In scuba diving, air is delivered to your lungs at the surrounding water pressure. Breathing air at high pressure can cause a number of illnesses:

• lung overexpansion in which high pressure air trapped in your lungs expands as you ascend, damaging your lungs and possibly forcing air into your bloodstream
• decompression sickness ("the bends") in which dissolved nitrogen in your body forms gas bubbles as you ascend, blocking blood supply to limbs or organs
• nitrogen narcosis in which nitrogen causes impairment similar to drunkenness
• oxygen toxicity in which oxygen poisons your nervous system or lungs

All of these illnesses and their prevention will be discussed during diver training. Not all of them are fully preventable, hence you must be alert to the risk and know how to seek appropriate treatment. Always know how to contact the local emergency services and the diver emergency services (if they exist) before a dive.