Table 1 below shows the balance between gas volume and gas density in a flexible container, like a lung. As the pressure increases with depth, the volume will decrease, and the density will increase. This is what happens with any gas space in your body and equipment, for example, your ears, sinuses, mask, and lungs.
Table 2 shows that even though a diver's lungs stay the same size at depth (the same volume), the volume of gas needed to fill them increases proportionally as depth increases (Boyle's Law in action). This is because as the ambient pressure surrounding a diver increases, the air in her or his lungs has to increase as well. Increased pressure means the gas is denser, and denser gas means more gas. Therefore, as a diver ventures deeper, he or she consumes more gas per breath.
As absolute pressure decreases during ASCENT, the gas volume in your lungs will increase proportionately. So, the volume of gas expands. If a diver should hold his or her breath at 10 meters/33 feet and ascend with 8 liters of compressed gas inside lungs that can hold only 4 liters (as in Table 1), the expanding gas would cause serious damage to the diver’s lungs.
Have you noticed anything? The biggest pressure and gas volume change is between 10 meters/33 feet and the surface. The volume of gas doubles on ascent or halves on descent. In only a couple of meters or a few feet, the pressure differential is large. Because of the rapid change in just 10 meters/33 feet, this is the area where you have to be the most cautious, especially with regard to barotraumas (lung overexpansion), which can occur when you hold your breath.