When you fly at a typical cruising altitude of 35,000 to 40,000 feet, the outside air pressure is extremely low—only about 20% to 25% of the pressure at sea level—making the air too thin for humans to breathe. To compensate, aircraft cabins are pressurized, but they are not pressurized to sea level. Instead, the cabin pressure is usually maintained at an "altitude equivalent" of 6,000 to 8,000 feet. This means the air inside the cabin is similar to the air at the top of a high mountain. The pressure difference between the cabin and the external environment is managed by the aircraft's structure and outflow valves. This drop in pressure (from sea level to 8,000 feet) causes the air in your middle ear and sinuses to expand, which is why your ears "pop" during ascent and descent. Modern aircraft like the Boeing 787 and Airbus A350 use carbon-fiber hulls to maintain a higher cabin pressure (lower altitude equivalent of about 6,000 feet), which significantly reduces fatigue and "jet lag" by allowing your blood to carry more oxygen.