When many people think of air, oxygen is the first gas that comes to mind. After all, this is the gas that our bodies depend on to survive.

But nitrogen is actually the major component, and makes up about 78% of the air we breathe. Nitrogen is completely inert, so it has no positive or negative effects on our bodies under normal conditions. But as you'll learn later in this course, nitrogen can have negative effects on divers as a result of pressure change.

Oxygen is the gas we depend on for survival, and it makes up about 21% of the air we breathe. Oxygen can also have negative effects on divers, and you'll learn about those effects later in this course.

About 1% of air consists of miscellaneous gases such as carbon dioxide.