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At approximately what altitude does night vision begin to deteriorate?

  • A

    10 000 ft

  • B

    3000 ft

  • C

    5000 ft

  • D

    7000 ft

Tolerance to hypoxia as a function of the altitude exposure is described based on the findings obtained both in laboratory and in flight as well as on the physiological and psychomotor effects observed. They can be divided into four altitude zones of tolerance:

Hypoxia StageAltitudeEffects
Indifferent Stage/ZoneGL -
10 000 ft
  • Night vision is impaired as low as 5000 ft.
  • Performance of new tasks is impaired.
  • Heart rate and breathing rate increase slightly.
Compensatory Stage/Zone10 000 ft - 15 000 ft
  • Increase in the respiratory volume.
  • Increase in cardiac output and blood pressure.
  • After a short time the central nervous system is affected causing: dizziness, impaired judgement and memory, reduced performance in tasks needing attention to detail or subtle movements.
Disturbance Stage/Zone15 000 ft - 20 000 ft
  • Euphoria
  • Dizziness and sleepiness
  • Headache
  • Fatigue
  • Memory impairment
  • Reduced performance
  • Loss of judgement
  • ‘Grey-out’ and tunnelled vision
Critical Stage/Zone20 000 ft - 23 000 ft
  • Mental clarity fades rapidly.
  • Confusion and dizziness set in within minutes.
  • Sudden loss of consciousness.

Note: The stages and respective altitudes may be expressed differently depending on the study source.

The night vision refers to the clarity of vision of the human eyes in a dark environment. The retina of the eye (inner layer) is extremely sensitive to oxygen deprivation. The retina contains cones responsible for daylight vision and rods responsible for night vision. The rods do not function well when oxygen is limited. As oxygen levels drop, visual acuity in the dark is impaired. This makes the night vision impairment one of the first symptoms of hypoxia even at altitudes as low as 5000 ft.

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