A one piece flexible sanitary seal (16) of uniform cross-section is provided for over pressure vent structure that protects a confined space meeting hygienic standards. The seal (16) has an elongated body (38) provided with a central section (40), a C-shaped terminal end (42) having a main lug portion (46) and a secondary lug segment (48) that project away from each other with a cavity (50) therebetween, and a long tail portion (44) cooperating with the central section (40) to present a longitudinally extending angular groove (62). The seal (16) is adapted to be mounted between a vent membrane unit (14) and a frame component (18) of the vent structure. Lug portion (46) and lug segment (48) sealingly engage the inner surface of the vent membrane (14) inboard of a line of weakness therein defined by a series of slots. A rectangular projection (52) extends from the central section (40) in disposition to engage the vent membrane unit (14), outboard of the line of weakness. The main lug portion (46) and the secondary lug segment (48) of terminal end (42) are moveable toward and away from one another while remaining in tight sealing engagement with the vent membrane unit (14) during in and out cycling thereof. Frame component (18) is received in the groove (62) to firmly affix the seal (16) to the vent structure.