ISO Classification of Cleanrooms

ISO 14644 classification for cleanrooms is based on the formula

Cn = 10N (0.1 / D) 2.08

Where 

  Cn = maximum permitted number of particles per cubic meter equal to or greater than the specified particle size, rounded to whole number 

  N = is the ISO class number, which must be a multiple of 0.1 and be 9 or less 

  D = is the particle size in micrometers

  An ISO 1 cleanroom has the lowest levels of contamination, while an ISO 9 has the highest allowable level. To give a perspective, the ambient air outside in a typical urban environment might contain as many as 35,000,000 particles per cubic meter, 0.5 um and larger in diameter, corresponding to an ISO class 9 cleanroom.

Cleanroom class comparison (ISO v/s Federal Std. 209)

ISO is based on metric measurements whereas Federal Standard 209 is based on imperial measurements. The classes, according to ISO14644, are in terms of class levels 3, 4, 5…of airborne particulate cleanliness corresponding to 1, 10, 100…..class Fed 209 standards. A Class 5 means that less than 3,520 particles (0.5 microns in size) are present per cubic meter, which equals 100 particles per cubic foot.

*In United States, Federal Standard 209E (FED-STD-209E) was used until the end of November 2001 to define the requirements for cleanrooms. On November 29, 2001, these standards were superseded by the publication of ISO specification 14644