Decompression sickness The air that you breath when scuba diving is approximately 21 percent oxygen and 79 percent nitrogen just like the earths atmosphere. As you descend and the pressure increases, this ratio stays the same however the amount of particles in one breath increases as the pressure also increases. Because the density and pressure are related by a constant, the term partial pressure is used to describe how many particles of a substance are in your body at anytime and at any depth. This means that the partial pressure of nitrogen goes up as you descend because as the pressure increases you inhale more nitrogen atoms with every breath. This can be dangerous because your body does not use nitrogen like it uses oxygen in its cells so it takes longer to get all the extra nitrogen out of your body. Because it takes longer to get this nitrogen out of your body, it is important to ascend slowly so that you give your body plenty of time to push out all of the extra nitrogen gas. If you rise too quickly however and there is too much nitrogen in your body without the pressure to keep it compressed. This can cause it to form little air bubbles inside of your blood and tissues. These little bubbles can then move around, join with each other to grow larger or get stopped in narrow parts of your bloodstreams like joints cutting off blood flow. Because of this, time at great depths is limited so that the amount of nitrogen you take in at high pressure is limited to the amount that your body can handle. This is also the reason why you take a 3 minute safety stop at 15ft so that your body has a little extra time to push out the nitrogen and prevent decompression sickness. Below is the chart that shows how long you can stay at a depth before you need to surface or face a high chance of decompression sickness. Note these times can be increased by not spending all the time at the max depth, instead you spend only a few minutes at the extreme depths then move up to a shallower depth for the rest of the dive. The time at depth must still be accounted for however so do not think you can spend the max time at each depth before you move up.