text: In high performance, high altitude flight, it is necessary to provide personnel with positive pressure breathing apparatus. Such systems are especially needed when the body is exposed to very low pressure. Then survival depends upon supplying correct gas mixtures at pressures which properly ventilate and pressurize the body's breathing system. Masks worn against the face, which cover the nose and mouth, are well known. These function acceptably at relatively small differential pressures between mask pressure and ambient pressure. Such masks frequently have a reflective seal which gently bears against the skin and is deflected by mask pressure to form a seal with the skin. In order to maintain the mask pressure, the seal must prevent blow-by of the breathing gases.
  Masks with reflective seals function acceptable at altitudes where the differential pressure is on the order of about 6 inches to 8 inches of water. However, at much higher altitudes, where the differential may be on the order of perhaps 35 inches of water, reflective seals are not sufficiently reliable, and blow-by becomes a risk.
  The shortcomings of reflective seals arise in part from the characteristics of skin and in part from the variability of facial configurations of the wearers. When the reflective seal contacts the skin, it does not meet a clean and flat surface. Instead, the skin may be pocked, pitted, locally enlarged, and wrinkled. Furthermore it is customary to provide only a limited number of mask configurations and sizes, and for the wearer to select the most suitable member of the group for his personal use. As a consequence, even though the mask will be made of relatively flexible and conformable material, still for many wearers, the flexing which occurs when the mask is pulled against the face may in some localized regions, especially over the bridge of the nose, permit blow-by. The more different the wearer is from the "average" face, the greater is the risk of such an event.
  It is theoretically possible to avoid all risk of blow-by by providing an individually and precisely conforming rigid or nearly rigid seal, and pressing it against the face so that it deeply indents the skin to form a force seal. This is of course an intolerable situation, if only because of the discomfort it will cause to the wearer and of the impediment to his blood circulation.
  It is an object of this invention to provide a breathing mask which can utilize the relatively gentle features of the reflective seal for operations which involve modest differential pressures, and a relatively more rigid secondary seal which is brought into operation at greater differential pressures to form a force seal, but without requiring excessive rigidity or full-time application that are likely to cause discomfort.
  There are additional problems involved in such high performance breathing masks. One is that the larger the frontal area of the mask, which develops with the larger differential pressure a force to force the mask away from the face, the larger is the force needed to hold the mask against the face. This can lead to discomfort. Another is that when the airman undergoes high negative G forces, in the sense of enduring tight high speed pulls out of dives, and high speed inside turns, forces are developed which tend to slide the mask down his face, which could lead to blow-by as well as to possible loss of the mask itself. It is a further object of this invention to provide a mask whose frontal area is minimized, and which is conformed so as to be supported by the mental protuberance of the chin during negative G events.