The HEPA filters are tested by the DOP-dioctylphthalate-method when manufactured. DOP, a liquid plasticizer is heated to the point of vaporization and reconstituted into 0.3 micron particles to form a mono disperse aerosol. These single size particles re diluted wit 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.