A tension pneumothorax usually happens because of an injury to the chest. For example, a knife or gunshot wound will cause a hole in a person's chest. Air then gets sucked in through that hole. The air starts to build up. It pushes on the lungs. If the knife or gun has also caused a hole in the person's lung, air will leak out of the lung. That air will also build up in the chest. When enough air is pushing on the lung, the lung will collapse.