What Is ISO Classification? Cleanroom Classes Explained

The practical answer: an ISO cleanroom class is a measured particle-cleanliness designation, not an all-purpose score for a room. To interpret it correctly, check the particle size, air sample results, room condition during testing, and the standard used.

What Is ISO Classification?

In the cleanroom context, ISO classification is a system for describing how clean the air is with respect to airborne particles. A class is determined by measuring particle concentrations and assessing whether they meet the relevant limits in a specified standard.

That makes the class a defined technical result—not a general rating of room quality, cleanliness, sterility, or suitability for every process. Two rooms with the same class may still differ in airflow, pressure, temperature, humidity, equipment, and operating procedures.

The phrase “ISO classification” can refer to different classification systems in other fields. In this guide, we use it specifically for cleanroom airborne-particle classification under ISO 14644-1.

What Does ISO Stand For?

ISO is the International Organization for Standardization. It develops international standards that help organizations apply consistent technical methods and requirements across markets.

Important distinction: ISO publishes the classification standard; it does not inspect, classify, or certify each individual cleanroom. A facility arranges the appropriate measurements and documents the results against the applicable requirements.

What Is ISO 14644-1?

ISO 14644-1 is the international standard used to classify cleanrooms and clean zones by airborne particle concentration. Its classification limits apply to specified particle sizes and are assessed using samples taken at defined locations in the space.

The standard sets particle concentration limits. Facility design and ongoing controls—such as filtration, airflow management, gowning, cleaning, and work practices—are what help a facility achieve and maintain its target class.

Engineering note

When reviewing a classification report, look for the edition of the standard, the particle sizes assessed, the sampling locations, the test condition, and the reported results—not only the class label.

How Does the ISO Cleanroom Classification System Work?

The cleanroom scale runs from ISO Class 1, the cleanest, through ISO Class 9, the least clean. As the class number increases, the permitted concentration of airborne particles increases. Counts are expressed per cubic meter of air and are evaluated at the particle sizes specified for classification.

The table below gives the maximum concentration limits in particles per cubic meter for the particle sizes shown in ISO 14644-1:2015, Table 1. An em dash indicates that the table does not specify a concentration limit for that class and size combination; it does not mean “zero particles.”

Maximum airborne particle concentration limits (particles/m³), by particle size.
ISO Class	≥ 0.1 µm	≥ 0.2 µm	≥ 0.3 µm	≥ 0.5 µm	≥ 1.0 µm	≥ 5.0 µm
ISO 1	10	2	—	—	—	—
ISO 2	100	24	10	4	—	—
ISO 3	1,000	237	102	35	8	—
ISO 4	10,000	2,370	1,020	352	83	—
ISO 5	100,000	23,700	10,200	3,520	832	29
ISO 6	1,000,000	237,000	102,000	35,200	8,320	293
ISO 7	—	—	—	352,000	83,200	2,930
ISO 8	—	—	—	3,520,000	832,000	29,300
ISO 9	—	—	—	35,200,000	8,320,000	293,000

What Do ISO Classification Codes Mean?

A designation such as “ISO Class 7” identifies the particle-cleanliness class being claimed under ISO 14644-1. The class number indicates the allowed airborne particle concentrations for the specified particle sizes; it does not, by itself, describe every room requirement or operating parameter.

For example, a class designation alone does not set the room’s temperature, humidity, pressure relationship, or process-specific microbial controls. Those requirements must be defined separately.

ISO Cleanroom Classes 1–9 at a Glance

The class descriptions below are general orientation, not universal industry rules. The required class depends on the product, process, applicable regulation, and the way work is performed. A facility may also use a cleaner zone around a critical operation within a less-clean supporting room.

ISO Classes 1–3

These are the most stringent particle-control environments. They are associated with specialized processes where very low airborne particle concentrations are essential, such as selected advanced semiconductor or precision manufacturing operations. The engineering, monitoring, and operating controls required are correspondingly demanding.

ISO Class 4

ISO Class 4 supports high-cleanliness requirements in sensitive manufacturing and related controlled processes. Its suitability should be confirmed against the actual process risk and product requirements rather than selected simply because a tighter class sounds safer.

ISO Class 5

ISO Class 5 is commonly associated with critical work areas, including some semiconductor production and pharmaceutical aseptic processing environments. The class describes airborne particle limits; pharmaceutical facilities must also address applicable regulatory, microbiological, process, and qualification requirements.

ISO Class 6

ISO Class 6 can be used in controlled manufacturing and as a supporting environment around a cleaner zone. It may form part of a room-to-room cleanliness strategy where process steps, material movement, and personnel access are carefully planned.

ISO Class 7

ISO Class 7 is used for some supporting rooms, assembly areas, and selected pharmaceutical or medical-device processes. Whether it is sufficient depends on product exposure, process controls, and the applicable customer or regulatory requirements.

ISO Class 8

ISO Class 8 is a less stringent controlled environment that may suit some support, preparation, or packaging areas. It is not a substitute for assessing product protection needs or any specific process standard.

ISO Class 9

ISO Class 9 is the least stringent class in the ISO 1–9 scale. Its particle limits are broadly comparable to some typical indoor environments, but ordinary room air varies considerably. The comparison is not a guarantee that a given office or factory room meets ISO Class 9.

What Do Class 100 and Class 1,000 Cleanroom Mean?

“Class 100” and “Class 1,000” are legacy U.S. cleanroom labels from FED-STD-209E. They refer to maximum airborne particle counts per cubic foot at a stated particle size, rather than the particles-per-cubic-meter convention used in ISO class labels.

In common comparisons based on particles at or above 0.5 micrometers, Class 100 is approximately ISO Class 5, and Class 1,000 is approximately ISO Class 6. The relationship depends on the particle size and the exact basis of comparison, so an old room label should not replace a current classification report.

Legacy terminology

FED-STD-209E has been withdrawn. It remains familiar in conversation and older facility documents, but current requirements should be checked against the applicable ISO standard, regulation, or customer specification.

ISO 14644-1 vs. FED-STD-209E

Both systems describe airborne particle cleanliness, but their labels and units differ. ISO 14644-1 uses ISO Classes 1–9 and particle concentrations per cubic meter. FED-STD-209E used class numbers such as 100 or 1,000 and counts per cubic foot. Comparisons commonly use a 0.5 µm particle-size basis.

Commonly referenced approximate equivalencies, compared at ≥ 0.5 µm.
FED-STD-209E label	Approximate ISO class	ISO limit at ≥ 0.5 µm (particles/m³)
Class 1	ISO Class 3	35
Class 10	ISO Class 4	352
Class 100	ISO Class 5	3,520
Class 1,000	ISO Class 6	35,200
Class 10,000	ISO Class 7	352,000
Class 100,000	ISO Class 8	3,520,000

Treat this as a practical cross-reference, not a universal conversion. The standards are not perfectly interchangeable in every context; particle-size thresholds, reporting conventions, sampling, and specified test conditions matter. For a project, state the standard and class explicitly in the specification.

How Is Cleanroom Classification Measured?

Classification measurements use an airborne particle counter to sample air and report particle concentrations by size. The applicable standard guides how many sampling locations are needed and how the locations are selected for the cleanroom or clean zone. The test plan and instrument setup should follow the applicable standard and facility requirements.

A classification survey is different from routine environmental monitoring. Classification determines whether measured particle concentrations meet a stated class under defined conditions. Routine monitoring is part of the facility’s ongoing control program and may use different locations, frequencies, alert levels, or additional measurements.

At-Rest vs. Operational Conditions

The room condition during sampling affects what a result means. Reports should identify the state in which measurements were taken:

1
As-built
The installation is complete, services are connected and functioning, but production equipment, materials, and personnel are not present.
2
At-rest
The installation is complete, equipment is installed and operating as agreed, and personnel are not present.
3
Operational
The room is functioning in its specified operating mode, with the agreed equipment and personnel performing normal work.

These states represent different operating realities. An at-rest result does not automatically demonstrate that a room remains within its target during production, so the intended test condition should be chosen and clearly reported.

Particle Size and Count Limits

A class limit only has meaning when read together with its particle size. For instance, a limit at 0.5 µm is not interchangeable with a limit at 5.0 µm. Classification results count airborne particles that meet or exceed the stated size threshold, so a report should name the size channels assessed.

What particle classification does not tell you

Airborne particle classification does not, by itself, measure viable microorganisms, prove product sterility, or establish that a process is microbiologically controlled. Those questions require the appropriate additional controls and monitoring for the application.

Cleanroom Classification vs. Certification

Classification determines whether airborne particle concentrations meet a stated ISO class under the specified test conditions.

Certification or qualification is often used for a broader assessment of room performance. Depending on the facility and requirements, that assessment may include particle classification as well as airflow, filter integrity, pressure relationships, or other performance tests.

Terminology and required tests vary by regulation, industry, and facility. We recommend defining the acceptance criteria and required test scope in project documents rather than assuming that the word “certified” means the same test package everywhere.

What Affects a Cleanroom’s ISO Class?

The measured class is influenced by the room’s engineering and by how people and processes use it. A strong design can be undermined by uncontrolled sources of contamination or inconsistent work practices.

Filtration and airflow

HEPA or ULPA filtration, airflow patterns, and air changes influence how particles are removed, diluted, or carried away from critical work. The design must suit the room layout and process.

Pressure relationships

Room pressure relationships help manage air movement between spaces. They should be considered alongside room function, material flow, personnel routes, and any containment needs.

People, garments, and work

People and their activities can introduce particles. Gowning, suitable protective garments, training, movement, and consistent work practices all contribute to contamination control.

Equipment, materials, and cleaning

Equipment, incoming materials, surfaces, cleaning supplies, and maintenance can be contamination sources. Material transfer and cleaning procedures should match the controlled environment and process.

Temperature and humidity may be important operational controls for people, materials, or the process, but they do not define the ISO airborne-particle class. Specify and control them separately where the application requires it.

How to Choose the Right ISO Cleanroom Class

Start with the process and its acceptance requirements, not with a class label copied from another facility. A practical selection sequence helps balance contamination risk, compliance, room design, and long-term operating cost.

1
Define sensitivity and process risk
Identify which products or operations are exposed, how contamination could affect them, and what level of control the process needs.
2
Check external requirements
Review applicable regulations, industry standards, customer specifications, and the target market’s documentation expectations.
3
Set the class for critical work
Focus on the area where the sensitive operation occurs. Where appropriate, consider a cleaner zone within a less-clean room instead of applying the tightest target everywhere.
4
Check real operating conditions
Account for occupancy, equipment, workflows, gowning, material transfers, room pressure relationships, and the condition in which the room will be classified.
5
Plan for lifecycle cost and maintenance
Consider the full operating and supply picture, including cleaning, consumables, protective clothing, equipment, maintenance, and ongoing verification.

For regulated or technically demanding projects, consult the relevant industry standards and qualified cleanroom professionals. At CleanroomLink, we work with customers to clarify application, market, and sourcing needs, then help coordinate compatible cleanroom protection products and facility solutions. We do not treat product sourcing as a substitute for engineering design or regulatory review.

Common Cleanroom Applications by ISO Class

Cleanroom classes are used across semiconductor and electronics manufacturing, medical devices, pharmaceuticals and biotechnology, laboratories, aerospace, and precision manufacturing. The examples below are illustrative: they do not prescribe a class for an entire industry.

For example, a pharmaceutical facility may have different classifications for separate operations and supporting spaces, while a semiconductor manufacturer may set controls around a particularly sensitive process step. The required class depends on what is manufactured, how the process is controlled, and which regulations or customer requirements apply.

Frequently Asked Questions About ISO Classification

What is an ISO classification?

For a cleanroom, an ISO classification describes airborne particle cleanliness by comparing measured particle concentrations with the limits in ISO 14644-1 for specified particle sizes.

What does ISO stand for?

ISO is the International Organization for Standardization. It develops international standards; it does not classify or certify individual cleanrooms.

What do Class 100 and Class 1,000 cleanroom mean?

They are legacy FED-STD-209E labels based on particle counts per cubic foot. Common approximate comparisons at 0.5 µm are Class 100 to ISO Class 5 and Class 1,000 to ISO Class 6; the comparison depends on particle size and basis.

What are the ISO classification codes?

Cleanroom classes run from ISO Class 1 through ISO Class 9. The number identifies a particle-cleanliness class under ISO 14644-1; a higher class number allows a higher particle concentration.

Which ISO cleanroom class is the cleanest?

ISO Class 1 is the cleanest class on the ISO 1–9 scale.

Is ISO classification the same as cleanroom certification?

No. Classification assesses whether airborne particle concentrations meet a stated class under defined conditions. Certification or qualification may involve a broader set of room-performance tests, depending on the applicable requirements.

How often should a cleanroom be reclassified?

Follow applicable standards, regulations, customer requirements, and the facility’s risk-based monitoring and reclassification program. The appropriate interval depends on the room, process, and governing requirements, so there is no single schedule that applies to every facility.

What Is ISO Classification? Cleanroom Classes Explained