What is a Cleanroom?

​A cleanroom is defined by ISO14644-1 as a room in which the concentration of airborne particles is controlled.
A cleanroom is defined by ISO14644-1 as a room in which the concentration of airborne particles is controlled, and which is constructed and used in a manner to minimize the introduction, generation, and retention of particles inside the room and in which other relevant parameters, e.g. temperature, humidity, and pressure, are controlled as necessary. 
Cleanroom specifications for particulate matter (such as dust) are defined according to the maximum allowable particle size (diameter), and also according to the maximum allowable number of particles per unit volume. For non-particulate contaminants, the maximum allowable density in terms of microbes per cubic meter, or molecules per cubic meter, is specified.

​The determination of how clean an area is depends on the class number that it is designed to. According to Federal Standard 209, A to D versions, class number refers to the maximum number of particles of 0.5 micron size or bigger that would be allowed in one cubic foot of cleanroom air. 
Maximum number of particles in Air (particle per cubic feet of air)

Table: A
​Classes and their Typical Uses
Class 1 & 10 - production laboratories for electronic integrated circuits...
Class 100 - production areas for photo labs, medical implants...
Class 10,000 - hospital operating ,production locales for TV tubes...
Class 100,000 - production of ball bearings...
ISO is based on metric measurements whereas Federal Standard 209 is based on imperial measurements.
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 
 Maximum Concentration Limits (particles/m3 of air) table B
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. ​
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*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.