Understand basic gas laws as it relates to freediving
Understand basic gas laws as it relates to freediving and the changes that occur to your body.
Understand the cause behind decompression illness (the bends) and how to avoid it.
A fundamental concept to understand for safe freediving practices is that your body will undergo changes because of increasing pressures, relative to the depth you dive. The deeper you dive, the greater these effects will be.
Pressure and Volume
When you are on the surface of the water, your body and its air spaces are at a normal size. The surface of the water can be referred to as at sea level or 1 atmosphere of pressure.
As you descend under the water, the water pressure increases and as it increases the size of the compartment, carrying a gas reduces.
For example, if you took a balloon full of air from the surface of the water, down underneath the water, it would become smaller as the water pressure increases. The further down you go, the smaller the balloon will become. Regardless of the size of the balloon, the amount of gas within the balloon will be the same, but it will be more compressed together. If you were then to take the balloon back up to the surface, then you will see the balloon expand back to its normal size.
What the picture above depicts is a normal size balloon at 0m, 1/2 size at 10m deep, 1/3rd the size at 20m and 1/4 the size at 30 m. This will continue to occur, the deeper that the balloon descends. Remember this concept in term of your lungs and other body airspaces, because this is exactly what occurs to them as well.
Pressure and volume as it relates to the body's air spaces
Your body is mainly made up of water and solid tissue matter, but there are airspaces, such as your lungs, ears, sinus's, dive mask and wetsuit that pressure and volume changes occur too.
It is important to compensate for these changes that occur in these spaces through a process known as equalisation, which will be detailed later on.
Effects of pressure on gases as it relates to freediving
The air you breathe in can be divided into 79% nitrogen and 21% oxygen, with trace elements of other gases. As you descend, the pressure of the water compresses these gases, making them smaller and more concentrated.
When you ascend, these gases will go back to their normal size and concentration, much like the diagram of the balloon. This brings us to an important concept, of tissue injury caused by expanding gases trapped within the tissues. Which is extremely rare for freediving but a significant risk if you SCUBA and freedive on the same day.
Nitrogen build up and tissue injury (The Bends / Decompression Sickness).
Nitrogen is known as an inert gas, which means that the body does not use it for anything other than to splint open gas passages. It is also highly dissolvable, so when you add increasing pressures due to depth, the nitrogen moves from your lungs and bloodstream into the body's tissues, commonly around joints.
When a SCUBA diver comes back to the surface, they perform what is known as safety stops along the way, which is determined by the depth dived and how long they diver has spent at that depth.
The purpose of safety stops is to allow the dissolved nitrogen in the tissues, to turn back into a gas and move into the lungs to be exhaled.
This occurs over and over again until the SCUBA diver reaches the surface. A SCUBA diver must never ascend too fast, unless it is an emergency. Otherwise, they run the very real risk of decompression sickness.
When only Freediving, potentially harmful effects from this occurring are extremely unlikely to occur because freedivers do not spend extended periods under the water constantly breathing from an external air source, as opposed to SCUBA divers. However, there have been reports of this occurring when the Freediver has performed frequent, repetitive deep dives without an adequate surface interval.
Additionally, you should never SCUBA dive either before or after freediving, as this can cause the nitrogen to build back up again rapidly, causing an emergency. Most experts agree that a time of 24 hours should be left between either activities.
Partial Pressures
Most gasses are made up of a mixture of gas types and each of these individual gases add to the total pressure of the mixed gas, this is known as Dalton Law.
For example, imagine the vial P1 has 100 pieces of gas and P2 has 200 pieces of gas. If you add them together, you have a total of 300 pieces of gas (in vial P3). Therefore, you could say that the partial pressure of vial P3 is 300, made up of the partial pressure of P1 at 100 and P2 at 200.
This is exactly how the air you breathe can be calculated, and now that were adding pressure changes due to depth, this is a principle that must also be remembered.
For example, if I was to take these vials down to 10m depth (2 atmospheres of pressure) their pressure would double (P1 = 200, P2 = 400 P3 = 600), and this will continue to occur with every atmosphere of pressure that you descend.
So why is this relevant to freediving?
Because your body does not rely only on the volume of gas, but the pressure of that gas!
This means that when you descend, the pressure of 'air' (made up of oxygen, nitrogen and trace elements of other gases) increases the deeper you go.
Think of this like the body getting super concentrated amounts of oxygen, that saturates the tissues and provides ample amounts of oxygen the deeper that you go.
However, you must ascend to the surface at some stage and this oxygen, has been getting used by the body to function.
So, as you, the freediver comes back to the surface and the gas pressure goes back to normal, you risk an immediate lack of oxygen, causing what is known as a blackout.
This is why the last part of the dive, the ascent, especially the last 10m is the most dangerous part of the dive, due to the sudden, massive changes in pressures.
Further Reading and References
Boyles Law
Daltons Law
Decompression Sickness
Stat Pearls
Cooper JS, Hanson KC. Decompression Sickness. 2020 Jul 29. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan–. PMID: 30725949.
DAN - Divers Alert Network.
Emergency Med Journal
Pollock NW, Buteau D. Updates in Decompression Illness. Emerg Med Clin North Am. 2017 May;35(2):301-319. doi: 10.1016/j.emc.2016.12.002. Epub 2017 Mar 15. PMID: 28411929.
