Classical appearance and presentation of a left upper lobe collapse.

The lobe collapsed anteriorly giving a 'veiled like' appearance to the hemithorax and often as in this case discernable volume loss.

In a smoker, the cause is considered lung cancer until proven otherwise.  It may be apparent on the chest x-ray as in this case with a central obstructing hilar mass.  In other cases CT chest or bronchoscopy reveal a endobronchial mass.

Other less common causes include mucus plugging.