As altitude increases, the air pressure decreases. The surface of the earth is the bottom of an ocean of air. The layers on top increase pressure so that at sea level a body is under 14.7 pounds of pressure per square inch. Moving up in altitude decreases weight of air that causes the pressure. At 18,000 feet, the pressure decreases to 7.4 pounds of pressure per square inch.
People experience this change in pressure when flying in an airplane. It is the force that causes pressure to build and become uncomfortable in the ears. The popping in the ear equalizes the pressure, allowing the traveler to be more comfortable.
The change in pressure presents special challenges for cooks. The air bubbles that cause baked goods to rise expand faster, reducing cooking time. Water boils at a lower temperature, so boiled foods need additional time for cooking.
Pressure changes affect humans due to a lack of oxygen. At the higher altitude, the individual tends to breathe heavily while his heart races. Athletes who practice at lower altitudes experience poorer performance when competing in Denver, which has an altitude of over 5,000 feet. Above 8,000 feet, the lack of oxygen and dehydration result in even more severe altitude sickness.