The classification of cleanrooms based on particle levels is one of the most important concepts for any cleanroom project. This classification is not arbitrary; it is based on a rigorous international standard known as ISO 14644-1, which provides a standardized methodology for measuring, evaluating, and certifying air cleanliness in controlled environments. Understanding how these levels are defined and when to apply them to your project is essential to ensuring that your cleanroom complies with regulatory standards and guarantees the quality of your products.
ISO 14644-1 is the internationally recognized standard for classifying air cleanliness in cleanrooms and controlled environments. This standard replaced the former U.S. standard FED STD 209E and now applies to all industries where controlling contamination from airborne particles is critical.
The standard defines specific requirements regarding:
The standard applies only to particles with sizes between 0.1 µm and 5 µm. Ultrafine particles (smaller than 0.1 µm) and macroparticles (larger than 5 µm) are addressed in other standards or specialized descriptors.
The standard is based on a logarithmic mathematical formula that determines the maximum permissible particle concentration for each ISO class at a given particle size:
CN =10N × (0.1/D)²⁰⁸
Where:
For example, for an ISO 5 cleanroom with a particle size of 0.5 µm:
CN =105 × (0.1/0.5)² × 0.08 = 3, 520 particles/m³
This approach ensures mathematical consistency across all classification levels, enabling standardized comparison on a global scale.
Cleanrooms are classified into nine categories, ranging from ISO 1 (the strictest and cleanest) to ISO 9 (the least strict). Here is a detailed overview of each class:
Maximum particle limit (≥0.5 µm): 10 particles/m³
Air change rate: 500–750+ changes per hour
Airflow type: Unidirectional (laminar)
Protective clothing requirements: Full-body coverall, safety goggles, face mask, double or triple layers of gloves, respiratory protection
Typical applications:
ISO 1 is so strict that it is used only in situations where even a single particle could ruin millions of dollars worth of equipment or research.
Maximum particle limit (≥0.5 µm): 100 particles/m³
Air change rate: 480–720 air changes per hour
Airflow type: Unidirectional (laminar)
Typical applications:
Maximum particle limit (≥0.5 µm): 1, 000 particles/m³
Air change rate: 400–600 air changes per hour
Airflow type: Primarily unidirectional
Typical applications:
Maximum particle limit (≥0.5 µm): 10,000 particles/m³
Air change rate: 300–450 air changes per hour
Airflow type: Mixed (unidirectional and turbulent)
Typical applications:
ISO 4 cleanrooms represent the threshold between ultra-critical environments and those that are simply highly controlled.
Maximum particle limit (≥0.5 µm): 3, 520 particles/m³
Air change rate: 240–360 air changes per hour
Airflow type: Mixed, with laminar flow zones
Typical applications:
ISO 5 is currently the most common class for processes where the product is directly exposed to the risk of particulate contamination.
Maximum particle limit (≥0.5 µm): 35, 200 particles/m³
Air change rate: 90–180 air changes per hour
Airflow type: Non-unidirectional (turbulent)
Typical applications:
ISO 6 strikes a balance between strict control and reasonable operating costs, making it a popular choice for support and preparation areas.
Maximum particle limit (≥0.5 µm): 352, 000 particles/m³
Air change rate: 60–120 air changes per hour
Airflow type: Non-unidirectional (turbulent)
Typical applications:
ISO 7 is often used as a buffer zone between the external environment and more stringent areas.
Maximum particle limit (≥0.5 µm): 3, 520,000 particles/m³
Air change rate: 10–25 air changes per hour
Airflow type: Non-unidirectional (turbulent)
Typical applications:
Maximum particle limit (≥0.5 µm): 35, 200,000 particles/m³
Air change rate: <5 air changes per hour
Airflow type: Standard HVAC systems
ISO 9 essentially represents the ambient air of a standard office, with slightly better control. This class is generally not considered a “cleanroom” in the strict sense of the term.
| ISO Class | Particles ≥0.5 µm (per m³) | Particles ≥1 µm (per m³) | Particles ≥5 µm (per m³) | Air changes per hour | Airflow | Typical industry |
|---|---|---|---|---|---|---|
| ISO 1 | 10 | 0.083 | 0.0029 | 500–750+ | Laminar | Ultra-critical aerospace, high-tech semiconductors |
| ISO 2 | 100 | 0.83 | 0.029 | 480–720 | Laminar | Ultra-sensitive pharmaceuticals, cutting-edge research |
| ISO 3 | 1,000 | 8.3 | 0.29 | 400–600 | Laminar | High-precision optics, Research laboratories |
| ISO 4 | 10,000 | 83 | 2.9 | 300–450 | Mixed | Medical devices, critical electronic components |
| ISO 5 | 3,520 | 832 | 29 | 240–360 | Mixed/Laminar | Pharmaceuticals (aseptic), Electronics, Medical, Biotech |
| ISO 6 | 35,200 | 8,320 | 293 | 90–180 | Non-unidirectional | Pharmaceutical packaging, Buffer zones, Electronics |
| ISO 7 | 352,000 | 83,200 | 2,930 | 60–120 | Non-unidirectional | Gowning, Support Areas, Storage |
| ISO 8 | 3,520,000 | 832,000 | 29,300 | 10–25 | Non-unidirectional | Hardware Preparation, Support |
| ISO 9 | 35,200,000 | 8,320,000 | 293,000 | <5 | Standard HVAC | Transition Areas, Offices |
Certifying a cleanroom according to ISO 14644-1 is not a one-time process; it is a series of rigorous procedures conducted by independent third parties or certification bodies.
This phase evaluates whether the cleanroom design is appropriate for its intended use. Engineers and experts review the plans, HVAC specifications, proposed materials, and filtration systems to ensure that the design can theoretically achieve and maintain the required ISO classification.
Once constructed, the cleanroom is inspected to verify that all components—including HEPA/ULPA filters, HVAC systems, construction materials, and seals—are installed correctly according to specifications. No equipment or personnel are present at this stage.
At this stage, the equipment is installed and the systems are operational, but no personnel are present. HEPA filter leak tests, airflow visualization tests (using smoke), and measurements of differential pressure and air change rates are performed.
This is the final test, conducted with personnel and equipment operating under normal conditions. Particle concentration measurements are taken at several locations within the cleanroom using particle counters calibrated to ISO 21501-4.
The reference method for measuring particle concentration is the use of light-scattering airborne particle counters (LSAPCs). These devices use a laser beam to illuminate airborne particles and count those larger than specific sizes.
To be valid, these counters must:
The counters must meet very strict criteria, including:
According to ISO 14644-2 (compliance standard), cleanrooms must be tested regularly to ensure ongoing compliance:
In addition, cleanrooms must be recertified after:
Organizations must maintain detailed records of all tests, certifications, and compliance results to demonstrate performance history and regulatory compliance.
Choosing the right ISO classification for your project is not an arbitrary decision; it depends on several critical factors related to your process, your products, and your regulatory requirements.
Various industries and regions impose minimum classifications:
The stricter the ISO class (the lower the number), the higher the costs:
For example, an ISO 5 room can cost 2 to 3 times as much as an ISO 7 room for the same floor area, due to stricter HVAC requirements.
Many projects use a tiered approach with multiple classes:
This approach optimizes costs while providing adequate protection for critical processes.
Your experience with similar products or your stability studies may reveal which ISO class is truly necessary. For example, some manufacturers find that ISO 6 is sufficient even when ISO 5 is recommended, which can result in substantial cost savings.
Classifying cleanrooms according to ISO 14644-1 is not simply a matter of compliance; it is central to your ability to produce quality products, remain compliant with regulatory requirements, and protect your customers and patients. By understanding how particle levels are defined, measured, and certified, you can make informed decisions regarding the design of your cleanroom.
Consab supports companies across all sectors—including pharmaceuticals, microelectronics, medical, food, aerospace, and biotechnology—in the selection, design, and construction of controlled environments that precisely meet their specific needs.
If you have questions about the appropriate ISO classification for your project or would like to explore design and cost options, check out our ultimate guide to cleanrooms. Additionally, our controlled environment experts are ready to assist you and provide a customized quote for your project.
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