The EN ISO 14644-1 standard establishes 9 classes (from 1 to 9) for the cleanliness of the air in controlled environments and for each class defines the maximum quantity of particles of different diameters (from 0.1 to 5.0 μm) admissible per cubic meter of sampled air. The class of reference for biological applications is ISO 5 (formerly Class 100, FED-STD-209E) which sets an upper limit of 3520 units per cubic meter of sampled air for particles of diameters equal to or greater than 0.5 μm.