Why some Gases are Used and How

Nitrogen is present in air and is used in Nitrox. There are several different mixtures of Nitrox: Nitrox 32(32% oxygen), Nitrox 36(36% oxygen). 

However, if not properly monitored Nitrogen Narcosis can occur. "The Rapture of the Deep" may begin around 30m when breathing air and about 40m when using Nitrox 32.

Symptoms include:

Slowing of reactions, Excitement, Euphoria, Giggling, Over Confidence, Memory Impairment, Difficulty with Concentration and a Tendency to Fixation of Ideas...to get an idea of what nitrogen narcosis is like then CLICK HERE

Other inert gases are also narcotic:

(Table taken from "The Physiology and Medicine of Diving")

Helium and Neon are virtually narcotic-effect free. Hydrogen is less narcotic than Nitrogen, but more so than Helium or Neon.

Oxygen makes up 21% of air, therefore the use of this gas inevitable when using compressed air. The partial pressure at which oxygen becomes toxic varies from person to person  but 1.6bar is about a safe maximum. Oxygen toxicity symptoms range from twitching to convulsions (which if in water can lead to drowning by loss of mouthpiece).

Heliox as mentioned earlier is a mixture of Helium and Nitrogen. However, if the body is exposed to Heliox at depths greater than 152m then High Pressure Nervous Syndrome (HPNS) may result. The symptoms of HPNS are dizziness, nausea, vomiting and trmors in the hands, arms and torso.

Most gas related diving injuries are cured by placing the patient in a hyperbaric chamber: