The DOP particles, being a liquid aerosol, impinge on the surfaces of the glass fibres making up the filter, and are trapped there, Each tiny droplet spreads out to form a mono-molecular layer on the surface, and since the surface area is extremely large, there is little chance of saturation. it would take an extremely high concentration of DOP, generated very close to the filter surface, to cause wetting out. Once the DOP is trapped, it stays on the fibres and is not released.