The ISO standard assigns a class ranging from ISO 1 to ISO 9, with the higher classes permitting more particulate contamination per unit volume of air. A cleanroom graded ISO 1 is permitted to contain no more than 10 particles of size greater than per 0.1 microns per cubic meter of air. A cleanroom graded ISO 2 increases this limit to 100 or fewer particles of size greater than per 0.1 microns per cubic meter of air. In addition, limits for concentrations of particles of 0.2 microns and 0.3 microns are added. Typically, as the ISO class number increases, the stated limits of particle concentration step up by a factor of 10 with each higher ISO class, and larger particle size contaminants are added to the standard limits.