It refers to altitudes above a certain point where the amount of oxygen is insufficient to sustain human life for an extended time span. This point is generally tagged as 8,000 m (26,000 ft, less than 356 millibars of atmospheric pressure).
At sea level, because air is compressible, the weight of all that air above us compresses the air around us, making it denser. As you go up in elevation (while mountaineering, for example), the air becomes less compressed and is therefore thinner.
It's not that there's less oxygen in the air around you, it's that the molecules are more spread out. Colorado Springs is about 6,000 feet above sea level. At that altitude, you are breathing in around 21 percent less oxygen than you would at sea level.
As altitude increases, the concentration remains the same but the number of oxygen molecules per breath is reduced. At 12,000 feet (3,658 meters) the barometric pressure is only 483 mmHg, so there are roughly 40% fewer oxygen molecules per breath.
The oxygen saturation of hemoglobin determines the content of oxygen in blood. After the human body reaches around 2,100 metres (6,900 ft) above sea level, the saturation of oxyhemoglobin begins to decrease rapidly.
Altitude sickness, also called acute mountain sickness, happens when your body is unable to adapt to a low-pressure, low-oxygen environment — typically at about 8,000 feet above sea level. This can cause breathing issues and a host of other symptoms which can range from very mild to life-threatening.
Above 12,000 feet MSL each member of the flight deck crew must use supplemental oxygen during the entire portion of the flight at those altitudes (14 CFR § 135.89, § 135.157, § 121.327, & § 121.329). For best protection, you are encouraged to use supplemental oxygen above 10,000 feet MSL.
Somewhere between 30,000 and 40,000 feet the pressure around you becomes far too low to push those oxygen molecules across the membranes in your lungs, and you get hypoxic (altitude sickness). If you try to breathe 100 percent oxygen above 40,000 feet for very long without a special type of mask, you'll die.
The amount of oxygen in the air goes down as you climb higher above sea level and becomes very low at altitudes above 8,000 feet. If you travel to a high altitude, you may feel ill because the air has less oxygen in it.
At elevations more than 5280 feet above sea level people may start to feel the effects of altitude. This varies for every individual so some people may not feel effects until they reach elevations greater than 8000 feet.
This is due in part to the fact that, at high altitudes, atmospheric pressure is reduced, and oxygen is less available (commonly referred to as the air being “thinner”).
As we go far from the Earth, the layers of air become thinner. The layers of air closest to the Earth are very close to each other and hence are thick. The layers of air at higher levels are not so close to each other and hence are thinner.
Travelers with medical conditions such as heart failure, myocardial ischemia (angina), sickle cell disease, any form of pulmonary insufficiency or preexisting hypoxemia, or obstructive sleep apnea (OSA) should consult a physician familiar with high-altitude medical issues before undertaking such travel (Table 3-05).
At a high altitude, there is less oxygen available to breathe. This can pose risks to people with chronic obstructive pulmonary disease. A study in Respiratory Research defines a high altitude as any elevation greater than 1,500 meters (m) — about 5,000 feet (ft) — above sea level.
The initial euphoria is a result of increased dopamine, the neurotransmitter contributing to feelings of pleasure, when entering high altitude. Dopamine is a short-burning fire, and then it's gone, Goodwin explained.
The general aviation pilot flying an unpressurized airplane will not normally operate above 25,000 feet. In fact, its rare to see a personal aircraft not legally restricted to flight at or below 25,000 feet (FL250).
The decreased atmospheric pressure and low levels of oxygen can have very dangerous effects on a climber's body. Even though there is less oxygen available to the climbers, their bodies can learn to compensate. The human body survives by breathing air into lungs.
Today's fighter pilots therefore operate in cabins pressurized according to a pressurization schedule,15 they breathe up to 100% oxygen,15 and they wear and use pressure breathing equipment.
The highest certified altitude of an airliner was Concorde's 60,000 feet. Today some of the corporate jets can fly at 51,000 feet. Q: What is the highest cruising altitude allowed? A: Most airliners are limited to 45,000 feet or less.
The higher the altitude, the less oxygen there is in the air and the lower the overall air pressure is. If flights were not pressurized, passengers would be at risk of various physiological aliments. Because of this, federal regulations require that all commercial flights over 8,000 feet be pressurized.
At 25,000 feet the average EPT is 3 to 5 minutes. After about 20 minutes without supplemental oxygen, you will be pronounced dead.
At altitude, a very common reaction is increased urinary output. The body's kidneys sense the lower level of oxygen immediately and kick into high gear. The kidneys release a hormone, erythropoetin, that commands the bone marrow to produce more red blood cells to increase the oxygen-carrying capacity of the blood.
Above about 8,000 meters (26,000 feet), the human body cannot survive at all, and starts to shut down. Mountaineers call this altitude the "death zone." To prevent severe altitude sickness, mountaineers bring supplemental (extra) supplies of oxygen and limit their time in the "death zone."
When you start climbing, you're essentially doing single-leg squats with some cardio mixed in, and your heart rate quickly skyrockets. Your body suddenly needs more oxygen -- hence the feeling of being winded.
In studies spanning the globe, researchers have seen several consistencies in people living at higher elevations: they weigh less, have less cardiovascular disease and some types of cancer and live longer.