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Airplanes are pressurized because the air is very thin at the altitude they fly. The average passenger jet has a cruising altitude of about 30,000 to 40,000 feet. At this altitude humans can’t breathe very well and our body gets less oxygen. This is called hypoxia and can cause slower thinking, dimmed vision, and death. Another effect of a depressurized cabin can be “the bends.” This is the same thing scuba divers can get if they resurface too quickly. A dramatic drop in pressure can cause excess nitrogen to come out of the blood stream as gas bubbles. This can result in joint pain, paralysis, and death. To prevent all of these complications, the cabins of many aircraft are pressurized to near sea level pressure, which makes breathing in an airplane about the same as breathing at sea level.