These risks are caused by :

• Being unable to breath fresh air naturally whilst submerged and relying on limited breathing gas supplies and fallable breathing equipment:
  • Drowning : being unable to inhale anything but water
  • Secondary drowning : can occur hours after a near drowning
  • Hypoxia : inhaling a breathing gas that contains too little oxygen to support consciousness
  • Anoxia : inhaling a breathing gas that contains no oxygen

• Barotrauma or pressure damage on descent:
  • Tympanum damage (damage to the eardrum caused by failing to equalize pressure in the inner ear).
  • Damage to the body's other air spaces, such as the paranasal sinuses
  • Squeeze damage to tissues adjacent to equipment air spaces, such as the eyes inside the air space of the mask

• Barotrauma or pressure damage on ascent:
  • Pneumothorax or collapsed lung
  • interstitial emphysema or gas trapped in the chest after lung damage
  • air embolism or air bubbles in the blood stream
  • Subcutaneous emphysema or gas under the skin tissue
  • Damage to other air spaces, such as the facial sinuses or ears

• Breathing gas at high pressure:
  • Decompression sickness is cuased by ascending too quickly to be able to eliminate excess nitrogen that had been adsorbed at depth
  • Nitrogen narcosis is caused by using breathing gases that contain nitrogen at depth
  • Oxygen toxicity is caused by using breathing gases that contain oxygen at depth
  • HPNS, High Pressure Nervous Syndrome or Helium Tremors caused by using breathing gases that contain helium at depth

• Impurities and poisons that may be present in breathing gas:
  • Carbon monoxide poisoning is due to compressor maintenance errors or input of impure air to the compressor
  • Carbon dioxide poisoning is due to incomplete elimination of carbon dioxide produced by the diver's own metabolism. This gas is normally safely exhaled to the environment and not re-inhaled in significant volumes.