Air is present at atmospheric pressure in many areas of the body. Hollow areas such as the ears, sinuses, intestines and others are directly affected by outside air pressure.

The pressure applied to the body decreases greatly as a person ascends closer and closer to the surface. The body may be unable to equalize the pressure between the water and the pressure inside a cavity, such as the middle ear, sinuses or lungs. As the pressure decreases, trapped air expands in the air cavities.

Less serious injuries include:

• Reverse ear squeeze
• Reverse tooth squeeze

The expanding air can cause serious injury to a body cavity. The diving term for this is called a pop.

More serious injuries include:

• Air embolism:
• Air collects in the bloodstream
• Pneumothorax:
• Collapsed lung
• Pneumomediastinum:
• Air collects around the organs in the chest
• Decompression sickness