ISO 21501-1:2025

Overview

ISO 21501-1:2025 - Determination of particle size distribution - Single particle light interaction methods - Part 1: Light scattering aerosol spectrometer (LSAS) defines the characteristics, calibration procedures and validation methods for light scattering aerosol spectrometers. The standard targets instruments used to measure particle size, number concentration and number-based size distributions of particles suspended in a gas using single-particle light scattering techniques. It updates the first edition (2009) and aligns requirements with related parts of the ISO 21501 series.

Key topics and technical requirements

• Instrument scope and principle: Describes the LSAS measurement principle (single-particle light scattering) and the optical particle size concept (light scattering equivalent particle diameter). (Annex A)
• Size range: Defines the applicable measurement range (approximately 0.06 μm to 45 μm) and related metrological concepts.
• Counting efficiency: Specifies how to determine counting efficiency η(x) as a function of particle size, and gives acceptance ranges:
  • Near the lower size limit: counting efficiency between 0.30 and 0.70 (50 ± 20 %).
  • From twice the lower size limit up to 10 μm (or instrument plateau): counting efficiency between 0.90 and 1.10 (100 ± 10 %).
• Lower/upper size limits and size range: Defines lower size limit (counting efficiency 0.5 ± 0.15), upper size limit (0.5 ± 0.2), and the size-of-quantification range.
• Sizing accuracy and size resolution: Requirements for the instrument’s ability to resolve neighboring size classes and to report sizing accuracy.
• Flow and concentration parameters: Calibration and tests for sampling flow rate, effective flow through the sensing volume, maximum particle number concentration and coincidence error handling.
• Test methods and calibration procedures: Detailed test methods (size calibration, counting-efficiency tests via parallel comparison and generator methods, effective flow rate measurement). (Annexes B–F provide standards, effects, sampling guidance and examples.)

Practical applications and users

ISO 21501-1:2025 is applicable to:

• Pharmaceutical testing and QA: MDIs, DPIs, nebulizers, and production control of active agents.
• Aerosol and environmental monitoring: atmospheric aerosols, bio‑aerosols, nebulized droplets, tunnels, stables/composting facilities.
• Industrial quality control: cut-off determination for impactors, cyclones and impingers, and fractional separation efficiency of filters.
• Sectors and users: instrument manufacturers, calibration and metrology laboratories, pharmaceutical R&D and QC teams, environmental monitoring organizations, filter producers, and aerosol researchers.

Related standards

• ISO 21501 series (other parts on single-particle light interaction methods and complementary techniques).
• Refer to Annex B for particle size standards and Annex E for an example calibration with PSL particles.

Keywords: ISO 21501-1:2025, light scattering aerosol spectrometer, LSAS, particle size distribution, aerosol spectrometer calibration, counting efficiency, sizing accuracy, size resolution, pharmaceutical aerosols, environmental aerosol measurement.