The device relates to a mask worn over a wearer's face for air filtration. The mask has an inturned lip that forms a seal on the edge of the mask with the wearer's face. The mask also has at least two other contact surfaces, internal to the edge seal, which vary in length to increase the seal integrity...http://www.google.es/patents/US4739755?utm_source=gb-gplus-sharePatente US4739755 - Respirator
