This part of ISO 14644 covers the classification of air cleanliness in cleanrooms and associated controlled environments exclusively in terms of concentration of airborne particles. Onlyparticlepopulationshavingcumulativedistributionsbased on threshold (lower limit) sizes ranging from 0,1 μm to 5 μm are considered for classification purposes.This part of ISO 14644 does not provide for classification of particle populations that are outside of the specified particle size range, 0,1 μm to 5 μm. Concentrations of ultrafine particles (particles smaller than 0,1 μm) and macroparticles (particles larger than 5 μm) may be used to quantify these populations in terms of U descriptors and M descriptors, respectively.This part of ISO 14644 cannot be used to characterize the physical, chemical, radiological, or viable nature of airborne particles.