US Federal Standard 209E was first published in 1963 as a definition and procedure for clean room classification, testing and monitoring. This standard was used in the United States and by other nations for many years and widely recognized in the US. The International Standards Organization (ISO) worked with the European Committee for Standardization to develop a set of standards for use by all European Nations. The need for a single standard for classification and certification of clean rooms was apparent and in 1992 the American National Standards Institute (ANSI) backed by the Institute of Environmental Sciences and Technology (IEST) petitioned the ISO to develop an a single international standard. The ISO formed Technical Committee 209 and began work on what became ISO 14644. In 1999 IEST published ISO 14644-1, the first internationally recognized standard for classification of clean rooms. A year later ISO 14644-2 was published identifying the monitoring and testing requirements for compliance with ISO 14644-1. The contrast between the FED STD 209E and ISO 14644-1 can be summarized as follows: ISO 14644-1 ·Nine Clean Definitions ·Class designations 1 – 9 ·Metric Measurement System ·Fewer test locations – standard 1 minute sample time ·Addition of standard particle size (1.0µm) ·Clean classifications are calculated ·Guidelines for periodic test schedule ·Adopted by The United States in November 2001 Federal Standard 209E ·Six Clean Definitions ·Class designations 1 – 100,000 ·English Measurement System ·More test locations – less than one minute sample time ·Cancelled and superceded by The United States in November 2001