Cleanrooms are classified by how clean the air is. In Federal Standard 209 (A to D) of the USA, the number of particles equal to and greater than 0.5mm is measured in one cubic foot of air, and this count is used to classify the cleanroom. This metric nomenclature is also accepted in the most recent 209E version of the Standard. Federal Standard 209E is used domestically. The newer standard is TC 209 from the International Standards Organization. Both standards classify a cleanroom by the number of particles found in the laboratory’s air. The cleanroom classification standards FS 209E and ISO 14644-1 require specific particle count measurements and calculations to classify the cleanliness level of a cleanroom or clean area. In the UK, British Standard 5295 is used to classify cleanrooms. This standard is about to be superseded by BS EN ISO 14644-1.

Cleanrooms are classified according to the number and size of particles permitted per volume of air. Large numbers like “class 100” or “class 1000” refer to FED_STD-209E, and denote the number of particles of size 0.5 mm or larger permitted per cubic foot of air. The standard also allows interpolation, so it is possible to describe e.g. “class 2000.”

Small numbers refer to ISO 14644-1 standards, which specify the decimal logarithm of the number of particles 0.1 µm or larger permitted per cubic metre of air. So, for example, an ISO class 5 cleanroom has at most 105 = 100,000 particles per m³.

Both FS 209E and ISO 14644-1 assume log-log relationships between particle size and particle concentration. For that reason, there is no such thing as zero particle concentration. Ordinary room air is approximately class 1,000,000 or ISO 9.

ISO standards are the industry norm for rating cleanrooms. ISO standards were adopted by the industry in 2001. If you do any serious research into ISO standards, you are likely to come across the Federal Standard 209E for cleanrooms, which was the industry norm until ISO standards were developed. The federal standards were officially cancelled by the US Department of Commerce in November 2001, but they are still widely referenced.

“ISO classifications expanded the horizon for classifying clean space,” according to Richard Matthews of Filtration Technology, Inc., in Greensboro, NC, who chaired the ISO board that developed these standards. Here are the Federal standards and their ISO equivalents. Note that ISO created three new levels that the Federal standard did not address.

  • ISO Class 9 = No comparable Federal standard
  • ISO Class 8 = Federal standard Class 100,000
  • ISO Class 7= Federal standard Class 10,000
  • ISO Class 6 = Federal standard Class 1,000
  • ISO Class 5 = Federal standard Class 100
  • ISO Class 4 = Federal standard Class 10
  • ISO Class 3 = Federal Standard Class 1
  • ISO Class 2 = No comparable Federal standard
  • ISO Class 1 = No comparable Federal standard

Whenever possible, refer to the ISO standards because they are internationally accepted. If you are dealing with partners in other countries, this will make issues much simpler