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What Are The Differences in ISO Cleanroom Classifications?

Designing clean rooms and labs

The International Standards Organisation uses the standard ISO 14644 to outline the approach for the design, classification and operation of cleanrooms. ISO 14644 has a ranking system for cleanrooms running from ISO 1 to 9. The system is rated on the number of airborne particles per m³ and the size of those particles, with 1 being the most hygienic cleanroom environment.

To put it more simply, the smaller the number, the cleaner the air in the room.

These highly controlled environments are used in a wide range of different industries, including pharmaceutical, biotech, medical device and advanced manufacturing. Provisions are made to reduce contamination and control other elements of the environment such as temperature, pressure and humidity.

One of the key elements to keeping this environment under control is the HVAC (heating, ventilation and air conditioning) system. The most important part of this is air filtration through HEPA (high-efficiency particulate air) filters, which are installed into the ceiling of a cleanroom and remove particles as small as 0.3 microns. This process creates an environment in which the pressure inside is greater than outside – known as positive pressure. The air is pushed towards the lower section of the room and out of vents installed in the bottom of the cleanroom walls.

When deciding on which cleanroom classification is right for you, it’s important to ask three questions:

  • What is the size of the particles I need to filter out?
  • What are the potential sources of contamination?
  • How much air needs to circulate to get the contaminated particles out of the room?

Particle Size

One of the most important things to understand when designing a cleanroom is what size of particles will need to be filtered out. It’s mostly calculated on a ‘larger than’ scale, and the table at the end of this article can help with that.

For example, an ISO 6 cleanroom must have less than 35,200 particles per m³ that are smaller than 0.5 microns. These are not visible to the human eye – to give you an idea, the diameter of a human hair is 100 microns and we can usually only see particles that are at least 50 microns in size.

ISO 1 cleanrooms are extremely rare, requiring an environment to contain no more than 0.23 particles at 0.1 micron per square metre at all times. Creating this type of controlled environment is very difficult, and only a few sites in the world meet these requirements.


The two main sources of contamination for a cleanroom environment are people (skin flakes, oil, cosmetics, clothing debris, hair) and the process that is happening inside (equipment, materials, water), so it’s important to determine what these are likely to be before designing your cleanroom. The position of air grills and filtration systems can help keep areas clean.

Cleanrooms with an ISO class of 7 or lower require a separate room for gowning, which prevents dirt from the outside getting into the main cleanroom. Controlling this environment is also important in any manufacturing environment such as electronics or automotive parts, to avoid possible cross-contamination.

Air Circulation

The amount of air circulation you need depends on your classification, and generally, that air is twice as much as the category above (so ISO 7 needs double the air circulation of ISO 8). This means more filters and space to circulate, as well as more air conditioning.

Air changes help to dilute the air. Therefore, the more air changes, the more diluted the air to an acceptable concentration.

Controlled and Classified

The classification system was introduced to help classify ISO 9 – a controlled environment which has filtered air, a pressurised environment at a certain temperature and humidity but that isn’t classified in terms of particle count.

It is important to build a cleanroom that fits your requirements at a competitive rate. If you have any questions about an upcoming cleanroom project within your business, please get in touch with our expert team either by sending us an email or calling us on 0117 316 7000.

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