Cleanrooms have always been held to a cleanliness standard and that standard has changed over the years. Many people still reference the old Federal 209E “Class System” which was developed in the early 90s by the U.S. Government. This standard was in place until 1999 when it was replaced by the ISO 14644-1 standards. The biggest reason for the in change in standards was to switch from the US standard of measurement to the metric standard.
More revisions to the ISO 14644 standards are on the way and Dr. Tim Sandle has written a detailed article as to what these proposed changes may include. You can read the article in its entirety via this link.