The lower the particle concentration, the higher the classification of the cleanroom. An international standard to classify cleanrooms is provided by ISO 14644-1. The International Organisation for Standardisation - ISO for short - is an independent organisation dedicated to setting international standards for procedures.
The ISO cleanroom classes are identified by the numbers 1-9. ISO 1 is the cleanest possible room, ISO 9 corresponds to almost normal room air. You can read about the permitted number of airborne particles per ISO class here.
