**5.5 Electrostatic and HEPA filters**

Electrostatic filters: Static electricity attracts dirt and dust to vertical and overhead surfaces. The static is often generated when two surfaces rub together and are then separated. Electrostatic filters generate a static electrical charge on all particles in the air that passes through eight filter layers. The discharged particles are then attracted to collector plates with an opposite electrical charge. These filters have the advantage of being washable.

High-efficiency particulate air filters: HEPA filters have a strong particletrapping capacity that facilitates the removal of a high percentage (99.97%) of airborne particles that pass through an air purifier and accordingly meet US government standards. This contrasts with the 60–90% efficiency of medium filters [27]. Furthermore, HEPA filters perform significantly better than electrostatic air cleaners, in which filtering is based on ionic processes. HEPA filters are therefore often used in medical facilities and in households in which the residents suffer from severe allergies.
