Patients with lung disease experiencing difficulty breathing can be treated with oxygen therapy. This involves the delivery of "extra" oxygen by a face-mask or through small tubes placed in the nose called nasal prongs. This extra oxygen can have concentrations as high as 100% pure oxygen. The concentration of oxygen in normal air is only 21%. The high concentration of oxygen can help to provide enough oxygen for all of the organs in the body. Unfortunately, breathing 100% oxygen for long periods of time can cause changes in the lungs, which are potentially harmful. Researchers believe that by lowering the concentration of oxygen therapy to 40% patients can receive it for longer periods of time without the risk of side effects.