A pneumothorax is air in-between the lungs and the chest wall, where there should not be any air. This air can press on the lungs and cause them to collapse. A pneumothorax is usually caused by a trauma to the chest like a rib fracture but it can also happen spontaneously by the bursting of alveoli from the inside out. If you have a pneumothorax, you will have a lot of chest pain and shortness of breath.