Oxygen is the gas your body uses to maintain life. Too little of it and you’ll pass out and eventually perish, but too much of it can also be harmful. As a gas on the periodic table, it is represented by the symbol O and has an atomic number 8. It is an important component of the air that you breathe. It makes up about 21% of normal air.
Nitrogen is what’s called a noble gas and at 1 atmosphere is inert, having very little effect on a diver. Air is about 78% nitrogen. Nitrogen gas is represented by the symbol N and has an atomic number of 7. It is a component of all living organisms. However, in its pure form, it has no vital use for humans above water. Underwater, nitrogen needs to be carefully tracked. Nitrogen gas that has built up in body tissues as you breathe at greater pressures at depth needs to be released from the body slowly as you ascend. If not, it can lead to decompression problems.
Many other gases make up the remaining 1% of compressed air. A few of them are argon, carbon dioxide, helium, hydrogen, methane, and neon. Except for carbon dioxide, most of these gases have no effect when underwater.