ISO 14644 cleanroom classification explained

When someone describes a room as "ISO 7" or "ISO 5", they are using the classification system in ISO 14644-1, the international standard for cleanroom air cleanliness by particle concentration. It replaced the older US Federal Standard 209E, whose Class 100, Class 10,000 and Class 100,000 labels are still heard in conversation.

Understanding what the class number means, and what it does not mean, helps owners write better requirements and avoid paying for cleanliness they do not need. This guide explains the classification system and how a room is tested against it.

The ISO 14644 family

ISO 14644 is a series of standards. The parts most relevant to HVAC design and testing are:

The current edition of Part 1 was published in 2015 and made several significant changes from the 1999 edition, particularly to sampling.

How classes are defined

ISO classes run from ISO Class 1 (the cleanest) to ISO Class 9. The class number N sets the maximum permitted concentration of particles at or above a given size, using this formula:

Cn = 10^N x (0.1 / D)^2.08

where Cn is the maximum concentration in particles per cubic metre and D is the particle size in micrometres. Each step in class number allows ten times more particles.

In practice, people use the table of class limits in the standard. The most commonly used rows are shown below.

Values are maximum particles per cubic metre at or above the stated size. For comparison, ISO 5 corresponds roughly to the old Class 100, ISO 7 to Class 10,000 and ISO 8 to Class 100,000, since those classes counted particles per cubic foot at 0.5 um.

Choosing particle sizes

A classification is always stated with the particle size or sizes considered, for example "ISO Class 7 at 0.5 um and 5 um, operational". One or more sizes can be used. The 2015 edition removed the 5 um limit for ISO 5, because counting very small numbers of large particles is statistically unreliable. Where large particles matter, the standard provides a macroparticle descriptor (the M descriptor).

Occupancy states

A class means little unless the occupancy state is stated. ISO 14644-1 defines three:

• As-built: the installation is complete with services connected and running, but no production equipment, materials or personnel are present.
• At-rest: equipment is installed and operating as agreed, but no personnel are present.
• Operational: the room is functioning in its specified manner, with the specified number of personnel working.

People are usually the largest source of particles, so the same room may achieve ISO 5 at-rest and only ISO 7 operational. Owners should specify the class required in each state. GMP grades, described in GMP cleanroom grades A to D, set separate limits for at-rest and in-operation conditions.

What classification does not cover

ISO 14644-1 classifies air by particle concentration only. It says nothing directly about:

• Microbial contamination, which is addressed by GMP rules and other standards.
• Chemical or molecular contamination, covered by ISO 14644-8.
• Surface cleanliness.
• Temperature, humidity, pressure or air change rates.

The standard also does not prescribe how a room achieves its class. Air change rates, airflow pattern and filtration are design choices. Guidance on these is found in ISO 14644-4 and industry recommended practices; see air change rates and airflow in cleanrooms.

How a classification test works

The 2015 edition changed the sampling method substantially. The main steps are:

1. Determine the number of sample locations. Instead of the old square-root-of-area rule, the 2015 edition uses a table in Annex A that sets the minimum number of locations based on cleanroom area. Rooms larger than 1,000 m2 use a formula. The table was derived statistically so that, with a stated confidence, most of the room meets the class limit.
2. Divide the room into sections of approximately equal area and place one sample location in each, at work height or another agreed position. Additional locations can be added at critical points.
3. Set the sample volume. Each sample must be large enough that at least 20 particles would be counted if the concentration were at the class limit for the largest particle size considered, with a minimum of 2 litres and at least one minute of sampling.
4. Count particles with a calibrated light-scattering airborne particle counter, calibrated to ISO 21501-4.
5. Evaluate results. Under the 2015 edition each location's average concentration must not exceed the class limit. The 95 percent upper confidence limit calculation used in the old edition for rooms with few locations is no longer required.

If a single location fails, the standard allows investigation and, with justification, additional sampling. The test report must record the class, particle sizes, occupancy state, locations, results and instrument calibration.

Before counting particles

Particle counts are only meaningful once the HVAC system is performing correctly. Airflow volumes, filter installation leak tests and room pressures are normally verified first using methods in ISO 14644-3. A full test sequence is described in cleanroom certification and testing.

Monitoring and re-testing

ISO 14644-2 covers ongoing monitoring and the interval for repeating classification. It ties the frequency to a risk assessment and the monitoring plan, with periodic re-classification typically at least annually. Industry rules often set shorter intervals: EU GMP Annex 1 calls for requalification of Grade A and B areas at least every six months, and USP 797 requires certification of compounding areas at least every six months.

Writing a good requirement

A useful cleanliness requirement states:

• The ISO class for each room or zone.
• The particle sizes to be considered.
• The occupancy state or states.
• Any additional criteria, such as microbial limits, recovery time or temperature and humidity tolerances.

For example: "Filling zone, ISO 5 at 0.5 um, at-rest and operational; surrounding room, ISO 7 at 0.5 um and 5 um, at-rest." This gives designers and testers a clear target.

Key takeaways

• ISO 14644-1 classifies cleanrooms from ISO 1 to ISO 9 by maximum particle concentration at stated sizes.
• Each class step allows ten times more particles; ISO 5, 7 and 8 correspond roughly to old Classes 100, 10,000 and 100,000.
• A class must be stated with particle sizes and occupancy state: as-built, at-rest or operational.
• The 2015 edition uses a lookup table for sample locations and requires every location to meet the limit.
• Classification covers particles only, not microbes, chemicals, pressure or air change rates.