Particle size is a measurement choice
- Particle size distribution
- Particle size distribution describes how particle count, mass, or volume is distributed among size intervals. In aerosol work, the full distribution is usually more useful than a single average because different size fractions can affect inhalation, deposition, filtration, and sensor response.3,4,5
The meaning of a particle-size result depends on the measurement principle. Laser diffraction derives a distribution from light-scattering behavior and an optical model. Health-related air sampling often uses inhalable, thoracic, and respirable fractions. Regulatory inhalation studies may focus on aerodynamic particle size because particle motion in air affects how emitted material separates within an impactor.1,3,4
Two instruments can therefore report different values for the same aerosol without either result being wrong. Each method captures a particular behavior: how a particle scatters light, moves in an air stream, travels through an electric field after charge conditioning, or deposits as collected mass 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 be used with many two-phase systems, including powders, sprays, aerosols, suspensions, emulsions, and gas bubbles in liquids. ISO 13320 also defines a method boundary. For non-spherical particles, the reported distribution is model-based and may differ from distributions measured 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 form part of the quality and performance evidence. FDA inhalation guidance addresses product quality considerations for MDI and DPI products. FDA nasal aerosol guidance also discusses cascade impactor particle or droplet size distribution and, for certain suspension products, drug particle-size distribution by microscopy.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 calls for quick comparisons. 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 work well for comparative questions. Does one nozzle produce a smaller droplet spectrum? Does a device setting change the fine fraction? Is the chamber background interfering with the measurement? Even at the screening stage, flow, timing, background conditions, and instrument checks need to be documented so the result can be repeated.3,6
Review-ready studies
Review-ready studies 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 either case, 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 starts with the decision the data need to support, then selects the particle-size method. A development screen might use laser diffraction or real-time aerosol sizing. A product-quality package may call for cascade impactor collection and assay-ready recovery. An exposure or indoor-air question may require chamber control, background subtraction, size fractions, and sampling records.1,3,4,6
The practical goal is a report that states what was measured, why the sizing basis was selected, which controls were used, and what the data can and cannot support. That context helps teams compare devices, troubleshoot aerosol behavior, plan follow-up testing, and avoid overstating a particle-size result.3,6