Which of the following is an early sign of hypoxia in aircrew?

• A. Cyanosis
• B. Euphoria
• C. Tunnel vision
• D. Dizziness

Answer: (D)

When oxygen delivery to the brain begins to fall, the first clues you’ll notice are changes in brain function. Dizziness is a classic early sign because the vestibular system and higher cognitive centers are very sensitive to even mild cerebral hypoxia. That mild hypoxia can make you feel lightheaded, unsteady, or as if you’re spinning, signaling impaired brain performance before other, more visually conspicuous signs appear.

Cyanosis is unreliable as an early warning because skin or mucous membrane color changes only after significant deoxygenation and can be masked by lighting, circulation, or pigmentation, so it tends to be a later indicator. Tunnel vision and other visual changes occur as hypoxia worsens and are not as consistently present in the initial stages. Euphoria can occur in some individuals with mild hypoxia, but it’s variable and can mislead, whereas dizziness more reliably flags the need to act. So dizziness stands out as the earliest practical cue to descend or use supplemental oxygen.