The chest x-ray shows a linear density along the left lung field due to a skin fold, which can sometimes be mistaken for a pneumothorax.
There are old healed rib fractures on the right as well as infiltrates in the left lower lobe.
A skin fold results from the compression of redundant skin by the x-ray cassette. The skin fold produces an edge, whereas pneumothorax exhibits a visceral pleural line. The edge produced by the skin fold rarely continues over the lung apex, and most importantly, normal lung markings can be traced beyond the skin fold to the chest wall.
Skin folds are more commonly seen in bedside chest x-rays. If in doubt, the chest x-ray can be reproduced with proper patient positioning.
Reference: Fundamentals of Diagnostic Radiology by Brant and Helms.