A cleanroom is an environment where airborn particulates are controlled through an exchange of highly filtered air, and through minimization of activities that generate particles. For example, a special gowning procedure is required prior to entering the cleanroom. In addition to particle control, the cleanroom is temperature and humidity controlled to 70F, 50% RH.
Protocols help mantain the best possible environmental conditions when something goes wrong with the air handling systems.

A comparison between Federal Standard 209 E classification and 1999 ISO 14644-1 classification schemes is given here.