For clean rooms where dust has to be arrested down to 0.3 microns, absolute (or HEPA) filters are used. They are usually located at the supply air outlet. In latest design of HEPA filters the filtering media is a sheet of extremely fine glass fiber interwoven in a diverse direction and mixed with resins. They offer resistance to the airflow and can achieve 99.97% efficiency down to 0.3 microns. The life of the HEPA filters depends on the amount of particulate matter being filtered and on the degree of pre filtering employed. Therefore in case of HEPA filters are used, pre filters installation is a must it should be well maintained to increase life span of HEPA filters.