Background
The region from sea level to around 10,000 ft (3,000 m) is known as the physiological-efficient zone. Oxygen levels are usually high enough for humans to function without supplemental oxygen and decompression sickness is rare.
The physiological-deficient zone extends from 12,000 ft (3,700 m) to about 50,000 ft (15,000 m). There is an increased risk of problems such as hypoxia, trapped-gas dysbarism (where gas trapped in the body expands), and evolved-gas dysbarism (where dissolved gases such as nitrogen may form in the tissues, i.e. decompression sickness). Above approximately 34,000 ft (10,000 m) oxygen-rich breathing mixture is required to approximate the oxygen available in the lower atmosphere, while above 40,000 ft (12,000 m) oxygen must be under positive pressure. Above 63,000 ft (19,000 m), also known as the Armstrong limit, fluids in the throat and lungs will boil away. Generally, 100% oxygen is used to maintain an equivalent altitude of 10,000 ft (3,000 m).
Read more about this topic: Pressure Suit
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