In some environments, it is necessary to limit the number of airborn particles. This can be a requirement in the pharmaceutical, electronic and food industries, as in some hospital environments. Two standards for classifying air cleanliness are US Federal Standard 209E and EN ISO 14644-1. The classification systems are exclusively based on the concentration of airborne particles and their sizes. The table shows the classes defined and how they approximately correspond e.g. ISO class 4 lies between M2 and M2,5. Included also are the zone classes defined in the guide "Good manufacturing practice for medicinal products" issued by the European Commission.