This part of ISO 14644 covers the classification of air cleanliness in cleanrooms, clean zones, and separative devices exclusively in terms of concentration of airborne particles. Only particle populations having cumulative distributions based on threshold (lower limit) particle sizes ranging from 0.1 µm to 5 µm are considered for classification purposes. Air cleanliness by particle concentration, whether of a cleanroom, clean zone or separative device, will in the rest of this document be referred to by the initials ACP. The use of light scattering (discrete) airborne particle counters (LSAPC) is the basis for determination of the concentration of airborne particles, equal to and greater than the specified sizes, at designated sampling locations. This part of ISO 14644 does not provide for classification of particle populations that are outside the specified lower threshold particle-size range, 0.1 µm to 5 µm. Concentrations of ultrafine particles (particles smaller than 0.1 µm) are addressed in ISO 14644-12:XXXX, which specifies air cleanliness by nano-scale particles, while an M descriptor (see Annex C), may be used to quantify populations of macroparticles (particles larger than 5 µm). This part of ISO 14644 cannot be used to characterise the physical, chemical, radiological or viable nature of airborne particles.