Measured or specified concentration, in particles per cubic meter of air, including the ultrafine particles. ISO 14644-1, ISO 14644-3.NOTE: The U descriptor may be regarded as an upper limit for the averages at sampling locations (or as an upper confidence limit, depending upon the number of sampling locations used to characterize the cleanroom or clean zone). U descriptors cannot be used to define airborne particulate cleanliness classes, but they may be quoted independently or in conjunction with airborne particulate cleanliness classes.