ISO Clean Room Classification
Airborne particulate cleanliness class limits — ISO 14644-1:2015.
Free · no sign-in requiredPurpose
Establish the maximum permitted airborne particle concentration for a cleanroom class before specifying filtration, air change rates or a particle-count acceptance test.
Inputs required
The target ISO class (1 to 9) and the particle sizes of interest. Concentration limits are returned per cubic metre of air for each considered size.
Standards followed
ISO 14644-1:2015, Cleanrooms and associated controlled environments — Classification of air cleanliness by particle concentration.
Class limits for ISO 7
- ≥ 0.1 µm
- 10,000,000 /m³
- ≥ 0.2 µm
- 2,365,144 /m³
- ≥ 0.3 µm
- 1,017,625 /m³
- ≥ 0.5 µm
- 351,676 /m³
- ≥ 1 µm
- 83,176 /m³
- ≥ 5 µm
- 2,925 /m³
ISO 14644-1:2015 (classification), ISO 14644-2:2015 (monitoring), ISO 14644-3 (test methods), ISO 14644-4 (design & construction), EU GMP Annex 1 (2022) for pharmaceutical mapping.
Engineering reference & methodology
The classification equation
Class limits follow the relationship where the maximum permitted concentration equals 10 raised to the class number, multiplied by 0.1 divided by the particle size in micrometres, raised to the power 2.08. The result is particles of that size and larger per cubic metre. Each class step is a factor of ten, so an ISO 7 room permits ten times the particle concentration of an ISO 6 room at the same size.
Considered particle sizes
Classification uses sizes between 0.1 and 5.0 micrometres, and only those sizes for which the standard gives a limit for that class. Very low classes have no meaningful limit at large sizes because the permitted concentration falls below what can be counted reliably, and high classes have no limit at the smallest sizes for the same reason. A class is therefore defined by the size or sizes actually specified for the project, not by every size in the table.
Occupancy state matters
A class means nothing without the state it applies in. As-built means the room is complete but without equipment or people. At-rest means equipment is installed and running but no personnel are present. Operational means the process is running with people present. Personnel are the dominant particle source in most rooms, so a room that meets a class at rest can fail the same class in operation. Always state the class together with the state and the occupancy assumed.
Sampling and verification
Classification is demonstrated by measurement, not by design calculation. The number of sampling locations is taken from the lookup table in Annex A of ISO 14644-1:2015, based on the room area; the square-root-of-area rule from the superseded 1999 edition must not be used. Each location is sampled for long enough to collect a statistically meaningful count, and every location must meet the limit.
Consequence of getting the class wrong
The class drives filtration grade, air change rate, room pressurisation cascade and the size of the air-handling plant, so an error propagates through the entire mechanical design and the room construction budget. Specifying a class tighter than the process needs adds fan power and filter cost for the life of the facility; specifying one too loose risks product contamination, batch rejection and, in regulated pharmaceutical or medical-device production, a failed qualification that cannot be remedied without rebuilding the room.
Relationship to regulatory grades
Pharmaceutical Grades A to D used in GMP guidance are defined with their own limits and occupancy states and are not interchangeable with ISO classes, even where the numbers look similar. Where a project is governed by a GMP annex, classify to that document and use the ISO class only as a cross-reference.
Results are preliminary design guidance. Verify every value against the edition of each standard adopted by your project or authority having jurisdiction, and against manufacturer performance data, before using it in design or procurement.
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