A cleanroom is a controlled environment engineered to limit the concentration of airborne particles, along with regulated temperature, humidity, and air pressure. Air is continuously filtered through HEPA or ULPA filters and exchanged many times per hour to keep contamination below a defined limit. Cleanrooms are classified by the ISO 14644-1 standard, which ranks them from ISO 1 (cleanest) to ISO 9 based on the maximum allowable particle count per cubic meter of air.
How a cleanroom works
A cleanroom controls contamination by combining high-efficiency air filtration, high air-exchange rates, and directional airflow. Supply air passes through HEPA filters, which capture at least 99.97% of particles at 0.3 microns, or ULPA filters, which capture 99.999% down to 0.12 microns. The filtered air floods the room and continuously dilutes and sweeps away particles generated by people, equipment, and processes.
The number of times the entire volume of air in a room is replaced each hour is called the air change rate. Cleaner classifications require far higher rates: a less-critical ISO 8 space may use roughly 10 to 25 air changes per hour, while a critical ISO 5 zone can require several hundred. Airflow patterns also matter. Turbulent (non-unidirectional) airflow mixes and dilutes contamination and is common in lower-grade rooms, while laminar (unidirectional) airflow moves in parallel streams that sweep particles away in one direction, protecting exposed product in the most critical zones.
Rooms are also held under controlled differential pressure. A pressure cascade keeps cleaner rooms at higher pressure than adjacent, dirtier spaces, so air always flows outward from clean to less-clean and unfiltered air cannot migrate inward. Personnel enter through gowning airlocks and wear controlled garments — coveralls, hoods, gloves, and boot covers — because people are the single largest source of particles and microbial contamination.
Cleanroom classification chart (ISO 14644-1)
ISO 14644-1 defines cleanliness by the maximum number of particles of a given size permitted per cubic meter of air. The table below summarizes the most common classes used in life-science manufacturing, the maximum particles at or above 0.5 microns per cubic meter, the roughly equivalent legacy US FED-STD-209E class, and typical uses.
| ISO class | Max particles ≥0.5 µm per m³ | ~FED-STD-209E | Typical use |
|---|---|---|---|
| ISO 5 | 3,520 | Class 100 | Aseptic filling, sterile product exposure, laminar-flow zones |
| ISO 6 | 35,200 | Class 1,000 | Support of aseptic areas, high-precision assembly |
| ISO 7 | 352,000 | Class 10,000 | Background rooms around critical zones, compounding |
| ISO 8 | 3,520,000 | Class 100,000 | General manufacturing, packaging, material staging |
Lower ISO numbers are cleaner. FED-STD-209E was the older US federal standard and was officially withdrawn in 2001 in favor of ISO 14644, but its Class 100 / 1,000 / 10,000 / 100,000 labels are still widely referenced in industry conversation. Classification measurements are also tied to the room’s operational state — as-built, at-rest, or in-operation — because occupancy and running equipment change the particle load.
Standards & references
- ISO 14644
- The international standard family for cleanrooms and controlled environments. Part 1 sets the airborne-particle classification (ISO 1 through ISO 9); other parts cover monitoring, testing, design, and operation.
- cGMP — 21 CFR Part 211
- The FDA’s current Good Manufacturing Practice regulations for finished pharmaceuticals. They require adequate control of air, environment, and contamination, making cleanroom design and monitoring a compliance requirement, not just a preference.
- USP <797>
- The US Pharmacopeia standard for sterile compounding. It sets environmental and engineering-control requirements — including ISO-classified air — for pharmacies and facilities preparing sterile preparations.
- USP <800>
- The US Pharmacopeia standard for handling hazardous drugs. It adds containment and negative-pressure requirements to protect personnel and the environment when compounding hazardous compounds.
Frequently asked questions
What ISO class is a cleanroom?
How is a cleanroom classified?
What is the difference between ISO 5 and ISO 7?
How is a cleanroom kept clean?
What industries use cleanrooms?
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