There is a small pneumothorax present on the radiograph to the left. It is located in the left pulmonary apex. If you compare the lucency of the lung fields in the first few intercostal spaces, you will notice that the left apex is slightly more lucent than the right. In the left third intercostal space, there is a thin white line (see magnified picture below) that represents the pleural surface of the lung. Increased lucency more peripheral to this line is the air trapped in the pleural space.