The particle size is an important factor for the cleanliness of a cleanroom. The particle size refers to the diameter of an individual particle in an air sample. The smaller the particle size, the higher the number of particles per unit volume and the lower the cleanroom class. The particle size is measured in micrometers (µm) or nanometers (nm). The cleanroom class defines the permissible number of particles per cubic meter of air for a specific particle size. The cleanroom class is defined according to international standards such as ISO 14644 or US FED STD 209E.