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.
Standards referenced: ISO 14644-1 · ASME BPE · 21 CFR 211 · USP 797 · USP 800
Related: ISO 7 Cleanroom Requirements · ISO 8 Cleanroom Requirements · ISO 5 Cleanroom Requirements
A room at rest vs a room in operation: the state a classification is measured in
A particle count without an occupancy state attached is not a classification. ISO 14644-1 recognizes three: as-built, meaning the room is finished but empty and idle; at-rest, meaning equipment is installed and running with nobody inside; and operational, meaning the process is being performed by the people who normally perform it. A certificate that claims ISO 7 and does not name the state has left out the variable that matters.
GMP grading carries the distinction explicitly. Under EU GMP Annex 1, Grade B is held to 3,520 particles of 0.5 µm and larger per cubic meter at rest, and to 352,000 in operation. Grade A is held to 3,520 in both states, because the critical zone has to maintain the condition while work is happening rather than before it starts.
The engineering consequence is that a room which passes at rest and fails in operation almost never has a filtration problem. It has a people and movement problem: gowning quality, headcount, how material is transferred in, and whether equipment or an operator’s torso is blocking the airflow path between a filter and an exposed product. Those are resolved in layout and in the operating procedure, not by adding filters.
The recovery test is the bridge between the two states. It shows how quickly the room clears a deliberate challenge, which is the only honest predictor of whether an operational count will hold once real work starts.
Where the class ladder gets expensive, and how delivery method changes it
Our cleanroom cost guide separates a modular or prefabricated installation from a full GMP facility build, and at the clean end of the ladder the two columns diverge sharply. ISO 5, historically Class 100, sits at $700 to $1,000 per square foot as a prefabricated installation, against $900 to $1,850 inside a GMP build. ISO 4 and cleaner runs $1,000 to $1,500 or more prefabricated, against $1,650 to $2,500 or more built in place.
The gap is not the walls. It is the base building work, the plant space and shafts, the utility capacity the suite needs, and the documentation depth a GMP build carries that a prefabricated enclosure dropped into a suitable shell does not. The same guide puts mechanical, HVAC and filtration at 30 to 50 percent of the project, scaling hard with class, and soft costs such as design, engineering, permitting and project management at 20 to 35 percent.
Read together, those numbers say the largest cost decision available on a cleanroom is usually made before any cleanroom vendor is contacted: whether the building can already serve the air, water and headroom the class requires.
What to require on a cleanroom certification report
A certificate is a test record, and an incomplete one cannot support a qualification. Require all of this on the document:
- The occupancy state named, and for an operational test the headcount present and the equipment actually running at the time.
- The standard and its edition, ISO 14644-1:2015, with the particle sizes considered and the limits applied, since the 2015 revision changed both the location count and the statistical treatment of the results.
- The number of sampling locations derived from the standard’s table for the room area, with a plan showing each location and the sampling height used.
- The volume sampled at each location and the raw count at each size, not a pass statement. The standard sets a minimum single sample volume of 2 L and a minimum sampling time of one minute.
- Measured supply air volume per filter with the calculated air change rate and the room volume used in that calculation printed alongside it.
- Installed filter leak testing to ISO 14644-3 on every terminal filter, scanned at the media face, the frame and the housing seal, with the challenge method and the aerosol concentration recorded.
- Pressure differentials measured door by door with doors closed, plus a statement of what happens when an airlock door opens. A cascade that inverts on door opening is not a cascade.
- A recovery test result for every non-unidirectional room, with the challenge concentration and the measured recovery time.
- Airflow visualization for unidirectional zones recorded as video, performed with the process equipment and an operator in position rather than in an empty room.
- Calibration certificates for the particle counter and the airflow instrument, dated so the calibration interval clearly covers the test date.
Frequently asked questions
What is a cleanroom?
What defines a cleanroom versus an ordinary clean space?
How does a cleanroom control airborne particles?
What is a HEPA filter and why does a cleanroom need one?
What does ISO class mean for a cleanroom?
Why does a cleanroom use positive air pressure?
What is an air change rate in a cleanroom?
Do cleanrooms control temperature and humidity as well?
Almost always, and on many projects that is the harder requirement. ISO 14644 classifies particle concentration only; it says nothing about temperature or humidity. Those come from the process: tablet compression needs controlled humidity to stop material sticking, analytical instruments need temperature stability tighter than comfort standards, and gowned operators generate heat the system must remove. A room can hold its particle class perfectly and still be unusable if humidity drifts outside what the process tolerates.
Why do people wear special garments in a cleanroom?
What is the difference between a cleanroom and a laminar flow hood?
How is a cleanroom certified as clean?
What are the ISO classes of cleanrooms?
ISO 14644-1 runs from ISO 1, the cleanest, to ISO 9, roughly ordinary room air, classified by maximum permitted particle concentration at specified sizes per cubic meter. Pharmaceutical work concentrates in ISO 5 through ISO 8: ISO 5 for the critical zone in aseptic processing, ISO 7 as its background, ISO 8 for non-sterile manufacture and support areas. A class quoted without the particle size and the occupancy state is an incomplete specification.
How is a cleanroom kept clean after it is built?
By monitoring and gowning discipline rather than by the construction. Classification is reverified on a schedule, typically annually and twice yearly for ISO 5 and cleaner, with routine particle and microbial monitoring in between, filter integrity testing, and pressure differentials logged and trended. The largest variable a building cannot control is people: gowning practice and traffic discipline account for more day-to-day variation than any mechanical factor, which is why they are audited rather than assumed.
Can a cleanroom be built inside an existing building?
How do I find out what cleanroom I need?
Ask your quality organization which regulation or customer requirement actually applies, then design to that rather than to a number someone suggested. If nobody will ever request a classification certificate, you probably need a controlled environment rather than a cleanroom. If a certificate will be requested, the class follows the process step it protects. Getting that answer before design is the single largest cost decision on a cleanroom project, because classification drives air change rate and therefore everything downstream.
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?
What is a unidirectional airflow cleanroom?
One where filtered air moves in a single direction at near-uniform velocity across the space, sweeping particles out rather than diluting them. It is used where the critical zone has to be protected continuously, and it requires near-full ceiling filter coverage.
What is a turbulent or non-unidirectional cleanroom?
One where filtered air enters at ceiling level and mixes with room air, diluting contamination and removing it through returns. It is the common arrangement for less critical classifications and is far cheaper to build and run.
How do people contaminate a cleanroom?
They shed skin particles continuously, and the rate rises sharply with movement. That is why gowning, personnel numbers and movement discipline affect particle counts more than most equipment decisions.
What is a gowning sequence and why is it ordered?
A defined order of donning garments so that clean items are not contaminated by items still being handled, typically progressing from the least to the most critical. It is trained and assessed because a correct garment donned in the wrong order does not work.
Does a cleanroom control anything besides particles?
Frequently yes. Temperature, humidity, pressure and viable organisms are all controlled where the process requires it, and the classification only addresses airborne particulate. A room can be correctly classified and still unsuitable on humidity or microbial control.
What is a cleanroom environment, as distinct from the room itself?
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