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. Only particle populations having cumulative distributions based on threshold (lower limit) sizes ranging from 0, 1 ?m to 5 ?m 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 ?m to 5 ?m. Concentrations of ultrafine particles (particles smaller than 0, 1 ?m) and macroparticles (particles larger than 5 ?m) 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. Note: The actual distribution of particle concentrations within incremental size ranges normally is not predictable and typically is variable over time.