SCUBA divers are taught the effects of increasing and decreasing pressure on their bodies and equipment as part of their initial training.
Except for the ears and the face mask, most other air spaces take care of themselves.
As a diver descends underwater, the increasing water pressure causes any air space to reduce in volume.
The space inside a diver's mask will also reduce in volume, causing the mask to press against the face.
This is especially true if the mask strap is too tight.
To equalise the pressure, a diver simply exhales into the mask via the nose and the pressure is released.
It is not uncommon to see a mark across the forehead of a diver after a dive.
This indicates either the diver didn't equalise the pressure correctly while descending or couldn't because his mask strap was too tight.
In severe cases of mask "squeeze”, eyes are noticeably red after a dive.
This generally occurs only when the diver has been poorly trained.
Often divers with ill-fitting masks will overtighten their strap to help seal their mask. This can be easily avoided by having a mask professionally fitted.
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