MMAD, GSD, fine-particle dose, or microscopy context depending on product type and study purpose.3,4,10
- Air-quality and exposure work often separates particles by health-related size fractions, including inhalable, thoracic, respirable, PM10, and PM2.5 frames.6,7,9
- Filtration and air-cleaning studies use size-resolved particles because removal, deposition, and sensor response can shift across particle-size intervals.6,9
- Consumer spray and device-development programs use particle-size data to compare formulation, actuator, orientation, plume behavior, and exposure-relevant fractions.5,8
Method selection follows the decision
Early in development, the priority is often a quick comparison. Optical particle sizing, laser diffraction, or real-time aerosol instruments can show whether a change in formulation, actuator, device setting, or chamber condition shifted the distribution enough to justify more controlled work.5,8
Screening and troubleshooting
Screening studies can show whether one nozzle generates a smaller droplet spectrum or a device setting changes the fine fraction. They can also check whether chamber background interferes with the measurement. These studies still need documented flow, timing, background, and instrument checks so the result can be repeated.5,8
Review-ready studies
Studies intended for review need a clearly defined method. Inhalation products may require compendial or regulator-aligned impactor work, while workplace or indoor-air programs may require sampling conventions that match the exposure question. In both cases, method records are part of the evidence.3,6,8,10
What to define before requesting testing
- Define the product or aerosol source, including device geometry, formulation state, actuation profile, chamber condition, and expected size range.3,5
- State the sizing basis needed for the decision, such as aerodynamic mass, optical volume distribution, mobility number distribution, or microscopy-based morphology.4,5,6
- Identify the report outputs that matter, such as full distributions, percent below a cutoff, MMAD, GSD, Dv10, Dv50, Dv90, or concentration-time data.3,5
- Decide whether the study must support development screening, product comparison, exposure review, quality documentation, or a regulatory submission.3,8
How ARE Labs connects size data to study design
ARE Labs starts with the decision the particle-size study needs to support, then selects the method. A development screen might use laser diffraction or real-time aerosol sizing. A product-quality package might need cascade impactor collection and recovery suitable for assay. An exposure or indoor-air question might call for chamber control, background subtraction, size fractions, and sampling records.3,5,6,8
When comparing devices, keep the sizing basis, sampling position, actuation timing, and sample handling consistent. Switching from a number distribution to a volume distribution can change the reported median even if the aerosol itself has not changed. Before drawing a conclusion, the report should distinguish a change in measurement from a change in the product.2,5,8