Which of these is a key sign of needing to use supplemental oxygen?

Drowsiness and confusion are significant indicators that a person may need supplemental oxygen. These symptoms often indicate that the brain isn't receiving enough oxygen, which can be caused by low levels of oxygen in the blood. Oxygen is vital for brain function, and when levels drop too low, cognitive functions can become impaired, leading to these signs.

While increased heart rate, dry mouth, and skin color changes can also be relevant in assessing a passenger's condition, they are not as directly tied to the immediate need for oxygen. Increased heart rate might indicate anxiety or excitement, dry mouth could be the result of various non-critical factors such as dehydration, and color changes in the skin can signal other conditions. However, drowsiness and confusion are more specifically associated with hypoxia, or insufficient oxygen, which makes them a clear warning sign that supplemental oxygen may be necessary.