FS209E and ISO Classes Explained

First developed in the 1960’s, a clean room is a controlled environment where products are manufactured. The reason for a clean room is to control airborne particles to a specified limit.

The particles are usually generated by people, process, facilities, and equipment. Particles must be continually removed to a certain level and the level is usually set to the industry federal 209E (FS209E) or ISO standards. They are not sterile environments, only particles are removed, not microbes.

The FS209E and ISO establish the various classes that rooms are designed and built to. Air flow rates, direction, pressure, temperature, humidity and filtration all need to be tightly controlled. They are also planned and built using the above standards. The newer ISO standards add two ‘cleaner’ standards and one ‘dirtier’ standard than the previous FS209E standards.


As an example, a typical office will contain approx. 500,000 to 1,000,000 particles per cubic foot of air (0.5 microns or larger). A typical class 100 clean room is to never allow more than 100 particles per cubic foot of air. The ISO class 1000 and 10,000, are limited to 1000 and 10,000 respectively.

Bearing in mind a human hair is 75-100 microns in diameter. A 0.5 micron particle (200 times smaller) can cause a major disaster in a clean room. The Hubble Telescope was damaged and completion was delayed due to a 0.5 micron particle.

ISO CLASS FS209E CLASS X 0.5 Micron (μm) per cubic meter
ISO 2 4
ISO 3 1 35
ISO 4 10 352
ISO 5 100 3,520
ISO 6 1,000 35,200
ISO 7 10,000 352,000
ISO 8 100,000 3,520,000
ISO 9 35,200,000
Class 1 allows only 2 particles @ 0.2 Micron/M³

Air-change per hour

Furthermore, a vital factor in cleanroom design is controlling air-change per hour (ACH) or air-change rate (ACR). This relates to the number of times that filtered outside air replaces the existing volume in a building or chamber per hour. In a cleanroom, depending on the classification and usage, air change can range anywhere from 10-600 times an hour. Determining the appropriate number of air changes for a company’s particular application requires evaluation of factors such as personnel, effectiveness, frequency of access and cleanliness of process equipment.

FS Cleanroom Class ISO Equivalent Class Air Change Rate

1 ISO 3 360-540
10 ISO 4 300-540
100 ISO 5 240-480
1,000 ISO 6 150-240
10,000 ISO 7 60-90
100,000 ISO 8 5-48

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