Cleanrooms, Cleanliness and Classifications

For many, the mention of the word “cleanroom” conjures up images of a space-age white room with workers moving about in full-body “bunny suits” and filtered facemasks working on items inside of glove boxes. Of course, Hollywood has a lot to do with that, but even many people within industries that utilize cleanrooms aren’t entirely certain about the various standards and classifications or what they mean.

The very short – and overly simplistic – answer is that cleanrooms are classified on the cleanliness of the air within the room with increasingly higher standards regarding the quantity and size of foreign particles within a given volume of air. To achieve the various classification levels, cleanrooms are designed to cycle the air within them a certain number of times, achieve specified filtration levels and have a minimum air flow, among other criteria. Further, cleanrooms must be tested at regular intervals to retain their certification.

What is a cleanroom?

The modern cleanroom dates back to 1960 when physicist Willis Whitfield sought a solution to the problem of particle contamination and random airflow while working at Sandia National Laboratories in New Mexico. Ranging from small alcoves to entire manufacturing floors, cleanrooms share some common features; air that is repeatedly recirculated and filtered while also being tightly controlled for temperature and humidity.

Other measures are taken to preserve the cleanliness of the environment. Workers wear protective clothing and undergo protocols designed to reduce the amount of outside contamination that they might introduce into the room. Within the cleanroom, equipment and fittings are designed to limit their contribution to contamination. In some cases, the rooms have “positive air pressure” where the air pressure within the room is higher than that of the surrounding area in order prevent contaminates from leaking into it from outside. Stainless steel, because of its durability and resistance to corrosion is heavily used as is aluminum.

It’s important to understand that a cleanroom is not sterile. Microbes may be present on surfaces, for instance. Instead, the focus is on removing airborne particles. But, the use of ultraviolet light to disinfect the air is becoming more common.


As mentioned, everything that goes into a cleanroom is carefully considered in light of its potential to shed debris or non-volatile residue (NVR). Items containing organic material, such as paper and pencils are replaced with alternatives. Even the cleaning supplies are specialized for use in cleanrooms.

An item cannot be “cleaned to a level that allows its use within a specific class of cleanroom.” Instead, it must be tested to ascertain whether it meets or exceeds a specified value for NVR and particle count. Fittingly, such tools or equipment themselves are tested in a cleanroom. If they meet the criteria, they are then packaged in material that also meets the standards for the intended cleanroom.

The standards themselves are measured in terms of the maximum size of particles, as well as the quantity detected in a cubic meter of air (roughly 35 cubic feet). For the highest standards, particulate matter must be equal or smaller than 0.2 microns. A micron, symbolized as “µm”, is 0.001 of a millimeter. By comparison, a human hair is roughly 75 microns thick and a bacterium measures between 0.2 and 3 microns. Also as a part of achieving that standard, designated ISO Class 1 by the International Organization for Standards (ISO), no more than 12 particles measuring less than 3 microns can be present in a square meter of the cleanroom’s air. A typical room in a building could be expected to have 35,000,000 particles measuring larger than 0.5 microns in every cubic meter. Within cleanrooms, specialized particle counters continually monitor the amount and size of airborne contaminants.



Prior to 2001, the United States along with many other countries, used the General Service Administration’s standards, referred to as FS209E. It had six categories, ranging from 1 to 100,000. The numbers related to the quantity of particles measuring 0.5µm or larger within a cubic foot of air, so a Class 1 room would be the cleanest, while a non-cleanroom environment would be considered Class 1,000,000.

These standards were replaced by ISO-14644-1 standards starting in 1999. The ISO standards added two higher standards for cleanliness. Like the Federal Standard, the classifications went from lowest to highest with ISO Class 1 being the cleanest and ISO Class 9 which still has a lower particulate level than a regular room. In addition to a desire for updated, common standards, the progress of cleanroom technology lead to the replacement of the old system. Class 1 under FS209E is the equivalent of ISO Class 3, as it allowed eight particles larger than a micron plus 1,344 particles of smaller sizes.

A majority of cleanrooms in use are ISO 7 or ISO 8. As the cleanliness level increases, so do the steps and protocols required to achieve those levels. The use of protective clothing, anterooms, improved filters, and other best practices are implemented with the ultimate goal of reducing airborne particles chief in mind. Ultimately, the level of air cleanliness is maintained by high-efficiency particulate air (HEPA) or ultra-low particulate air (ULPA) filters with the air being changed at higher rates to match the higher classification. An air change is calculated by the volume of air filtered in one hour divided by the volume of the room itself. Where a conventional building might see two to four air changes per hour, an ISO 7 cleanroom would see between 30 and 60 such changes. And, an ISO 5 cleanroom’s system would achieve 240-360 air changes within that time.

Bottom Line

While the cleanroom needs of an aerospace company differ from those of a biopharmaceutical manufacturer, understanding the internationally-agreed upon set of standards makes meeting the required level of cleanliness a goal that can be achieved with confidence.

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Picture of Jonathan Hopkins

Jonathan Hopkins

Jonathan serves as Vice President of Cleanroom Services and as a member of the company’s Senior Leadership Team. He has over a decade of leadership experience in the precision cleaning sector as well as a strong background in manufacturing management. As both a business and technical leader, Jonathan focuses on providing customers with solutions to the most demanding cleaning challenges. Jonathan is a veteran of the United States Navy where he served in Surface Warfare and Mine Countermeasures. He is also an active member of the AIAA (American Institute of Aeronautics and Astronautics) and holds a Lean/Six Sigma Black Belt certification.

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