This is Respros new polymer seal the PRO-SEAL. It provides an even better seal to your Respro mask. The PRO-SEAL can be retroactively fitted into any Respro mask no matter when you bought it last week or 23 years ago. It provides enhanced sealing which reduces inward leakage Inward leakage (IL) is unfiltered air that gets in around the sides of a mask & filter because a mask fits poorly or just doesnt make a proper seal (think ear loop masks) The PRO-SEAL helps keep the filter away from the face which in turn increases the over all surface area of filter material to draw air through which not only reduces inhalation resistance but makes it easier to breathe through & a more comfortable experience The PRO-SEAL is more compliant & accommodates a wide range of face shapes