What is the makeup of normal diving air in terms of nitrogen percentage?

• A. 79% Oxygen, 21% Nitrogen
• B. 79% Nitrogen, 21% Oxygen
• C. 60% Nitrogen, 40% Oxygen
• D. 90% Nitrogen, 10% Oxygen

Answer: (B)

Normal diving air is basically the same as the air we breathe on the surface: nitrogen makes up about 79% and oxygen about 21%, with trace amounts of other gases. This means the breathing mix has a nitrogen fraction near 79%, not 21% or any other value. The reason this matters is that while the percentage stays roughly the same, depth changes the pressures involved, which affects how much gas is dissolved in the body, particularly nitrogen. So the correct understanding is that nitrogen is the major component, around 79%, with oxygen about 21%.