Oxygen levels begin to decrease at altitudes above 1,500 meters (4,900 feet). This is where the International Society for Mountain Medicine defines "high altitude." While the percentage of oxygen in the air remains the same (approximately 21%), the partial pressure of oxygen decreases as the total atmospheric pressure decreases with increasing altitude. This means that there is less oxygen available to your lungs with each breath you take.
Here's a breakdown of altitude regions and the corresponding oxygen availability:
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High Altitude: 1,500–3,500 meters (4,900–11,500 ft). Oxygen decrease becomes noticeable, and altitude sickness can occur in susceptible individuals.
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Very High Altitude: 3,500–5,500 meters (11,500–18,000 ft). Significant oxygen decrease; acclimatization is necessary to avoid serious altitude sickness.
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Extreme Altitude: Above 5,500 meters (18,000 ft). Profound oxygen decrease; long-term survival is difficult without supplemental oxygen.
It's important to note that the effects of decreasing oxygen levels vary from person to person and depend on factors such as acclimatization, physical fitness, and pre-existing health conditions.
In summary, while oxygen percentage in the air doesn't change with altitude, the amount of oxygen your body can absorb begins to decrease at around 1,500 meters (4,900 feet), becoming more pronounced at higher elevations.
