Iron lungs and cNEP devices use negative pressure to aid breathing, but the similarities end there. An iron lung is a cylindrical chamber that encloses a person’s entire body except their head. The air pressure inside the respirator is alternately decreased and increased to manually cause the lungs to inflate and deflate when a person has difficulty breathing on their own. They were often used during the early 20th century for people with polio and others requiring breathing assistance.