Pharmaceutical cleanrooms are used for manufacturing sterile and nonsterile drugs and medical products and often require a high level of cleanliness to maintain the aseptic environment surrounding the manufacturing processes and products.1, 2 The specifications for the required supply airflow rates or air changes per hour (ach) are often based on the engineer's judgement or the owner's quality assurance (QA) specification.2, 3, 4 However, these rates are also often excessive and cannot address the required cleanliness levels in the critical zone of a cleanroom. The computational fluid dynamics (CFD) study in this article systematically evaluates the impact of ach and the locations of supply diffusers on the airflow patterns and the resulting distribution of particles in the critical zone of a pharmaceutical cleanroom. © 2023 Amer. Soc. Heating, Ref. Air-Conditoning Eng. Inc.. All rights reserved.