Marc Machauer and Mark Hallworth, both with 25 years of experience in cleanrooms and pharmaceutical clean and aseptic manufacturing, discuss the newly published ISO 14644-21 standard for pharmaceutical contamination monitoring. They discuss the misinterpretation of the standard, the difference between classification and monitoring, and the importance of ISO classification. They also provide guidance on materials and valves in particle transport. The discussion includes practical examples and a decision tree for better understanding. They also touch on the importance of measuring losses in particle transport and discuss the worst-case scenario. The episode ends with the announcement of an upcoming white paper and webinar.