High-altitude flatus expulsion
High-altitude flatus expulsion (HAFE) is a gastrointestinal syndrome which involves the spontaneous passage of increased quantities of rectal gases at high altitudes. First described by Joseph Hamel in c. 1820 and occasionally described afterward. A landmark study of this phenomenon was published in 1981 by Paul Auerbach and York Miller. The phenomenon is based on the differential in atmospheric pressure, directly correlated with the observer's frequency of and level of experience in high-altitude metabolism. As the external pressure decreases, the difference in pressure between the gas within the body and the atmosphere outside  is higher, and the urge to expel gas to relieve the pressure is greater. Consistent with Boyle's law, controlling for dietary variance, the amount of gas produced is constant in mass, but the volume increases as the external pressure decreases. The feeling of fullness or need to expel brought on by this differential in atmospheric pressure has been verified by studies involving military pilots subjected to pressure changes simulating flight.