What issue could arise due to sinus problems during flight?

Sinus problems during flight can lead to ear or sinus pain primarily because of changes in air pressure. As an aircraft ascends or descends, the external air pressure changes quickly. The sinuses and the middle ear need to adjust to this pressure to maintain equilibrium. If the sinus passages are blocked due to congestion or other sinus issues, this adjustment becomes difficult, causing discomfort or pain in the ears and sinuses.

The pain arises from the inability to equalize pressure, leading to a painful sensation known as barotrauma. Symptoms may include a feeling of fullness, pressure, or sharp pain during ascent or descent. This phenomenon is commonly experienced by passengers when they have a cold, sinus infection, or allergies that affect the nasal passages.

In contrast, while the other options touch on relevant aspects of flight operations, they are not related to sinus problems. Increased speed and decreased fuel efficiency pertain to performance characteristics of the aircraft rather than physiological conditions of passengers. Carbon monoxide poisoning relates to air quality and engine performance but does not connect specifically to sinus issues that would arise during a flight.