Particle and PM counters

Particle counters are instruments dedicated to measuring the concentration and size distribution of airborne particles, used for monitoring cleanrooms and controlled pharmaceutical environments, ambient air quality monitoring, HEPA/ULPA filter inspection and the certification of contamination-controlled chambers. They work on the laser scattering principle: a focused laser beam passes through a sampled air volume; particles crossing the beam scatter the light, generating a light pulse detected by a photodetector. The amplitude of the pulse is proportional to the particle size; the number of pulses per volume gives the concentration.

Types of counter

Particle counters differ in: sampling flow rate (0.1 CFM · 2.83 L/min for rapid spot measurements; 1 CFM · 28.3 L/min for measurements over significant volumes); minimum detectable particle size (0.1 µm for critical pharmaceutical environments, 0.3 µm for standard industrial applications); portable or benchtop (for manual inspections) versus fixed remote units (for continuous cloud-based monitoring with alarms). Airborne models (for atmospheric air) and liquid models (for pharmaceutical and injectable liquids) are intended for different applications.

Cleanrooms and ISO 14644 classification

ISO 14644-1 classifies cleanrooms according to the maximum permitted concentration of particles per cubic meter: ISO class 1 (≤ 10 particles ≥ 0.1 µm/m³); ISO 3 (≤ 1,000); ISO 5 (≤ 3,520, formerly Class 100 in the US standard); ISO 7 (≤ 352,000, formerly Class 10,000); ISO 8 (≤ 3,520,000, formerly Class 100,000); ISO 9 (≤ 35,200,000, filtered ambient air). Certification requires measurements following the ISO 14644-3 procedure, with sampling on regular grids across the whole room, in the “as built”, “at rest” and “in operation” states. The counters used must comply with ISO 21501-4.

HEPA filter integrity testing

One of the most important applications is integrity testing of the HEPA/ULPA filters installed in cleanrooms: the PAO test (using PAO oil aerosol or equivalents) involves injecting an aerosol of known concentration upstream of the filter and scanning downstream with a particle counter to identify any leaks (localized escapes of particles). The scan is performed by slowly moving the counter probe along a grid of passes over the entire filter surface. Compliance is required for the certification of pharmaceutical GMP rooms (EU Annex 1) and is standard practice in IQ/OQ/PQ qualifications.

Industrial and environmental applications

Beyond cleanrooms, particle counters are used for: monitoring workplace air quality (checking exposure to dust from machining, pharmaceutical products or ceramics); monitoring industrial environments with controlled contamination (electronics, semiconductor and microelectronics production lines); inspecting HVAC ventilation and filtration systems; outdoor environmental monitoring of PM2.5 and PM10 (for scientific research and smart cities); advanced IAQ applications (indoor air quality in buildings); and monitoring emissions from industrial processes.

Reference standards

The main standards are: ISO 14644 (parts 1-12 – classification, testing and monitoring of cleanrooms); ISO 21501-4 (requirements for optical particle counters); EU GMP Annex 1 (pharmaceutical, microbiological and particulate contamination); IEST RP-CC006 (USA, recommended practices for cleanrooms); UNI EN 12469 (biological safety cabinets). Periodic calibration of counters follows ISO 21501-4 using certified reference particulate of known size (PSL – polystyrene latex spheres). The interval is annual for regulated applications and six-monthly for intensive critical use.

Selection criteria

The choice of particle counter depends on: the ISO 14644 class of the environment to be certified (minimum particle size, sampling flow rate); the application (cleanroom, IAQ, environmental, filters); portable versus fixed remote use (for continuous monitoring); compliance with 21 CFR Part 11 (for FDA pharmaceutical use); integration with cloud-based remote monitoring systems; management software compliant with GMP procedures; and an ISO 17025 accredited calibration service from the supplier.