How are the High-efficiency particulate air filters ( HEPA filters) tested ?

The HEPA filters are tested by the DOP(dioctylphthalate)method when manufactured. DOP, a liquid plasticizer is heated to the point of vapourization and reconstituted into 0.3 micron particles to form a monodisperse aerosol. These single size particles are diluted with air until a concentration of 100 micrograms per litre is reached, and the aerosol-air mixture is passed through the filter. The amount of penetration is measured on the downstream side with a forward light scattering photometer, giving the familiar readings of 0.03% or better, The material used to make the filter material is tested in the same way by the filter material manufacturer.