I think this the most popular accident in [Scuba diving](Scuba%20diving.md), not because many people are victim, but more because this is the one mainstream documentary are relating to by saying that divers need to make decompression stop. Divers are breathing with a mechanism called a [regulator](https://en.wikipedia.org/wiki/Diving_regulator). The regulator is giving air at the ambient pressure. When we're on the surface, the pressure is at 1 atmosphere \(ATM\), so your regulator gives air at 1 ATM ==The pressure increase of 1 ATM every 10 m==. That mean that, when we're at -10 m, we're facing 2 ATM of pressure \(1 ATM of surface + 1 ATM of depth pressure\), -20, 3 ATM, -30, 4 ATM... and so on. One of the basic mechanism with pressure is that, ==when the pressure increase, volumes decrease and vice versa==. Deeper you go, higher is the pressure, and more your body is compressed. So where's the problem? Because the regulator is giving you gas at the ambient pressure, when you're -30 m deep, you're breathing air at 4 ATM. That means you're breathing 4 times more nitrogen and oxygen in your lungs than you were at the surface. Normally, nitrogen is a neutral gas at 1 ATM, it doesn't transfer to the tissue. But, here at -30 m, we're breathing 4 times more nitrogen, so the gas is forced to move to the tissue and then dissolve in our blood. If we're exposed to too much nitrogen, during a too long period, we start to have the [nitrogen narcosis](Nitrogen%20narcosis.md), which cause at first, the feel of drunkenness. But the biggest problem come when we're start to ascend. The pressure decrease causing the increase of volume, and here, the nitrogen that were dissolved in our blood because of pressure is forming bubbles that ascend our blood system \(just like what we're experiencing when opening a soda bottle\) in their turn. This is what cause the decompression sickness : the bubble that are ascending can go to the brain and cause a stroke \(which can lead to death 😱\). This is why divers need to make decompression stops. When we're doing a decompression stop, we're eliminating those bubble \(we call this desaturating\) to avoid the decompression sickness. The longer you stay at high pressure, the longer the stop need to be. In recreational scuba diving limited to -20 m, the pressure and the exposition time are not sufficient to force divers doing a decompression stop. We can lower the decompression stops time by using differents gas mixtures such as [Nitrox](Nitrox.md) or a [Trimix](Trimix.md). **Links :** * [Diving physiology and the respiratory system - gas exchange under high pressure](https://www.youtube.com/watch?v=9IZGNzj9S0w) * [About pressure in diving](http://www.scuba-tutor.com/dive-physics/pressure/) * [Decompression sickness - Wikipedia](https://en.wikipedia.org/wiki/Decompression_sickness)