• Welcome to the DeeperBlue.com Forums, the largest online community dedicated to Freediving, Scuba Diving and Spearfishing. To gain full access to the DeeperBlue.com Forums you must register for a free account. As a registered member you will be able to:

    • Join over 44,280+ fellow diving enthusiasts from around the world on this forum
    • Participate in and browse from over 516,210+ posts.
    • Communicate privately with other divers from around the world.
    • Post your own photos or view from 7,441+ user submitted images.
    • All this and much more...

    You can gain access to all this absolutely free when you register for an account, so sign up today!

Exhale diving, theory and practice, a laymans view

Thread Status: Hello , There was no answer in this thread for more than 60 days.
It can take a long time to get an up-to-date response or contact with relevant users.


Well-Known Member
Jan 21, 2003
Sebastian Murat pioneered the theory and practice of exhale diving and posted a great deal of information about it on these forums. I have gone back and looked at those threads repeatedly, always find something I had missed.

I can no longer find some of those threads. If they are gone, it represents a great loss of information to all divers. Today there isn't any clear source to explain the theory or practice of exhale (FRC) diving. Below is my layman's attempt to summarize both theory and my kind of practice. My apologies in advance for any errors or omissions.

What is exhale (FRC) diving? Freediving with substantially less than a lung full of air. FRC is a specific definition, the amount of air in the lungs following a passive exhale. It is not a good measure to use in exhale diving because it varies so much among divers and with the conditions the diver is in. In some FRC is nearly the same as empty. In others it is around 40 percent.

Most exhale divers, no matter what they call it, dive with around 50 percent of a lung full, somewhere in the 40-60 percent range. Exhale diving also requires a certain style that uses as little energy as possible during the first half of the dive, allowing time for DR to set in hard, best combined with a breathup that minimizes ventilation and exertion, maximizes relaxation and eliminates hyperventilation.

What it is not:

This will spark some disagreement, but exhales for training purposes are not exhale diving. They are training for depth and warm up for whatever kind of diving you are doing. These exhales are usually full exhales, although not always. They speed up the aclimitization process so longer and deeper dives are more comfortable. They do not take advantage of physics like exhale diving, nor do they(when used with full lung diving) have the long term physiological effects of exhale diving.

Why does it work?

It adapts freediving to take advantage of the physics of the dive and the physiology of the dive response (DR). By doing so, it allows much more efficient use of the O2 available to the diver and reduces the negative impacts of c02. The difference in both is more than enough to make up for lower lung volume, 02 and co2 storage capacity.

How does it work?

Physics: Diving with half a lung full(compared to full lung) implys that the diver is less buoyant at the surface, that there is less buoyancy change as he descends and the the depth of neutral buoyancy is less.

In practice, the exhale diver uses much less energy (and o2) to make the same descent. As an example, full lung divers doing 30 m dives are very buoyant at the surface and should be neutral at 10 meters. They have to swim down hard, fighting buoyancy to reach 10 m and continue kicking lightly to 20 meters on the way to a 30 meter dive. A half lung diver would be neutral about 4 meters, a good surface dive and two soft kicks is all he will exert to reach the same depth. The difference in exertion, 02 consumption and c02 production is large. At depth, because buoyancy change is so much less, both divers will be about the same level of negative buoyancy. The swim up takes roughly the same level of exertion until the divers are above10 meters. From there up the exhale diver will be exerting himself more, but the difference is not great and, so late in the dive, it has little if any effect on o2 concentration in the brain.

Physiology: Blood shift is the primary aspect of DR that makes exhale diving work. Blood shift occurs when the peripheral arteries are contracted while the peripheral veins remain open. The result is minimal blood flow to the skeletal muscles and collection of blood volume in the core, primarily the lung blood vessels. Remaining O2 in that blood and in the lungs is effectively reserved for the brain and heart. At that point, exertion of the skeletal muscles in the legs and arms occurs in a low O2 environment and exertion shifts from aerobic to anaerobic, further conserving O2.

Blood shift is promoted by many factors including rising co2 levels, small lung size, negative pressure in the lungs, low levels of exertion and relaxation. All of these are promoted or more effective during a correctly executed exhale dive. The result is a much faster onset of blood shift and a much stronger shift, conserving 02 for the brain and heart.

The combination of taking advantage of the physics and blood shift can more than make up for the lesser quantity of O2 in the lungs of an exhale diver.

To put it all together, comparing full lung to exhale diving:

In full lung diving, at the onset of the dive there is plenty of O2 and minimal co2 in the lungs, the body doesn't know it needs to conserve o2, Further. most full lung divers breathup is less relaxed,more aggressive and energy intensive than an exhale diver,.so their burn rate of 02 (and co2 production) is higher. Swimming down against full lung buoyancy burns more O2 before the body realizes it needs to conserve and the c02 from that exertion flows easily and quickly back to the core where sensors in the chest feel the surge of co2 and finally begin conservation efforts, blood shift, etc. But its a little late, much(or more) of the extra o2 in the lungs has been burned. Worse, the co2 from that burn has gotten back to the chest sensors, making the diver uncomfortable, cutting the dive short unless the diver is capable of resisting a strong urge to breathe, which puts him at risk of BO because, at this point the full lung diver has less 02 available to the brain than the exhale diver.

Contrast this with a half lung exhale diver who has a very minimal breathup such that the co2 level in his blood is slightly elevated relative to most full lung divers. He is much more relaxed and has spent much less energy breathing up. He dives and descends much more relaxed with much less energy expenditure. Therefore, his rate of c02 production(and 02 burn) is much lower, both at the onset of the dive and thereafter. Increasing pressure (and decreasing lung size) plus the slightly higher starting c02 level aided by greater relaxation kick off blood shift faster and harder, conserving o2 in the core and readying the skeletal muscles to operate in primarily anaerobic mode once the divers begins to exercise. Once the exhale diver begins to exercise, blood shift has reduced the degree to which co2 and other waste products can return to the core, muscles operate at near anaerobic levels, using less o2 even though effort may be higher than the full lung diver as he begins his ascent. Heart rate slows, using less core 02. Co2 level that started out slightly elevated rises much more slowly than in a full lung diver, keeping the exhale diver comfortable for a longer period, extending dive time. O2 conservation allows that extra time to be safe with less chance of a BO.
Last edited:
Thats the basics, but there is much more, some of which I understand and some not. Here is some of it.

Exhale diving works best when you do a lot of it over a long period, because the body goes through various physiological changes the promote exhale diving. One is thickening of the corpuscles surrounding the alveoli in the lungs. This sets up a virtuous circle. Repeated blood shift stretches (stresses) the corpuscles, which grow thicker as they recover. That thickening means they can stretch even more, allowing greater blood shift(and greater depth) and allowing longer dive times.

Diving exhale reduces transfer of N2 and CO2 from the lungs to the blood stream as pressure increasing during the descent. This reduces nitrogen narcosis and the co2 hit that many divers get if they descend quickly into the 30 meter range. Narcosis, both from co2 and nitrogen is present, but not recognized by most divers in relatively shallow depths, like 25-30 meters. Many exhale divers report much clearer heads at depth that appears to be a result of less narcosis. Those doing much deeper dives report this to be a major factor.

Blood shift in exhale divers is cumulative, slowly building up through multiple dives. It may be cumulative in full lung divers as well, but to a much lessor degree. No one I know has explored what this means for exhale diving performance, but my experience and that of other exhale divers strongly suggests it aids performance.

Genetic factors affect exhale diving. Dive reflex(blood shift) is critical to exhale diving. How strong an individual divers DR is has a substantial genetic component. Some divers are just better at this. Training DR helps, but genetics are important..

Where does it work best ?

It has been reported that it does not work for static. Dynamic may close to the same, although some very long dynamics have been done with exhale.

IMHO,the best application is serial, recreational diving in moderate depths, down to 30 meters. Some exhale divers do comfortable recreational dives to 40-50 meters, but these are the exception.

Where does it work least well?

When current, rough conditions, generally the need to make very aerobic dives, all prevent the conditions that correct exhale diving requires. That said, at least one exhale diver uses the technique for underwater hockey, about as aerobic an activity as you can imagine. Most exhale divers find themselves making a smooth transition from fully exhale technique to full lung technique as conditions change. This also takes some practice.

What are the advantages of exhale diving? In the right setting, with the right technique, exhale dives are longer, more comfortable, and safer. Something that is more difficult to quantify is the “feeling” exhale divers get of being more in tune and part of the underwater environment, not fighting it, going with the flow.

What are the drawbacks of exhale divng? Restricted depth and the long adjustment period needed to get the full benefit of the technique. Exhale diving “feels” quite different from full lung diving. The warning signs divers recognize in their bodies are different and it takes a while to learn a new set of feelings. Squeeze is a major danger for beginning exhale divers. Squeeze can happen much faster and at much shallower depth than in full lung diving. The danger declines with practice as the exhale divers depth range increases and he learns how to deal with the early warning signs of squeeze.

Correct technique and practice are a necessity to make exhale diving work and a buddy is even more important than normal during the learning process. Exhale diving is less 'flexible”, has a narrower area in which it works well than does full lung diving. This type of diving must allow for relaxed breathups and near static conditions in the first part of the dive. Both things are critical, but especially making the first part of the dive a near static

Depth restriction is the most obvious and most annoying and potentially most dangerous drawback to exhale diving. If you can equalize full lung to a maximum of 40 meters, all of a sudden your depth limit at half lung is 20 meters and pushing beyond that can lead to squeeze very quickly. With time and practice, chest flexibility improves and the lungs become capable of storing more blood(occupying space that would have been air). The combination allows the diver to go deeper. It is normal for divers to recover all or nearly all of their depth range, again with time and practice.
Last edited:
Thank you so much for this useful summary and exposition, cdavis.

As a beginner, I'm particularly concerned with equalization in depth-oriented freediving. With regard to the last paragraph, a purely physics-oriented approach would suggest that if you can equalize full lung to 40m, at half lung the depth is reduced to 15m (2.5 atm) -- I wonder if you and other exhale divers would confirm this, or whether some other factors come into play that make it possible to push a bit deeper (and of course I realise that "half lung" is an approximation, but the effect could be even stronger if you say that it's up to 40%). Thanks in advance!
Way too many approximations. The 40/20 example was purely for example purposes. When I started, I could equalize fairly easily to about 50 ft, but 70 was a killer and required me to strain hard, not safe. Thats slightly over half of my full lung pb and I was diving about 60 percent of full lung. So, roughly, it fits.

In practice, you can get funny results. When I had been exhale diving for something over a year and had recovered a substantial portion of my depth range, I tried a full lung max dive, figuring I could easily hit a full lung pb and maybe get to 150 ft or so. Did not work that way. The whole dive, from the surface down, felt weird and uncomfortable. I bailed out of the dive at 115 ft, less than my full lung pb.
cdavis this is a great summary! Thank you!
Can you explain the difference in contractions timing (I assume you get it much earlier in the dive)?
Another question is: do you know if exhale diving is used for spearfishing?
Spearfishing: yes, I do. Others as well. Spearfishing requires care with exhale diving. You need to make sure your DR is strong before beginning to exercise fighting or chasing a fish.

My contractions come earlier, are much softer to start and build up more slowly. That seems to be typical. Mine tend to be very short interval between contractions, much more so than full lung diving, not sure why.
I like to call it "underwater Wim Hof" I plan to give it a go one day but know how easy it is to black out "comfortably" with that method v/s screaming co2 levels so certainly don't want to try that one alone in the water.
I wasn't familiar with Mr Hof, so did a little google. Unless I'm missing something, Hof method is the exact opposite of what you do in exhale diving. And you are right, way too much risk of BO with his methodology.
I was going to say.. Wim Hoff is claiming (watch his podcast on the "Joe Rogan Experience" ) that hyperventilation can drastically increase your body's capacity to store oxygen which is false. The reason he does his breath holds on an exhale are because after the hyperventilation his blood is extremely alkaline and he looses the bohr effect to a degree that an full inhale would cause him to blackout within the first 5-10 seconds of the breath hold. An exhale hold allows you to keep conscious despite the loss of the bohr effect. (i've messed around with this, DRY, after seeing his interview and came to those conclusions after a few hypocapnic BOs on inhale).

For exhale diving, especially depth and dynamic, its important to breath as little as possible because for multiple reasons the CO2 will help keep you conscious. With static its much less noticeable but as soon as you start moving on an exhale hold, any sight over breathing will knock off a huge portion of your capacity.

also, you shouldn't try anything ALONE the water
  • Like
Reactions: Kodama
Ahaaaaaa.... "I see," said the blind woman. Less guru's and more clinical science from now on.
DeeperBlue.com - The Worlds Largest Community Dedicated To Freediving, Scuba Diving and Spearfishing


ISSN 1469-865X | Copyright © 1996 - 2024 deeperblue.net limited.

DeeperBlue.com is the World's Largest Community dedicated to Freediving, Scuba Diving, Ocean Advocacy and Diving Travel.

We've been dedicated to bringing you the freshest news, features and discussions from around the underwater world since 1996.