As altitude increases, the air pressure and oxygen levels decrease, which can affect aircraft performance and passenger comfort. At high altitudes, the reduced oxygen levels can cause hypoxia or oxygen deprivation, which can lead to symptoms such as dizziness, fatigue, and confusion. Aircraft are pressurized to maintain a comfortable cabin environment and ensure adequate oxygen levels for passengers and crew. Pilots are also trained to recognize and respond to hypoxia symptoms and use supplemental oxygen as needed.