Changes in absolute pressure also affect air density. For example, if the pressure doubles, the volume decreases to one half. Since the same amount of air now occupies half the space, the density is doubled.

Air density is directly proportional to absolute pressure. So if pressure doubles, density doubles as well. The chart to the left illustrates the relationship between pressure, volume, and density.

As the air becomes denser, it doesn't flow as easily through your regulator and body's air passages. This increases breathing resistance, which means you have to work harder to breathe than you do on land. While you'll notice the change in breathing resistance, for most divers it's not a problem unless they exert themselves.