Looking back at the equation V_f/V_i = (Number of Feet Descended) / (33ft for freshwater or 30ft for saltwater) + 1, notice that if one breaths in as much air as they can at the surface, submerges to 33 feet freshwater, the percent of the original volume of air in your lungs is 50%. If you ascend to the surface, you would have 100% of the original volume of air in your lungs. This is twice as much as you had at depth. As such, if a diver breaths in as much air as their lungs can hold at 33 feet freshwater, and then ascends to the surface without exhaling, when they get to the surface, their lungs will be holding twice as much air as they can hold. Obviously, the air would have to escape somehow and that is what a lung overexpansion injury is.

Types:(Daves Outdoor Adventures, <http://www.doalasvegas.com/html/diving_injuries.html>)
There are 4 types. Damages to the lungs occur in all 4 of them. They are Mediastinal Emphysema which is when air gets into the chest cavity near the heart. This may cause a heart attack. Also there is subcutaneous Emphysema. This is when air gets near the neck. There are poke-able bubbles and a rash usually forms. However, nothing too serious happens besides the initial lung damage. Second to last is Pneumothorax. This is when the air that is released from the lungs gets into the chest cavity around the lung(s). It can cause one or both of your lungs to collapse. Last, is the arterial gas embolism. This is when the air gets into the bloodstream and causes a blockage. The effects and treatments of this variant are similar to DCS. The treatments are 100% oxygen until they get to a hospital. You give the person hyperbaric oxygen therapy if the injury is an Arterial Gas Embolism(Piantadosi). Otherwise, you wait and try and get the bubbles out of the chest, sometimes using a tube(Piantadosi).