December 2025: ISO 21501-1:2025 Updates Particle Size Testing for Aerosols

Ensuring accuracy, reproducibility, and reliability in measuring airborne particle size has never been more vital. In December 2025, a significant step forward emerges with the adoption of ISO 21501-1:2025, a standard that redefines particle size analysis using advanced light scattering aerosol spectrometers (LSAS). Covering everything from technical definitions to calibration methods and compliance demands, this critical update affects organizations in pharmaceuticals, environmental monitoring, filtration technology, and more. This article unpacks the new standard, explores what’s required for compliance, and analyzes what changes mean for industry professionals.

Overview / Introduction

The testing industry is foundational to ensuring product quality, regulatory compliance, and safe operations across sectors like pharmaceuticals, manufacturing, environmental science, and healthcare. Standardized testing for particle size distribution directly impacts product efficacy, cleanroom environments, environmental safety, and accurate research outcomes.

ISO 21501-1:2025 delivers a globally harmonized framework for measuring size and concentration of particles within aerosols. By establishing requirements for light scattering aerosol spectrometers (LSAS), it pushes for greater measurement precision, comparability, and process control. In this article, you'll discover what’s new, how to comply, and why these developments are crucial for engineers, laboratory managers, and QA specialists.

Detailed Standards Coverage

ISO 21501-1:2025 - Light Scattering Aerosol Spectrometer Requirements

Determination of particle size distribution — Single particle light interaction methods — Part 1: Light scattering aerosol spectrometer

ISO 21501-1:2025 specifies the characteristics and technical requirements for light scattering aerosol spectrometers (LSAS). These instruments analyze the size and number concentration of particles suspended in gas by detecting and evaluating scattered light. The standard covers comprehensive aspects of LSAS design, calibration, performance verification, and operational procedures.

Scope and Application

This standard applies to diverse application areas, including:

• Pharmaceutical characterization of metered dose inhalers (MDI), dry powder inhalers (DPI), and nebulizers
• Quality control in the production of active pharmaceutical ingredients (APIs)
• Cut-off determination for impactors, cyclones, and impingers
• Atmospheric studies: bio-aerosol measurement, composting facilities, street tunnel emissions, and more
• Fractional separation efficiency testing for air filters

The standard is essential for:

• Pharmaceutical and medical device manufacturers
• Environmental laboratories and monitoring agencies
• Filtration technology companies
• Research organizations studying aerosols and air quality

Key Requirements and Specifications

ISO 21501-1:2025 mandates strict requirements for LSAS, including:

• Counting efficiency: Specifies how accurately instruments must count and size particles; counting efficiency varies with particle size and must be within 0.3–0.7 (50 ± 20%) for smaller particles and 0.9–1.1 (100 ± 10%) above twice the lower size limit.
• Size resolution and accuracy: Requires that LSAS distinguish closely spaced particle sizes with a resolution of ≤0.15 (15% of size), and mandates specific calculation methods for sizing accuracy, verified against certified reference particles.
• Sampling flow rate: The volumetric flow entered into the LSAS must be accurate within 5%. Typical values range from 0.5 to 5 liters per minute.
• Effective flow rate through the sensing volume: Defines how gas is moved through the exact region where particles are detected, requiring careful calibration for precise measurement.
• Maximum particle number concentration: Specifies that LSAS must operate correctly up to high concentrations, maintaining coincidence loss (overlapping particle detections) under 10%.

Key Technical Procedures

• Calibration using monodisperse particles: LSAS must be calibrated with reference aerosols of precisely known size and refractive index, commonly monodisperse polystyrene latex (PSL) particles.
• Validation methods: Includes both parallel comparison and generator methods to validate counting efficiency.
• Maintenance of metrological traceability: Instrument calibration and data evaluation require traceability to international measurement standards.
• Annexes: Informative annexes delve into underlying light scattering physics, influence of instrument parameters, sample generation methods, and advanced calibration examples.

Notable Changes From Previous Editions

• Alignment with ISO 21501-4: Creating a more unified measurement methodology across related standards.
• Addition of Annex F: Introduces guidance and test methods for counting efficiency determination.

Practical Implications

Organizations using or specifying LSAS instruments must review their current calibration protocols, update standard operating procedures, and ensure instruments are compliant with the latest requirements. Laboratories should also verify that reference materials and calibration aerosols meet new traceability and accuracy standards.

Key highlights:

• Precisely defines minimum performance characteristics for LSAS across multiple industries
• Includes detailed calibration and validation procedures for robust measurement traceability
• Broadens application scope to pharmaceutical, environmental, and filtration sectors

Access the full standard:View ISO 21501-1:2025 on iTeh Standards

Industry Impact & Compliance

The updates in ISO 21501-1:2025 present measurable advantages for organizations relying on aerosol particle analysis:

• Assured Data Quality: Enhanced calibration and performance requirements result in higher data fidelity and better comparability across equipment and labs.
• Regulatory Alignment: Pharmaceutical firms, environmental labs, and filter manufacturers gain an internationally recognized compliance baseline, easing audits and regulatory submissions.
• Broader Applicability: By incorporating technical advances and harmonizing approaches with other standards, the updated ISO 21501-1 suits a wider range of real-world scenarios, from clean air monitoring in urban tunnels to testing inhalers in drug development.

Compliance Timelines and Considerations

New and existing LSAS installations should be reviewed for conformance. Typically, a transition period allows adaptation of internal documentation, requalification of equipment, and updated training for personnel. Non-compliance could jeopardize data validity, risk regulatory nonconformities, and undermine product or public safety.

Benefits of Adopting This Standard

• Competitive differentiation through best-in-class measurement practices
• Reduced inter-laboratory variability and data disputes
• Streamlined procurement and clearer specifications for new instrumentation

Risks of Non-Compliance

• Critical data inconsistencies, leading to failed audits or product recalls
• Decreased confidence in environmental, pharmaceutical, or quality assurance reporting
• Increased laboratory costs due to retesting and process delays

Technical Insights

Common Technical Requirements

• Precision in Measurement: LSAS instruments must deliver particle size and concentration data within finely specified tolerances, achieved via rigorous calibration and quality-controlled sample preparation.
• Use of Certified Reference Materials: Traceable particles with known sizes (typically PSL) are central to accurate calibration.
• Flow and Sensing Volume Definition: Proper flow calibration—volumetric and effective flow through the sensing volume—is essential for reliable data, especially at high particle concentrations that increase coincidence error risks.
• Signal Processing and Optical Alignment: Instruments depend on robust electronics and precise optical design—deviations can introduce sizing errors or reduced counting efficiency.

Best Practices for Implementation

1. Instrument Qualification: Regularly confirm LSAS compliance with ISO 21501-1:2025 through complete calibration and documentation.
2. Personnel Training: Laboratory staff and quality managers should understand both the operating principles and the revised calibration/validation procedures.
3. Reference Material Management: Use only certified and traceable reference particles for calibration; verify certificates are up to date.
4. Routine Maintenance: Ensure instruments are clean, optics are aligned, and flow meters are routinely calibrated.
5. Documentation: Maintain clear records of instrument validation, calibration, and traceability as required by the standard.

Testing and Certification Considerations

• Accredited Laboratories: Whenever possible, use laboratories accredited for ISO/IEC 17025 to perform instrument calibrations.
• Instrument Documentation: Ensure vendor-supplied LSAS documentation explicitly references conformity with ISO 21501-1:2025.
• Periodic Re-qualification: Implement and document routine re-calibration cycles per manufacturer and standard recommendations.

Conclusion / Next Steps

ISO 21501-1:2025 marks a significant advancement in the standardization of particle size distribution and concentration measurements for aerosols, directly impacting pharmaceutical, environmental, and filtration industries. Professionals should verify that their testing equipment, procedures, and quality systems are updated to conform with the new standard. This proactive approach not only assures compliance but underpins accuracy and trust in product and environmental data.

Key recommendations:

• Review and update standard operating procedures for particle size measurement.
• Verify LSAS instrument compliance and ensure calibrations are traceable to SI units.
• Train laboratory and QA staff on new requirements and validation methods.
• Stay connected with iTeh Standards for up-to-date resources and access to the latest international standards.

Access ISO 21501-1:2025 in its entirety for detailed requirements and calibration procedures:View ISO 21501-1:2025 on iTeh Standards

By staying abreast of such key developments on iTeh Standards, professionals can ensure their organizations lead in compliance, quality, and measurement innovation.