What you should know
- ISO 14644-1 classifies air cleanliness by airborne particle concentration, not by appearance or ACH.
- The classification must identify the room state, particle size or sizes and defined sampling locations.
- Classification, qualification and routine monitoring answer different questions.
- The process and regulatory framework determine which class and room state are appropriate.
What an ISO cleanroom class actually means
ISO 14644-1 provides a method for classifying air cleanliness using the concentration of airborne particles within a defined size range. A lower ISO class number represents a lower permitted particle concentration and therefore a cleaner classification condition.
The class does not by itself define viable contamination limits, temperature, humidity, room pressure, recovery time, air-change rate or how a particular product must be protected. Those requirements come from the user requirement, risk assessment and applicable process or regulatory framework.
| Class | General context only | Important design question |
|---|---|---|
| ISO 5 | Very high control or critical zones | Is unidirectional protection or a local critical-zone strategy required? |
| ISO 6 | High-control production or support | What sources and activities occur in the classified state? |
| ISO 7 | Controlled processing and background spaces | How will airflow, zoning and gowning manage operational sources? |
| ISO 8 | Controlled production and clean support spaces | What product and process risks justify the selected state and particle sizes? |
| ISO 9 | Controlled indoor environment at the less stringent end of the ISO scale | Would another controlled-environment specification describe the need more clearly? |
As-built, at-rest and operational states
| State | Typical condition | What the result tells you |
|---|---|---|
| As-built | Installation complete, services operating, production equipment and people absent | Baseline performance of the completed facility and air system |
| At-rest | Equipment installed and operating as agreed, but operating personnel absent | Performance with installed equipment but without routine people and process activity |
| Operational | Facility operating with the agreed number of people and activities | Performance under the defined real operating condition |
What a classification plan should define
- The room or clean zone boundary and a clear drawing reference.
- The target ISO class and the classification state.
- The particle size or sizes used for classification.
- Sampling locations, location identification and probe position.
- Minimum sample volume, instrument suitability and calibration status.
- Operating conditions, occupancy, equipment status and unusual events during testing.
- Acceptance decision, deviations and the final traceable report.
Classification, qualification and monitoring are not the same
ISO 14644-2 addresses monitoring to provide evidence related to air-cleanliness performance. Monitoring locations and frequency should be selected from risk, process understanding and performance history, not simply copied from the classification map.
| Activity | Primary question | Typical output |
|---|---|---|
| Classification | Does the defined room or zone meet its stated airborne particle class? | ISO class result for a declared state and particle size |
| Qualification | Does the facility and its systems perform as intended across defined tests? | Evidence covering filters, airflow, pressure, recovery and other specified tests |
| Routine monitoring | Does the controlled environment remain in control during use? | Trends, alerts, action limits, investigations and periodic review |
Common classification mistakes
- Naming an ISO class without specifying the room state.
- Treating a recommended ACH range as the classification criterion.
- Testing immediately after a disruption without following the approved stabilization condition.
- Using an unsuitable particle counter, tubing arrangement or sample flow for the intended test.
- Ignoring process-generated particles that appear only in the operational state.
- Using a copied limit table without confirming the applicable standard edition and regulatory context.
- Assuming one successful classification replaces routine monitoring and contamination control.
From user requirement to routine control
Define the process risk
Identify what must be protected, from which contaminants and during which activities.
Set the classification requirement
Choose the class, room state and particle sizes under the applicable framework.
Design the control system
Coordinate airflow, filtration, pressure, architecture, people and material flows.
Commission and qualify
Verify installed systems and execute approved performance tests.
Classify
Perform particle sampling using the current approved method and report the defined state.
Monitor and improve
Trend results, investigate excursions and review the strategy when the process or facility changes.
Frequently asked questions
Does ISO 7 mean the room needs a fixed ACH?
No. ISO 14644-1 classifies airborne particle concentration. The design airflow is selected from the sources, operating state, recovery target, distribution, thermal loads and applicable process requirements, then verified by testing.
Can a room pass at-rest and fail in operation?
Yes. People, materials and process activity can add particles or disrupt airflow. That is why the required state must be stated and why operational monitoring may be important.
Is room pressure part of the ISO class?
No. Pressure differential is a separate contamination-control parameter. It can help protect the room or contain a hazard, but it does not determine the airborne particle class.
Technical sources
Confirm the current controlled edition of every standard and the requirements that apply to your facility.
- ISO 14644-1:2015: Classification of air cleanliness by particle concentrationInternational Organization for Standardization
- ISO 14644-2:2015: Monitoring to provide evidence of cleanroom performanceInternational Organization for Standardization
- ISO 14644-3:2019: Test methodsInternational Organization for Standardization
Prepared by 360S Projects as a practical learning reference. Final design, validation and compliance decisions should be made by competent professionals using current project requirements, standards and manufacturer data.