This document (EN ISO 14644-1) has been prepared by Technical Committee ISO/TC 209 "Cleanrooms and associated controlled environments" in collaboration with Technical Committee CEN/TC 243 "Cleanroom technology", the secretariat of which is held by BSI (United Kingdom). The committee responsible for the German version is Working Committee NA 041-02-21 AA "Reinraumtechnik (SpA CEN/TC 243 und ISO/TC 209)" ("Cleanroom technology (Mirror committee CEN/TC 243 and ISO/TC 209)") at Standards Committee Heating and Ventilation Technology and their Safety (NHRS) at DIN, the German Institute for Standardization e. V. The series DIN EN ISO 14644 "Cleanrooms and associated controlled environments" consists of the following parts: - Part 1: Classification of air cleanliness by particle concentration - Part 2: Monitoring to provide evidence of cleanroom performance related to air cleanliness by particle concentration - Part 3: Test methods - Part 4: Design, construction and start-up - Part 5: Operations - Part 7: Separative devices (clean air hoods, gloveboxes, isolators and mini-environments) - Part 8: Classification of air cleanliness by chemical concentration (ACC) - Part 9: Classification of surface cleanliness by particle concentration - Part 10: Classification of surface cleanliness by chemical concentration. This edition is the result of a Systematic Review and includes changes in response to user and expert feedback validated by international enquiry. The title has been revised to "Classification of air cleanliness by particle concentration" to be consistent with other parts of the standard. The nine ISO cleanliness classes are retained with minor revisions. Table 1 defines the particle concentration at various particle sizes for the nine integer classes. The use of Table 1 ensures better definition of the appropriate particle-size ranges for the different classes. A retained formula is used to define intermediate decimal classes. The standard retains the principle of ultrafine and macroparticle descriptors for particle sizes that are outside the range suitable for measurements with particle counters. The most significant change is the adoption of a more consistent statistical approach to the selection and the number of sampling locations; and the evaluation of the data collected. The number of sampling locations has been changed compared to the 1999 edition of the standard. The new approach allows each location to be treated independently with at least a 95 % level of confidence that at least 90 % of the cleanroom or clean zone areas will comply with the maximum particle concentration limit for the target class of air cleanliness. A reference table is provided to define the required number of sampling locations. Since a random sample size is required, the cleanroom or clean zone area is then divided into equally sized areas and the sampling location is randomly placed in each area. Finally, the annexes have been reordered to improve the logic of this standard.