--- title: "Particle Count (by size) Sensors & Limit Values | Aethair" description: "Particle count measures how many airborne particles are present per unit volume of air, resolved into size channels rather than converted into a mass…" url: https://aethair.io/parameters/particle_count/ --- Measurements · Particulate Matter # Particle Count (by size) Also known as particle counter, particle number concentration, airborne particle counting Airborne particle number concentration counted in discrete size bins (e.g. 0.3–10 µm); used for cleanrooms and aerosol characterization. [Schedule a call](https://calendly.com/d/ctdd-mkr-v9v/aethair-demo-website) [Configure Particle Count (by size) Monitoring](https://aethair.io/build/?params=particle_count) At a glance particles Particulate Matter 3 ways to measure it - **Aethair PRO** Built-in sensor - **Aethair IAQ** Built-in sensor - **Thiamis** 1 external instrument 3 standards set limits - [USP USP 797](https://aethair.io/standards/usp_797/) - [ISO ISO 14644-1](https://aethair.io/standards/iso_14644_1/) - [TC9.9 ASHRAE TC 9.9](https://aethair.io/standards/ashrae_tc99/) What it is ## What is particle count? Particle count measures how many airborne particles are present per unit volume of air, resolved into size channels rather than converted into a mass concentration. Instead of asking "how many micrograms of dust," it asks "how many particles, and how big," and typically reports counts in size bins such as 0.3, 0.5, 1, 5, and 10 microns. This number-based view is the language of cleanliness: it is how cleanrooms, controlled environments, and precision-manufacturing spaces are specified, classified, and verified, because in those settings a handful of small particles can ruin a product long before they add up to any measurable mass. Where PM metrics report the weight of particulate, particle count reports its population. The two can diverge sharply, since the tiniest particles dominate by number but barely register by mass. ### Health and operational effects In most particle-count applications the concern is contamination rather than health. In semiconductor, pharmaceutical, medical-device, optics, and aerospace manufacturing, a single particle landing in the wrong place can cause a defect, so the count of fine particles is a direct measure of process risk and yield. Particle counting also underpins HEPA filtration validation and filter-leak detection, and, because it resolves the finest particles that mass-based instruments understate, it is increasingly used to study exposure to them. For cleanroom operators the count is not merely informative but contractual: the room's certified class, and its right to run, depend on staying within the per-channel limits. Limits ## Particle Count (by size) Limit Values The published values, by averaging period; each links to the standard that sets it. ISO Class 5 primary engineering control, ≥0.5 µm [**3520 particles/m³** USP 797](https://aethair.io/standards/usp_797/) ISO Class 7 buffer room, ≥0.5 µm [**352000 particles/m³** USP 797](https://aethair.io/standards/usp_797/) ISO Class 5, ≥0.5 µm [**3520 particles/m³** ISO 14644-1](https://aethair.io/standards/iso_14644_1/) ISO Class 6, ≥0.5 µm [**35200 particles/m³** ISO 14644-1](https://aethair.io/standards/iso_14644_1/) ISO Class 7, ≥0.5 µm [**352000 particles/m³** ISO 14644-1](https://aethair.io/standards/iso_14644_1/) ISO Class 8, ≥0.5 µm [**3520000 particles/m³** ISO 14644-1](https://aethair.io/standards/iso_14644_1/) [**3520000 particles/m³** ASHRAE TC 9.9](https://aethair.io/standards/ashrae_tc99/ "the white space, per ISO 14644-1") - **ASHRAE TC 9.9** 3520000 particles/m³: the white space, per ISO 14644-1 Particle count is reported as a number of particles per unit volume, channel by channel. Cleanroom cleanliness is expressed through the ISO 14644-1 classes (Class 1 to 9), with the older US Federal Standard 209E classes still in common use; each class sets a maximum count for each particle size. The applicable class limits appear in the table on this page, and classification requires the counts at every sampling location to meet the limit for the relevant channels: a single point can fail a room. Methods and guidance [ISO 21501-4](https://aethair.io/standards/iso_21501_4/) How it is measured ## How Particle Count (by size) Is Measured **Optical particle counters (OPC)** are the standard instrument: they draw a metered sample of air through a focused laser beam and count and size each particle from the pulse of light it scatters. For classified cleanrooms the counter itself must meet **ISO 21501-4**, the calibration standard that governs sizing accuracy and counting efficiency, and the room is then assessed against the ISO 14644-1 class limits. Accurate sample flow, and isokinetic sampling where the air is moving, are essential so that the metered volume and the particle population are represented correctly. Built into Aethair PRO and Aethair IAQ 2 Measured by the monitor's own sensors, calibrated before it ships: nothing to add. An instrument on Thiamis 1 A third-party instrument connected to a Thiamis gateway, reporting into the same Environet account. ### Placing particle counters Sampling follows a defined scheme rather than convenience: ISO 14644-1 sets the number and position of sampling points according to the area of the cleanroom, spread to represent the whole space at working height. Probes are placed at critical process points and at the face of HEPA filters for leak testing, with isokinetic inlets where airflow is directional. Classification also distinguishes the "at-rest" and "operational" states, so it matters whether people and processes are running when the count is taken. ### Every device and option that measures Particle Count (by size) 3 | Device / option | Type | Base platform | Measures | | - | - | - | - | | **Aethair PRO built-in sensors** | Built-in sensors | Aethair PRO | Particle Count (by size) [Dew Point](https://aethair.io/parameters/dew_point/) [Heat Index](https://aethair.io/parameters/heat_index/) [Relative Humidity](https://aethair.io/parameters/humidity/) [Illuminance / Light](https://aethair.io/parameters/light/) [PM1](https://aethair.io/parameters/pm1/) [PM10](https://aethair.io/parameters/pm10/) | | **Aethair IAQ built-in sensors** | Built-in sensors | Aethair IAQ | Particle Count (by size) [Formaldehyde (CH₂O)](https://aethair.io/parameters/ch2o/) [Carbon Dioxide (CO₂)](https://aethair.io/parameters/co2/) [Dew Point](https://aethair.io/parameters/dew_point/) [Heat Index](https://aethair.io/parameters/heat_index/) [Relative Humidity](https://aethair.io/parameters/humidity/) [PM1](https://aethair.io/parameters/pm1/) | | [Particles Plus](https://aethair.io/sensors/particles-plus-particulate-dust-monitors/) · Particles Plus 5301, Particles Plus 5301-AQM1, Particles Plus 5301-AQM2… | Particulate & Dust Monitors | Thiamis | Particle Count (by size) [Carbon Dioxide (CO₂)](https://aethair.io/parameters/co2/) [Relative Humidity](https://aethair.io/parameters/humidity/) [Barometric Pressure](https://aethair.io/parameters/pressure/) [Air Temperature](https://aethair.io/parameters/temperature/) [VOC (Total)](https://aethair.io/parameters/voc/) | [Configure Particle Count (by size) Monitoring](https://aethair.io/build/?params=particle_count) Where it is monitored ## Solutions That Monitor Particle Count (by size) Each one links to the solution page showing what that industry measures it for. ### [Cleanrooms, Labs & Manufacturing](https://aethair.io/solutions/cleanroom/#measure-particle_count) Core parameter Airborne particle counting by size is the defining measurement of cleanroom classification, the basis on which spaces are certified to ISO 14644 classes for pharma, electronics, and other critical manufacturing. ### [Healthcare Air Monitoring](https://aethair.io/solutions/healthcare/) Often added Clinical air quality, pressurization, and patient safety Article Library ## Reading on Particle Count (by size) ### [Hospital Air Quality Monitoring: Standards by Space](https://aethair.io/resources/healthcare-air-quality-monitoring/) Hospital air quality standards from ASHRAE 170 and The Joint Commission explained, plus what to measure in operating rooms, isolation rooms, and labs. ### [ASHRAE 170 Requirements and Which Edition Applies to You](https://aethair.io/resources/ashrae-170-requirements/) ASHRAE 170 requirements explained: what the standard sets, which edition your facility is held to, what changed in 2025, and how each value gets verified. [All articles](https://aethair.io/resources/) Related ## More in Particulate Matter [Total PM / Mass Conc.](https://aethair.io/parameters/pm_total/) [PM2.5](https://aethair.io/parameters/pm2_5/) [PM10](https://aethair.io/parameters/pm10/) [PM1](https://aethair.io/parameters/pm1/) [PM4 (Respirable)](https://aethair.io/parameters/pm4/) [Sample Flow](https://aethair.io/parameters/sample_flow/) [Respirable Crystalline Silica](https://aethair.io/parameters/silica/) [Black Carbon](https://aethair.io/parameters/black_carbon/) Questions ## Particle Count (by size) FAQ What is the difference between particle count and PM mass concentration? Particle count reports the number of particles per volume, split by size, while PM reports their combined weight in µg/m³. The two answer different questions: count is about how many particles there are, which matters for contamination, whereas mass is about how much material, which matters for health exposure. Because fine particles are light, a space can have a high count but a low mass, or the reverse. What is ISO 21501-4, and how do cleanroom classes work? ISO 21501-4 is the calibration standard that optical particle counters must meet to size and count particles reliably. Cleanroom cleanliness is then classified under ISO 14644-1, which sets a maximum particle count per size channel for each class from 1 to 9. The counts are taken at a defined set of locations, and every one must stay within the class limit. Why count particles by size rather than in total? Different processes care about different sizes: a sub-micron particle that is harmless in one setting is a fatal defect in a semiconductor fab. Resolving the count into channels shows which sizes are present and lets each be compared against its own class limit. A single total count would hide that detail. How do I monitor particle count? The Aethair family includes built-in particle counting by size channel for continuous, size-resolved monitoring of a space. For formal cleanroom classification to ISO 14644-1, a Thiamis gateway can carry an ISO 21501-4 certified optical particle counter with the required sampling regime. Use the configurator to combine particle count with PM and the other parameters your environment needs.