An iron lung is a large machine that enables a person to breathe when normal muscle control has been lost or the work of breathing exceeds the person's ability. Properly, it is called a negative pressure ventilator.

Nearly the whole body, except for the head, fits into one of these machines. The machine is airtight and creates a negative pressure inside which increases the volume of the chest cavity and pulls in air through the nose or mouth. Then the pressure inside the iron lung increases, causing the air to be expelled again. Therefore, it is considered a non-invasive therapy.

Iron lung ward filled with Polio patients, Rancho Los Amigos Hospital, ca. 1953

Iron lungs were used in the past for victims of polio, a virus that paralyzes muscles, including the diaphragm (needed for breathing).

Many polio patients used the machines only during the acute phase of the disease; the muscle action would later slowly return. Some would go on to use the machine only at night, others had to use it permanently. The first time an iron lung was used was on October 12, 1928 at Children's Hospital, Boston. The vaccine, made by Dr. Jonas Salk, was available to the public in 1955.

The iron lung has a marginal place in modern respirathory therapy. Most patients with paralysis of the breathing muscles use mechanical ventilators that push air into the airway with positive pressure.

The negative pressure ventilator is still indicated for certain patient groups.