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.

• A class 100 cleanroom maintains less than one hundred particles larger than 0.5 microns in each cubic foot of air space. It is equivalent to ISO class 5 cleanroom.
  This cleanroom has 250 square feet of class 100 space available.

• A class 1000 cleanroom has less than 1000 such particles per cubic foot. It is equivalent to an ISO class 6 cleanroom.
  This cleanroom has 600 square feet of class 1000 space available.

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.