Particle size is a measurement choice
- Particle size distribution
- Particle size distribution describes how many particles, or how much particle mass or volume, falls into each size interval. In aerosol work, that distribution is more useful than a single average because different fractions can drive inhalation, deposition, filtration, or sensor response.3,4,5
A particle-size result is tied to the measurement principle. Laser diffraction reports a distribution from light-scattering behavior and an optical model. Health-related air sampling often uses particle-size fractions such as inhalable, thoracic, and respirable fractions. Regulatory inhalation studies may focus on aerodynamic particle size because particle motion in air affects how emitted material separates in an impactor.1,3,4
That is why two instruments can report different values for the same aerosol without either result being wrong. Each method asks a narrower question: how the particle scatters light, how it moves in an air stream, how it behaves under an electrical mobility field, or where collected mass lands in a staged impactor.3,6
Diameter basis changes the answer
| Basis | What it represents | Useful when |
|---|---|---|
| Aerodynamic diameter | Motion of a particle in air compared with a unit-density sphere | 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 makes the method boundary visible: for non-spherical particles, the reported distribution is model-based and can differ from distributions produced by sedimentation, sieving, or other physical principles.3
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, while FDA nasal aerosol guidance discusses cascade impactor particle or droplet size distribution and drug particle-size distribution by microscopy for certain suspension products.1,2,8
- 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.1,2,8
- Air-quality and exposure work often separates particles by health-related size fractions, including inhalable, thoracic, respirable, PM10, and PM2.5 frames.4,5,7
- Filtration and air-cleaning studies use size-resolved particles because removal, deposition, and sensor response can shift across particle-size intervals.4,7
- Consumer spray and device-development programs use particle-size data to compare formulation, actuator, orientation, plume behavior, and exposure-relevant fractions.3,6
Method selection follows the decision
Early development often needs speed and comparison. Optical particle sizing, laser diffraction, or real-time aerosol instruments can show whether a formulation, actuator, device setting, or chamber condition changed the distribution enough to justify more controlled work.3,6
Screening and troubleshooting
Screening studies are useful when the question is comparative: does one nozzle generate a smaller droplet spectrum, does a device setting change the fine fraction, or does a chamber background interfere with the measurement? These studies still need documented flow, timing, background, and instrument checks so the result can be repeated.3,6
Review-ready studies
Review-ready studies need a clearer method frame. 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, not administrative extras.1,4,6,8
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.1,3
- State the sizing basis needed for the decision, such as aerodynamic mass, optical volume distribution, mobility number distribution, or microscopy-based morphology.2,3,4
- Identify the report outputs that matter, such as full distributions, percent below a cutoff, MMAD, GSD, Dv10, Dv50, Dv90, or concentration-time data.1,3
- Decide whether the study must support development screening, product comparison, exposure review, quality documentation, or a regulatory submission.1,6
How ARE Labs connects size data to study design
ARE Labs scopes particle-size studies from the decision back to the method. A development screen might use laser diffraction or real-time aerosol sizing. A product-quality package might need cascade impactor collection and assay-ready recovery. An exposure or indoor-air question might need chamber control, background subtraction, size fractions, and sampling records.1,3,4,6
The practical goal is a report that explains what was measured, why that sizing basis was selected, what controls were used, and what the data can and cannot support. That distinction helps teams compare devices, troubleshoot aerosols, plan follow-up testing, and avoid overstating a particle-size result.3,6