A ventilator is a machine that helps someone breathe when their lungs can't work properly on their own. In a hospital, doctors use ventilators to pump air in and out of a patient's lungs through a tube, doing the work that healthy lungs normally do automatically. Someone might need a ventilator after a serious accident, during a major surgery, or when fighting an illness that makes breathing difficult or impossible.

You ventilate a stuffy room by opening windows to let fresh air flow through. A ventilator does something similar for the body: it keeps fresh, oxygen-rich air flowing into the lungs and removes stale air that's full of carbon dioxide.

Buildings also have ventilators (often called ventilation systems) that move air throughout rooms and hallways, preventing spaces from becoming stuffy or filled with stale air. These ventilators use fans and ducts to keep air circulating, which is especially important in places like schools, offices, and submarines where many people share the same air for hours at a time.