On ascent, the air in the paranasal sinuses will expand according to Boyle's law, contracting during descent. Normally, the sinuses drain into the nasal cavity through small ostia, which permit mucociliary clearance and ventilation that equilibrates pressure. However, when the opening is obstructed due to inflammation, polyps, mucosal thickening, anatomical abnormalities, or other lesions, pressure equilibration is impossible. Squeeze is produced on descent when trapped air in the sinuses contracts and produces negative pressure. The pressure differentials are directed to the center of the sinuses producing mucosal edema, transudation, and mucosal-or submucosal-hematoma, leading to further occlusion of the sinus ostium. The sinus will fill with fluid or blood unless the pressure differential is neutralized.