A flail chest occurs when a segment of the chest wall breaks uner extreme stress and becomes detached from the rest of the chest wall. It occurs when at least two ribs are broken in at least two places, so a part of the chest wall moves independently from the rest of it. The flail segment moves in the opposite direction as the rest of the chest wall: because of the ambient pressure in comparison to the pressure inside the lungs, it goes in while the rest of the chest is moving out, and vice versa. This so-called "paradoxical motion" can increase the work and pain involved in breathing. Studies have found that up to half of people with flail chest die. Flail chest is invariably accompanied by pulmonary contusion, a bruise of the lung tissue that can interfere with blood oxygenation.