The requirement for supplemental oxygen at altitude is not a simple, fixed number, but a threshold determined by various factors, including regulatory mandates, physical exertion, and acclimatization. This need is driven by hypoxia, a condition where tissues are deprived of adequate oxygen delivery, primarily caused by decreasing atmospheric pressure as elevation increases. The percentage of oxygen in the atmosphere does not decrease significantly at typical altitudes; the problem lies in the diminishing force that pushes oxygen into the bloodstream.
How Lower Air Pressure Affects the Body
The physiological need for supplemental oxygen stems from the reduction of the partial pressure of oxygen (\(\text{PO}_2\)) in the air we breathe. While oxygen consistently makes up about 21% of the atmosphere, the total atmospheric pressure drops significantly at higher altitudes. This means the absolute number of oxygen molecules in a given volume of air is lower, lessening the driving force that allows oxygen to diffuse from the lungs into the arterial blood.
At sea level, the partial pressure of oxygen is sufficient to ensure near-complete saturation of hemoglobin, the protein that transports oxygen. As altitude increases, the lower \(\text{PO}_2\) reduces the efficiency of this transfer, causing a drop in blood oxygen saturation (\(\text{SpO}_2\)). When \(\text{SpO}_2\) falls, the body experiences hypoxia, which impairs mental and physical function.
Early symptoms of hypoxia can begin to manifest around 10,000 feet, often starting with subtle impairment of judgment, difficulty with complex tasks, and fatigue. As the altitude increases, symptoms progress to include headache, dizziness, euphoria, and tingling sensations. Without intervention, continued exposure to low \(\text{PO}_2\) leads to severe confusion, loss of consciousness, and eventually death.
Legal Requirements for Aircraft Oxygen Use
Mandatory use of supplemental oxygen is strictly regulated in aviation, with requirements varying significantly for unpressurized and pressurized aircraft. These rules, set by bodies like the Federal Aviation Administration (FAA), establish clear altitude thresholds to prevent crew impairment and passenger distress. Commercial airliners are typically pressurized, meaning their cabins are artificially maintained at a pressure altitude equivalent to 8,000 feet or less, negating the routine need for supplemental oxygen.
In unpressurized aircraft, the rules apply to both the flight crew and passengers. Flight crew must use supplemental oxygen continuously when flying above 12,500 feet mean sea level (MSL) for any period longer than 30 minutes. They must use oxygen for the entire duration of the flight when operating above 14,000 feet MSL.
For passengers in unpressurized aircraft, oxygen must be made available if the flight lasts more than 30 minutes between 10,000 and 15,000 feet MSL, though only for a fraction of the occupants. Above 15,000 feet MSL, enough supplemental oxygen equipment must be available for every passenger for the entire duration of the flight at that altitude. Pressurized aircraft flying above 25,000 feet must also carry a minimum 10-minute supply for all occupants in case of emergency depressurization.
Voluntary Use for High-Altitude Exposure
Outside of aviation, the need for supplemental oxygen in activities like mountaineering and high-altitude trekking is determined by physiological response and the ability to acclimatize. Acute Mountain Sickness (AMS), the mildest form of altitude illness, typically begins above 8,000 feet (2,400 meters). Symptoms such as headache, dizziness, and fatigue are common in this range, especially with rapid ascent, and mild cases can often be treated by resting.
The decision to use oxygen in this context is voluntary and depends on the rate of ascent and individual physical tolerance. Acclimatization, the body’s gradual adjustment to lower oxygen levels, allows mountaineers to tolerate higher elevations over time. However, the body’s ability to adapt is finite, and above 18,000 feet (5,500 meters), physiological function deterioration becomes pronounced.
The most extreme limit is the “Death Zone,” generally considered to be any altitude above 26,000 feet (8,000 meters). In this zone, the atmospheric pressure is so low that the body consumes oxygen faster than it can be absorbed, making long-term survival virtually impossible. Supplemental oxygen is required for anyone attempting to stay at this extreme elevation, as the environment inevitably leads to severe deterioration.