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The Case for Biochemical Decompression

How much do you fart during decompression? How about your teammates? It turns out that those may be critical questions if you’re decompressing from a hydrogen dive, or more specifically hydreliox, a mixture of oxygen, helium, and hydrogen suitable for ultra-deep dives (Wet Mules, are you listening?). Here the former chief physiologist for the US Navy’s experimental hydrogen diving program, Susan Kayar, gives us the low down on biochemical decompression and what it may someday mean for tech diving.



by Susan R. Kayar, PhD
Header courtesy of A. Tocco Comex

Thirty years ago, the Naval Medical Research Institute (NMRI) in Bethesda, Maryland, hired me for what at the time I thought was the coolest job I could ever be asked to do.  I still think so.  I was hired to be the physiologist for their experimental hydrogen diving program.  Why dive with hydrogen?  A recent InDepth article by Reilly Fogarty, “Playing with Fire: Hydrogen as a Diving Gas”, does an excellent job of explaining this subject.  The short answer: because hydrogen is the smallest molecule. 

One  might think that in an era with excellent one-atmosphere hard suits, and multiple forms of submersibles and robotics, there is no need to send bare-naked divers to the sorts of depths involved in hydrogen diving, as will be described shortly.  If these alternatives to divers are so great, why do we still use commercial divers at all?  One needs to ask an operational person this question, rather than a scientist like me.  But I think the words “logistics”, “costs”, “safety,” and “the direct human touch” would figure in the answers.  

Just a snapshot of the enormous efforts needed to send works to the oceans floor. Photo courtesy of A. Tocco Comex.

Once a diver dives deep enough to exceed safe limits with regard to nitrogen narcosis, the usual gas switch for the diluent to oxygen is helium.  However, if a diver keeps on going into the range of 1000 to 2000 feet of seawater (roughly 300-600 msw), a helium-oxygen gas mixture becomes dense enough that the work of breathing becomes difficult.  Divers fight to move this dense gas into and out of their lungs, making the effort to breathe a serious source of fatigue and a distraction to their assigned jobs. (See “Maintaining Your Respiratory Reserve,” by John Clarke). Hydrogen is a diatomic molecule (i.e. H2) with two protons and no neutrons, and is therefore half the molecular weight of helium, a monatomic molecule with two protons and two neutrons.  Therefore  replacing helium with hydrogen, eases a diver’s respiratory distress i.e. work of breathing.  

There is also a phenomenon of ultra-deep diving known as High Pressure Neurologic Syndrome, or HPNS, (also known as High Pressure Nervous Syndrome) which is evidently a function of high pressure interfering with the transmission of signals in the nervous system.  Symptoms of HPNS can range from tremors to confusion to psychosis and are highly variable in depth at onset and from diver to diver.  For unknown reasons, hydrogen at high pressure is narcotic and can suppress HPNS.  Past a very high pressure that again varies with the diver, but generally on the order of 23 atmospheres partial pressure of hydrogen, its narcotic properties can become overwhelming and have their own psychotic effects.  

There are also serious issues involving the explosivity of hydrogen in combination with oxygen, but these issues are manageable with the care one always uses in handling oxygen and other combustible and hyperbaric gases.  Hydrogen and oxygen can be combined safely if the oxygen content is less than 4% of the gas mix, with dive operations usually opting for 2% oxygen as their safe upper limit.  A 2% oxygen mixture is breathable if the total pressure is 10 atmospheres (roughly 90m/295 f) or more.  This is normally accommodated by starting a pressurization with helium and then switching to hydrogen after 10 atm.  As a final consideration, the price of helium is rising, and may make hydrogen substitution increasingly attractive.  Consequently, for a variety of practical reasons, hydrogen has a potential place in ultra-deep diving beyond 10 atmospheres of pressure.

Investigating Biochemical Decompression

As the physiologist to the hydrogen diving program at NMRI, my assignments were two-fold: first, to determine if there are any dangerous biological effects that had been previously overlooked of breathing hyperbaric hydrogen,  and second, to look into something that NMRI was calling “biochemical decompression,” or “biodec,” a term they had coined themselves.  

Susan Kayar at her workplace. Photo courtesy of Susan Kayar.

The unknown dangerous biological effects portion of the research was addressed first.  The short answer to that was “none”.  We found no evidence that inhaled hydrogen could participate in any unwanted biochemical reactions in the body, discounting whatever reactions eventually make hydrogen narcotic.  We still do not know exactly why hydrogen becomes narcotic, but it is unlikely from the physical properties of hydrogen that its narcotic effects are permanently harmful post-dive.

Then we got to the really exciting part of the hydrogen research program at NMRI: biochemical decompression.  A few years before I was hired in 1990, a biochemist at NMRI, Dr. Lutz Kiesow, heard it was possible for divers to use hydrogen as a breathing gas.  He knew there were many microbes that possessed a hydrogenase enzyme allowing them to consume hydrogen gas as a metabolic source equivalent to the consumption of oxygen as a metabolic source for most land organisms.  End products for hydrogen metabolism can vary with the microbe, but is often methane (CH4).  Hence, as a class, such microbes are called “methanogens”.  

Dr. Kiesow proposed that NMRI establish a research project to isolate the hydrogenase from a methanogen, and insert it somewhere in the body of a diver to effectively create  a chemical scrubber unit for hydrogen.  If a diver could continuously scrub out some of the hydrogen going into solution in his body during the dive, the diver would have a reduced body burden of inert (to the diver) gas, and could subsequently decompress more rapidly with lower risk of decompression sickness (DCS).  

What a cool concept!  I loved it from the moment I heard it.  But the real challenge was to resolve Dr. Kiesow’s “somewhere in the body” requirement into a safe, readily reachable, functionally useful body location.  The director who hired me understandably warned me that divers would be opposed to receiving routine injections, or any sort of biological implant making them Bionic Men, permanently different from their former selves or from other divers. So what was left?  

Susan Kayar today, sharing her knowledge with the world. Photo courtesy of Susan Kayar

On my first musings with the scientific head of NMRI when I was hired, I wondered if we could perhaps encapsulate the hydrogenase enzyme, or better yet just whole methanogens, and swallow the capsules down for delivery to the large intestine as the working location for this scrubber unit. The scientific head instantly responded he had been thinking the same thing, but had not wanted to bias my thinking by saying it first.  The approach met all our criteria. Taking capsules by mouth is as easy and as non-invasive a way to get things into the body as there can be.  The large intestine has many microbial species living there safely and performing many jobs that we are slowly realizing are important to our health.  

Trust Your Gut?

Methanogens typically are anaerobic organisms that would die quickly if exposed to oxygen, and the large intestine is the only part of the body that provides an anaerobic environment.  Some species of methanogens are even a normal part of our intestinal flora, where they consume traces of hydrogen manufactured by other intestinal microbes.  We were therefore confident that adding more methanogens should do no digestive harm. The amplified population of methanogens in the intestine would be likely to stay high only for as long as the divers breathed hydrogen, and return to baseline shortly after the exposure to hydrogen ended. The methane end product of this hydrogen scrubbing has a safe means of escaping from the intestine.  

The methane-releasing issues were the only parts of this research that got a little weird at times. I was very carefully coached by Navy people to use lengthy euphemisms such as “the methane is released to the environment following the path of least resistance,” or “methane has an obvious means of egress from the intestine.”  I was warned never to use what I have come to refer to as “the four-letter f-word” for methane release.  But the euphemisms never helped.  All audiences instantly understood the euphemisms as such.  

The first dive to 701m. Photo courtesy of A. Tocco Comex.

Indeed, I came to consider it a sign that my audience was truly listening to me and following the science when they suddenly started squirming in their seats and trying with greater or lesser success to cover their laughter when I started explaining the fate of methane. Jokes followed. One Navy brass listener asked me if the implementation of hydrogen biochemical decompression meant a negation of the stealth intended for Navy SEALs when they used closed-system (i.e., non-bubbling) breathing rigs.  The only sensible thing for me to do was laugh along with the room.  

An interesting phenomenon happened as soon as people got over their initial laughter at this childishly scatological word that I did not say but that they obviously thought of themselves. They started thinking about the physiology and the gas transfer physics I was describing, and they liked it.  No more laughter after that moment of enlightenment arrived. So go ahead and laugh now. “Better out than in” applies to laughter also.  I got a million of ’em. I am known in some circles as the “Queen of Farts” with good reason. 

Measuring Flatulence err Farts

I retired from Navy civilian service years ago, so I can say whatever I wish.  I measured farts. Measuring farts is funny. And measuring farts in rats and pigs is exactly how my NMRI team and I succeeded in demonstrating the feasibility of hydrogen biochemical decompression to reduce the incidence of DCS following hydrogen dives by roughly half. As far as we know, methane release rate is the only variable that can be biologically manipulated with a measurable effect on DCS incidence following any kind of dive. There is nothing humorous about reducing DCS incidence.  

Photo courtesy of Aqua Magazine, Susan Kayar.

The methanogenic species we chose has a rather grand first name but oddly mundane last name: Methanobrevibacter smithii.  It is native to the intestinal flora of many mammals, including humans and pigs, and thus does not cause digestive issues when added to the intestines.  The metabolic equation for M. smithii is the following: 

4H2 + CO2 = CH4 + 2H2O

To speed things along in the lab, we surgically injected M. smithii cultures into the upper end of the large intestines of our lab animal models of divers, which were initially rats and later pigs.  The animal-divers were then placed in a hyperbaric chamber which we pressurized with hydrogen and oxygen.  Some hydrogen and oxygen breathed by an animal-diver dissolves in the blood for transport throughout the body.  When the blood circulates through the vasculature of the intestinal wall, some hydrogen diffuses down its partial pressure gradient into the intestinal cavity, where the M. smithii are housed.  

Figure 1. Sample hydrogen dive with a rat using biochemical decompression.  A rat with a culture of M. smithii in its intestines was placed in a hyperbaric chamber.  As the pressure of hydrogen (green squares) increased in the chamber, increasing quantities of methane (red dots)  were released from the rat.  When the chamber was decompressed, methane release initially spiked as hydrogen became super-saturated in the rat, and then fell as hydrogen was removed from the chamber. Diagram courtesy of Susan Kayar.

Oxygen is taken up by the cells of the intestinal wall and aerobically metabolized to carbon dioxide (CO2), some of which also diffuses into the intestinal cavity. M. smithii metabolizes the hydrogen and carbon dioxide to methane and water. The animal-diver safely absorbs the water. It is a real scientific benefit that the methane exits the body as easily as it does. Since no mammalian cell manufactures methane, we could track the metabolism of our methanogens inside our animal-divers simply by measuring the rate of release of methane from them to the surrounding environment by gas chromatography. As the hydrogen pressure in the chamber increased, we measured increasing quantities of methane in the chamber gases 

Figure 2. Risk of DCS was significantly reduced in rats with methanogens following dives in hydrogen. Rats with M. smithii in intestines had significantly fewer cases of DCS (5/20) compared to untreated control rats (28/50) and rats undergoing the same surgical procedure as the treated rats but without M. smithii injections (13/20). Diagram courtesy of Susan Kayar.

When we then decompressed our animal-divers, on average, the animals with supplemental methanogens had approximately half the incidence of DCS as those without supplements. As the volume of methane they released during the dive increased, their incidence of DCS decreased.

Figure 3.  Injected activity of methanogens correlates with methane release rate and lower incidence of DCS in pigs following hydrogen dives (Kayar et al., 2001).  

Knowing from the metabolic equation above that four hydrogen molecules are consumed for each methane molecule manufactured, we could easily estimate the rate of hydrogen-scrubbing inside our animals.  Based on the solubility of hydrogen in body tissues (which we guesstimated as being similar to water), and the time at pressure of the dive, we could estimate how much hydrogen would dissolve in an animal of a given body mass by the end of the bottom time, and what fraction of that body burden of hydrogen had been eliminated by our process.  We computed that when M. smithii eliminated approximately 5% of the hydrogen dissolved in our animal-divers’ bodies, DCS incidence was reduced by 50% (Fahlman et al, 2001).   

Human Biodec

Having succeeded in demonstrating hydrogen biochemical decompression in a small animal model, the rat, and a larger animal model, the pig, we are at least scientifically prepared to extend this work to human divers.  A diver would make a saturation dive (commonly abbreviated to “sat”, meaning a dive sufficiently long i.e. 24 hours or more, to saturate the diver’s tissues with the breathing mixture) using a hydrogen-oxygen blend we usually call “hydrox”, or a hydrogen-helium-oxygen trimix which goes by the awkward name of “hydreliox”, depending on practicalities.  

Dive operations may even opt for a quad-mix including nitrogen.  The ultra-deep diving trials at Duke University found the narcotic properties of nitrogen helped to suppress HPNS, which was so problematic for their divers breathing heliox. However, the interaction is complex. Since we are still working out the exact mechanisms that make nitrogen and hydrogen narcotic under pressure, it remains to be determined if combining nitrogen and hydrogen for deep sat dives makes narcotic issues better or worse.  The issue deserves testing.

What oral supplements might look like. Photo by JESHOOTS.com from Pexels.jpg.

Regardless of the other gases in the sat diver’s mix, if there is hydrogen, then hydrogen biochemical decompression could be considered.  A couple of days before the end of the bottom time, the diver would prepare to biochemically decompress as a supplement to the physical decompression.  The basic process would be identical to that for our animal models, except for a gentler way of delivering the methanogens to the diver.  We would freeze-dry cultures of M. smithii and pack them into oral-delivery capsules designed to dissolve only under the conditions inside the large intestine.  It would take around 24-36 hours to have a capsule arrive in the intestine, dissolve, and re-activate the methanogens.  We would know that the M. smithii were on site and sufficiently active by chemically analyzing the sat chamber gases for methane output.  Then we would get to watch the diver not bend as he decompressed faster than divers in other hydrogen diving operations without biochemical decompression.  As I said, coolest job ever, or what?

But wait!  

There is one more really exciting finding to report.  We have evidence that even the quantity of methanogens native to the intestinal flora of a pig can provide sufficient hydrogen-scrubbing activity to reduce DCS incidence from a hydrogen dive (See Fig. 4 below).  Humans and pigs are similar in many respects, including basic intestinal flora.  It may well be that any human divers on a hydrogen dive, such as those at COMEX , have already benefited from hydrogen biochemical decompression without realizing it.  They have only to test for methane in their chamber gases to know.  

Figure 4.  Native methanogens in untreated pigs significantly reduced DCS incidence.  As untreated pigs were exposed to various dive profiles in hydrogen, increasing pressures of hydrogen elicited increasing quantities of methane released by methanogens native to the pigs’ intestinal flora.  Open circles represent pigs with subsequent DCS, closed circles represent pigs without DCS.  DCS incidence was significantly lower as the pigs released more methane.  

Skeptics have argued that the relatively small percentage of hydrogen scrubbing we have computed may be far too little to have any impact on DCS risk in human divers or to make a worthwhile reduction in decompression times. In addition to pointing to our DCS incidence data, we note that all divers are familiar with how important small differences in gas loads can be in DCS risk. If we dive within the time at depth limits of our chosen algorithms, we are confident to a very high level of probability that our dive will end safely. But exceeding our planned no-decompression limits by even a few minutes, and thus adding only a relatively small percentage increase in our inert gas load beyond what we think of as safe, makes our dive profile much riskier.  [Ed. Note: These are computational risks not necessarily operational ones i.e. small changes in times/depths are unlikely to result in DCI] Likewise, we are all in the habit of making what we term a “safety stop” in 3-5m/10-15 ft even from a low-risk, no decompression time-requiring dive. 

Sat dive operations currently using heliox and contemplating a shift to adding hydrogen will be dismayed to realize that hydrogen is considerably more potent at inducing DCS than is helium (Lillo R.S., E.C. Parker, W.C. Porter, 1997 Decompression comparison of helium and hydrogen in rats. J. Appl. Physiol. 82(3) 892-901). This would mean that costs saved by substituting relatively inexpensively manufactured hydrogen (by electrolysis of water) for increasingly expensive imported helium could be overwhelmed by the costs added in significantly longer decompression time. This is where hydrogen biodec may provide its greatest advantage: in shaving down the extra time needed for safe decompression from a hydrogen dive to something closer to that of a heliox dive.  Until someone takes the step of testing hydrogen biodec in human subjects, we will not know to what extent operational decompression times could be reduced.  

Nitrogen Biodec?

What comes next?  In an ideal scientific world, our research in animal models would be followed by equivalent studies in human divers.  However, for the time being in the post-Russian Cold War Era, the US Navy has expressed no further interest in hydrogen diving and has not offered to support human studies in hydrogen biochemical decompression.  To assuage my disappointment, I wrote a novel in which hydrogen biochemical decompression is used to help save the day in a submarine rescue scenario.  The novel is entitled “Operation SECOND STARFISH, A Tale of Submarine Rescue, Science, and Friendship,” available as a paperback and Kindle e-book on Amazon.  

But I am still dreaming bigger than that.  Since hydrogen biochemical decompression works, why not shoot for something everyone in the diving world could use?  Nitrogen biochemical decompression!  There are nitrogen-metabolizing microbes native to our intestinal flora.  But the problems of experimentally making nitrogen biochemical decompression work are staggeringly complicated.  One of many is that in nitrogen metabolism, usually referred to as nitrogen fixation, the end-products are molecules such as nitrites, nitrates, and ammonia, which are not gases that would just come bubbling out for us to measure.  

Susan submerged. Photo courtesy of Susan Kayar.

These fixed nitrogen compounds would stay dissolved in the fecal material and join many more such molecules already there from protein digestion.  (If you think the fart jokes are bad, consider the fecal jokes. “No shit!”-Ed.) The presence of fixed nitrogen products in feces (also known as “fertilizer” under other circumstances) suppresses the nitrogen-fixing microbes from fixing even more, since the process is energetically expensive to the microbes and done only by necessity.  It would take some genetic manipulation of the microbes to get them to work for us, and some form of special molecular labeling to measure how much end products they are making.  I leave those problems to future scientists to solve, while I enjoy my retirement in New Mexico, the Land of Enchantment, and go on dive vacations to Hawaii, Papua New Guinea, Tahiti, Fiji, and Raiatea to keep my vestigial gills damp. I may even write another novel. 

Additional Resources

Operation SECOND STARFISH, A Tale of Submarine Rescue, Science, and Friendship


Bennett, P.B., R. Coggin, M. McLeod, 1982.  Effect of compression rate on use of trimix to ameliorate HPNS in man to 686 m (2250 ft).  Undersea Biomed. Res. 9(4)335-51.

Fahlman, A., P. Tikuisis, J.F. Himm, P.K. Weathersby, and S.R. Kayar, 2001.  On the likelihood of decompression sickness during H2 biochemical decompression in pigs.  J. Appl. Physiol. 91:2720-2729.  

Imbert, J.P., C. Gortan, X. Fructus, T. Ciesielski, and B. Gardette, 1988.  Ch. 13.  Hydra 8: Pre-commercial Hydrogen Diving Project.  Advances in Underwater Technology, Ocean Science and Offshore Engineering, Vol. 14, pp 107-116.  

Kayar, S.R., M.J. Axley, L.D. Homer, and A.L. Harabin, 1994.  Hydrogen gas is not oxidized by mammalian tissues under hyperbaric conditions.  Undersea Hyperbaric Med. 21(3):265-275. 

Kayar, S.R. and M.J Axley, 1997.  Accelerated gas removal from divers’ tissues utilizing gas metabolizing bacteria.  U.S. Patent No. 5,630,410.  

Lillo R.S., E.C. Parker, W.C. Porter, 1997 Decompression comparison of helium and hydrogen in rats. J. Appl. Physiol. 82(3) 892-901

Kayar, S.R., T.L. Miller, M.J. Wolin, E.O. Aukhert, M.J. Axley, and L.A. Kiesow, 1998.  Decompression sickness risk in rats by microbial removal of dissolved gas.  Am. J. Physiol. 275 (Regulatory Integrative Comp. Physiol. 44):R677-682.  

Kayar, S.R., A. Fahlman, W.C. Lin, and W.B. Whitman, 2001.  Increasing activity of H2-metabolizing microbes lowers decompression sickness risk in pigs during H2 divesJ. Appl. Physiol. 91:2713-2719.  

Kayar, S.R. and A. Fahlman, 2001.  Decompression sickness risk reduced by native intestinal flora in pigs after H2 dives.  Undersea Hyper. Med. 28(2)89-97.  

Valée, N., Weiss M., Rostain JC, Risso JJ, A review of recent neurochemical data on inert gas narcosis. Undersea Hyper. Med. 38(1)49-59

Susan grew up in the St. Louis, Missouri, area.  An early fascination with the films of Jacques Cousteau inspired her to become certified as a scuba diver while still in high school.  Her diving in Missouri was confined to artificial lakes with sunken rowboats, lost Coke bottles, and a few carp as the thrills.  She persevered in her interests in marine sciences and attended the University of Miami as a biology major, remaining at that institution all the way through to a doctorate.  After graduation, it did not take long to realize she would starve if she insisted on a job in marine biology, so she moved into studying physiology in extreme environments and exercise stress.  Postdoctoral research appointments sent her from Colorado to Switzerland to New Jersey.  Her dream job finally materialized in an appointment with the US Navy in the Washington, DC area, where she studied decompression sickness risk in animal models of ultra-deep diving.
Susan was inducted into The Women Divers Hall of Fame in 2001 in recognition of her Navy diving research.  When funding for her Navy program ended, she managed research funding efforts for the National Institutes of Health (NIH), Defense Advanced Research Programs Agency (DARPA), and the Office of Naval Research (ONR).  Now in retirement, she has written a diving-themed novel, “Operation SECOND STARFISH.”


Is Freediving Safe?

According to DAN, breath-hold diving fatalities accounted for nearly a third, or 52 of the 162 recreational scuba deaths in 2017, and four times the number of tech diving fatalities that year. Is freediving actually more dangerous than tech diving? Former USA Freediving Team captain, record holder, and PFI instructor-trainer Ted Harty explains what’s happening and what’s required to improve freediving safety. Take a deep breath.




by Ted Harty

All photos courtesy of Ted Harty unless specified.

Background: Similar to the early days of tech diving, freediving suffers from an alarming number of fatalities. According to the 2019 DAN Annual Diving Report, there were at least 955 breath-hold diving incidents between 2004-2017, with 73% fatal outcomes—or an average of at least 51 fatalities per year. DAN rigorously collects data from public media, breath-hold diving associations, DAN’s Diving Incident Reporting System (DIRS), and individuals. 

However, the editors point out that it is highly likely that the data they captured underreports the actual number of breath-hold fatalities. Note also, that freediving competitions, which have a strong safety record, are not the culprit; there have only been two fatalities during competitions in the last 30 years.

In 2017, (the last year reflected in the DAN report) there were at least 52 freediving fatalities worldwide. By contrast, technical divers accounted for 13 fatalities in 2017, and have ranged between 15-25 a year worldwide—the majority involve rebreather diving—while annual scuba diving fatalities total roughly five times more. In 2017, there were 162 deaths involving recreational scuba diving, 70 in North America. Freediving fatalities, though likely underreported, still accounted for nearly a third of overall recreational diving fatalities. 

Which begs the question: Is freediving safe? 

The answer is yes. And no. But truthfully, that depends on you. Is scuba diving safe? The best answer to that question is, it depends on how closely you follow safe diving practices. People ask me all the time, isn’t freediving dangerous? My standard response is, “The way most people do it, yes it can be dangerous.” Don’t worry, I teach people how to freedive safely, and you are about to get some insight into the process. If you want to find out if you or your buddy is actually freediving safely, keep reading.

My name is Ted Harty. I’m the founder of Immersion Freediving, and my pride and joy is www.FreedivingSafety.com, a free, online course that teaches all of the safety information that I teach in my in-person classes. 

Performance Freediving International’s (PFI’s) annual Deja Blue freediving competition. Photo by Eiko Jones

I became a scuba instructor in 2005, and later became an instructor for Performance Freediving International (PFI), eventually achieving the rank of Freediving Instructor Trainer. I am a past USA Freediving record holder, and was the captain of the USA Freediving Team during world championships in France. My deepest freedive is 85 m/279 ft, and my longest breath-hold is seven minutes. I’ve trained a Basic Underwater Demolition/SEAL (BUD/S) instructor, and fitness guru Ben Greenfield. I have also worked with the CEO of Twitter, and I appeared on the Discovery channel with Tim Kennedy.

To most readers of InDepth, my freediving performance likely seems unreal, but when my freediving performances are compared to the world’s best freedivers, my performances are average at best, which I’m fine with. My main focus is on education, specifically safety education for freedivers.

Comparing Freediving To Scuba

To answer the question regarding freediving safety, let’s compare it to something that the majority of you are familiar with: scuba diving.

What does every single scuba diver on the planet have in common? They took a scuba class from a certified instructor. I would argue more than half of a scuba class is what to do when something goes wrong, and how to avoid those situations in the first place.  What to do if your mask comes off, or your regulator comes out, or you run out of air, or you run out of air and there is no one nearby, or your regulator is free flowing, etc.

In fact, to become a scuba diver, an instructor will demand that the student jump into the water and convince the instructor that they could easily handle all of these emergency situations. Only when the instructor is convinced the student could handle these emergencies are the students allowed to go scuba diving.

Let’s compare that to freediving. 

You walk into a dive shop, and say,“Yeah, gimme those fins—no the long ones. And that wetsuit. No, no, the camo one. And I want that teeny, tiny mask, that camo snorkel, oh yeah and that gun. No, the big one with three bands.”  You swipe your credit card, and out the door you go. It’s now up to you to figure out how not to kill yourself.

 See the difference?

There is no gatekeeper in freediving. In scuba, you have to have a card to get your tank filled or get on a dive boat. There’s nothing like that in freediving, really. With that being said, most people who freedive have never taken a class. This means they don’t know the rules—the dos, the don’ts, the safety procedures, or even how to rescue their buddy in the event of a problem.

The biggest danger in freediving is a blackout; you may have heard it called shallow water blackout. If the oxygen level in your blood drops below a certain level, the brain can no longer maintain consciousness, and a blackout is the result.  Blackouts are not common, just like running out of gas is not common. By the way, I bet you have a solid plan for if you run out of gas, don’t you?

Blackouts can easily be fixed with proper safety protocols and just result in ceasing diving for the rest of the day.  If proper safety is not in place, that same blackout can result in the diver sinking to the bottom of the ocean and dying, resulting in yet another Facebook post about someone dying while freediving. 

Let’s go over some things most people don’t understand about freediving blackouts.

Where Do Blackouts Happen?

Ninety percent of blackouts happen at the surface after the diver surfaces and takes two to three breaths. Many times, the diver will look fine at the surface. I’ve seen freedivers hit the surface look fine, take a few breaths, give a strong OK sign and then blackout at the surface. Most freedivers don’t know blackouts can happen like this, because most learned from their buddy, not an instructor.

Nine percent of blackouts happen between 4.6 m/15 ft and the surface. So, 99% of blackouts are in the top 4.6 m/15 ft of the water column—what I call swimming pool depths—and can be easily handled, assuming divers are following safe freediving practices and are trained in freediving rescue techniques. Unfortunately, many freedivers do not follow such procedures. More on this later.

What Would You Feel Prior To A Blackout?

Here is the part that most people not only don’t understand, but don’t believe me when I tell them. On a dive that results in a diver having a blackout, they would likely feel fine. I’ve said this for over ten years, and often I’m not believed.

When I was sitting in my PFI Freediving class with Kirk Krack as an instructor in 2008, he told me the same thing, and I didn’t believe him either. I thought, come on, on a dive where I lose consciousness, you are telling me I’d feel fine? No way! You’re just saying that to try to scare us into being more concerned about all this safety stuff you keep talking about.

When asked why this happens and why you wouldn’t feel it coming, I typically say something like this: The reasoning behind that is rather complicated and beyond the scope of this discussion. If I’m teaching an intermediate class, I spend 20 minutes discussing the partial pressure of oxygen and hemoglobin dissociation curves.

But, in this case, because you are tech divers, you are already aware that the partial pressure of oxygen drops extremely rapidly the last 10m/33 ft to the surface. In fact, the partial pressure of oxygen drops by half as you go from 10m/33 ft to the surface. 

It takes a freediver roughly 10 seconds to get from 10m/33 ft to the surface. This is why freedivers can blackout so suddenly, as the partial pressure drops very rapidly near the surface.

There is nothing like seeing it first hand for yourself.

Watch the video below to see two separated blackouts while freediving spearfishing. 

Note: If you don’t want to see a fish getting shot, don’t watch the video!

This first video shows a spear fisherman with 25 years of experience. He has never had a blackout before. This happened on a 15 m/50 ft dive, which is a totally normal depth for him; in fact, he did seven dives on this same spot before the blackout happened.

As you can see, he clearly had no idea anything was wrong, and the person who rescued him was freediving instructor Ren Chapman, from Evolve Freediving, who will tell you that the diver told him on the boat that he felt fine on the dive.

This spear fisherman has 25 years’ experience spearfishing, so let’s say that’s a total of 50,000 individual freedives, which I think is a completely reasonable assumption. I do 60 dives during an ocean session when I’m teaching. That means on 49,999 of his individual freedives, he was fine, but on 1 out of 50,000 freedives he had a blackout.

This diver typically dives by himself or with people who are not watching him properly. So if this blackout had happened during any of his 49,999 other freedives he would have died. On this dive, he was being watched by Ren Chapman of Evolve Freediving, so he was fine.

I have a question for you: Did you catch the moment the spear fisherman asked Ren for help?  Of course you didn’t, because he didn’t ask for help, because he didn’t know anything was wrong.

The second video shows one of my students rescuing his buddy after a blackout. In the second video, they were diving in a two-person team. My student went back to the boat and told his buddy, “Hey I gotta head back to the boat. I will be back in just a second, don’t dive till I get back.”  His buddy got tired of waiting and thought, I don’t need a babysitter, and did the dive.  

As you can see from the video, the only reason he’s alive is because the boat was tied up to the rig, so while he was on the boat, he was extremely close to the diver. Once his buddy surfaced and my student saw what was happening, he jumped off the boat and performed the rescue he learned in my class, and the diver was fine. You can learn how to rescue someone from a blackout in my free online safety course.

The most common thing that people say after being rescued from a blackout is that they didn’t black out. They often don’t believe anything happened. In the second video, the guy that had the blackout refused to believe he had a blackout, and continued to dive. My student was forced to follow him around to make sure it didn’t happen again.

You are much more prone to have a worse blackout that day if you continue diving after a hypoxic issue like a blackout or a loss of motor control.

If you talk to people that have had a blackout, they will more often than not say they felt fine on the dive and didn’t feel that anything was wrong. So here is what we have: With most blackouts, you wouldn’t feel anything was wrong. Why? Because the partial pressure drops extremely rapidly on the ascent. You also saw proof of this with your own eyes in the first blackout.

Now here is where it gets interesting.

If you ask the average freediver or spearo if they are worried about freediving safety or worried about blacking out, 90% will say something like this: “I don’t push myself. I dive shallow. I know my limits. I’m in tune with my body, it’s those people that push themselves and dive deep that need to worry; not me.”

One week ago in Florida, there was a freediving fatality in shallow water while lobstering. Shallow water doesn’t make you immune. When you dive shallow, you stay longer.

Many freedivers are not worried about having a blackout because they are in tune with their body; yet on a dive that results in a blackout, you would likely feel fine. See the issue here? Let’s say you were, in fact, genetically superior to us mere mortals, and you had some sort of internal blackout sensing device: Let’s put it to use.  

You are coming up from a 15 m/50 ft freedive, and your blackout sensing device goes off, and you know you’re going to black out when you hit the surface.  How will that information help you in any way? The only thing that will save you at this point is having a buddy who is waiting for you when you surface. 

So Now The Question Is Where Is Your Buddy?

First, let’s look at where the buddy needs to be. We teach that divers never freedive alone, but it’s more than that. We teach that when they surface from a freedive, a buddy needs to be what I call, “close enough to grab.” If you have 15 m/50 ft long go-go gadget arms, you can be 15 m/50 ft away from your buddy. If you have normal-sized human arms, you need to be 1 m/3 ft away so you can grab them and immediately stop their airway (mouth and nose) from going into the water if they blackout.

It’s not the blackout that causes the problem, it is the water in the lungs that kills them. You need to be close enough to grab someone to rescue them effectively. This is what we mean when we say “One Up-One Down.” One diver makes a dive, and the second diver stays on the surface and commits to being close enough to grab the diver when they surface. This is the core principle of safe freediving, which is called “direct supervision.”

In my opinion, failure to follow this very simple rule is what leads to most freediving fatalities. If you surface from a dive and have a blackout, and your buddy is 15 m/50 ft away, not paying any attention to you because they just saw a fish and are chasing after it to harvest it or take a photo of it, you’ve got a significant problem, right? How can your buddy save you from a blackout unless they are right next to you when it happens?

Just having a buddy “in the area” is essentially useless, unless they are committed to direct supervision. When you blackout, you are not going to be hollering for help, you are going to hit the surface, and then, if you are overweighted, slowly sink to the bottom. I’ll talk about weighting a bit later.

We also teach that when your buddy surfaces, you need to watch them for 30 seconds. After 30 seconds of breathing and looking normal, you can be assured they are oxygenated and will not blackout.

Bullet Proof Buddy SystemFor Scuba Or Freediving

I first started teaching this system when I would divemaster trips in the Florida Keys. I remember a couple who was doing lots of fighting on the boat. They made several trips with us. They would always come up arguing about getting lost and losing each other. The husband came up to me and said they need to take a navigation class because they always lose each other.

I told them I was happy to teach them a navigation class, but I could stop them from losing each other in just 60 seconds. They asked, “How is that possible?” I then asked them, “When you go into the water, who is the designated leader, and who is the designated follower?” They both looked at each other with a dumbfounded look, and I said, “Exactly. You’re both leaders.”  

So back to freediving. You jump in the water with your buddy. One of you is the designated follower; the other is the leader. Now the leader gets to go wherever they want. As the follower, you have one goal—stick your face in the water and follow their fin tips. No arguing about why I want to go this way, no discussion; just follow. When the leader makes a dive, your job is to keep an eye on the diver, and when they surface, be close enough to grab and watch for no less than 30 seconds. Then you are the leader. Now you get to go wherever you want, and the other diver has to follow you. 

What’s great about this system is it requires no discussion. You don’t have to say, “Hey, I’m about to make a drop. Make sure you’re watching me.”

My spearfishing students love this system because it’s very easy to implement. It works great for scuba as well. Switch leader and follower roles between dives one and two, and this way, both divers learn leadership and navigating skills instead of one diver always being reliant on following someone.

Blackouts Vs Running Out Of Gas

Blackouts are not common, just like running out of gas is not common. In scuba, you have a set of procedures set in place so that if you run out of gas it’s easily fixable. If you don’t have a plan to deal with that, and you run out of gas..…oops.

It’s the same with freediving. We have a set of procedures, diving in a team, One Up-One Down, be close enough to grab, and be trained in blackout rescue. This way, if a blackout happens, it’s easily fixable. The problem is many, if not most, people do not follow these procedures. When a blackout happens, it unfortunately often leads to a fatality.

Divers Alert network (DAN) started tracking freediving breath hold fatalities in 2005, but I assure you they are incredibly underreported. In my estimation, there are 50-75 fatalities from breath-holding in the USA alone per year. I’ve heard of four in the past couple of weeks.

Blackout fatality numbers are hard to track. They are not reported nearly as rigorously as scuba fatalities. Divers Alert network (DAN) started tracking freediving breath hold fatalities in 2005, but I assure you they are incredibly underreported. In my estimation, there are 50-75 fatalities from breath-holding in the USA alone per year. I’ve heard of four in the past couple of weeks.

Proper Weighting For Freediving

This is such an important concept, and I find that almost every untrained freediver—and even some that have taken a freediving course—doesn’t fully understand and properly implement safe weighting.

As I said earlier, it’s not the blackout that causes the problem, it’s the water in the lungs that does. When there is a freediving fatality, where is the body found? Typically on the bottom. Why? Because they were wearing too much weight. If you were to blackout, instead of the bottom of the ocean, where would be a better place to end up? On the surface!

I’ve seen countless Youtube videos of spear fisherman blacking out at the surface and then rapidly sinking to the bottom as a result of wearing too much weight.

When you blackout, you let out a very specific amount of air, but it’s not all the air in your lungs. To find out exactly how much air you would let out, follow along with me here.

Take a big breath, then do a relaxed passive exhale like a sigh. That’s exactly how much air you would let out if you blacked out. So you let out some of the air, but not all of the air. Sure, if you forcefully exhaled, you could force more air out, but is a blacked-out freediver going to do anything forcefully? Nope.

When you blackout, you will either float on the surface or sink to the bottom, and that outcome will be determined by the amount of lead you are wearing.
In the below video I simulated a blackout while intentionally wearing too much weight and you can see how fast I was sinking.

Surface Safety Test 

Get in the water wearing whatever gear and weight you usually wear. Take a big breath, do a relaxed exhalation like a sigh, don’t kick your feet, don’t move your hands. If you sink, now you know that if you were to blackout, you would end up on the bottom of the ocean. Does this seem like a good setup? Nope!

Take a pound off and repeat the test. Continue taking weight off until you can do the exhalation and not sink. This is what I call passing the surface safety test.

I tell my students that by doing this test, they will make sure they are not overweighted when they’re wearing the exact same gear that they did the test in. If you change your wetsuit, gain or lose weight, switch from fresh to salt, or decide to ditch your wetsuit bottoms, you will have to redo the test and change your weighting.

I tell my students that they should do this test every single time they jump off the boat. If your weight is correct, it will take all of 10 seconds. By being correctly weighted, you will end up floating on the surface if you blackout. 

Imagine rescuing someone who had a blackout, which is easier to rescue?

Your buddy who is floating on the surface, or your buddy who is sitting on the bottom at 18m/60ft.

I think the answer is obvious.

Wetsuits Are Not Just For Warmth But For Safety

Here is something most people never consider. Yes, a wetsuit is designed for warmth, but in freediving, it’s also designed for safety. This is a conversation I have all the time with my students. 

In this example, let’s say I have a very fit student with muscles and little body fat. I ask them how much weight they wear when they go freediving, and they say, “Oh Ted don’t worry. I don’t wear any weight, I’m super safe.” 

I then ask, “What type of wetsuit do you wear?” and they say, “None, I don’t need one.” Then I tell them, “Well that’s a problem because you’re overweighted,” and they always respond, “How can I be overweighted if I’m not wearing any weight?”

If you are a person I would call a sinker—with muscles and low body fat—it’s possible that if you jump in the water with no wetsuit and no weights, and do a relaxed exhalation, you will sink. This means you are overweighted, and you would end up on bottom if you had a blackout out, so you need a wetsuit not just for warmth but for buoyancy.

If I jump in the water with no wetsuit and do a relaxed exhalation, I float, because I have a body built with beer, bourbon, and BBQ. This is not a problem I have.

Should your snorkel be in your mouth while underwater?

Students always have a hard time breaking this snorkel habit, and most untrained freedivers have their snorkels in their mouths when they are underwater. So they are typically a bit surprised by how adamant I am that they remove their snorkels from their mouths when they dive.

I teach my students to take a breath at the surface using their snorkels and then remove the snorkels from their mouths as soon as they take their breath. Like any good instructor, if I’m going to tell students to alter something they have done for a long time, I better have a good reason for why I’m asking them to change, and I’ve got a good one.

 Freediving with your snorkel in your mouth is a drowning hazard!

If you are like most people, you retain the snorkel in your mouth when you are freediving underwater. So I have a question for you. What is stopping the water from rushing into your lungs?

The most common answer I get is my throat is shut. That is not what’s stopping it, because if it was your throat, that would mean when diving underwater with the snorkel, your entire mouth is full of water up to the back of your throat. I doubt you are diving like that.

The actual reason the water isn’t going into your throat is your tongue. Your tongue is actively plugging the hole of the snorkel, and that’s what stops the water from going into your lungs. The tongue stops the water from even getting your mouth. Okay, now you’re thinking, why are we talking about my tongue? 

Next question: If you were to blackout underwater with the snorkel in your mouth, will your tongue continue to actively block that hole? Nope! Why? Because you are unconscious, your tongue is going to go limp like everything else. You can’t count on it continuing to block that hole.

 So if you blackout with the snorkel in your mouth, the snorkel will turn into a funnel that channels the entire Atlantic ocean directly into your lungs. 

Does this sound like a good idea? Not to me!

That is why having the snorkel in your mouth while underwater is a drowning hazard.  It’s why most freediving courses will teach you to remove your snorkel upon descent. Even if your attentive buddy rescues you quickly if you blackout, if you had a snorkel in your mouth, you could still end up in the hospital for a week or more, because water got into your lungs. 

 When you don’t have the snorkel in your mouth, and you blackout, your mouth is going to stay closed. Why? Because it was likely closed to begin with, and the water pressure will help keep it closed. When you blackout, you are not going to open your mouth because that is an active process and when you are unconscious, you are not going to be doing anything active.

One of the main rules in freediving is to protect the airway. As long as we keep water from going in the mouth or nose, we protect the lungs. As long as no water gets in the lungs, the diver will be fine. As I said before, it’s not the blackout that causes us the problem; it’s the water in the lungs.

My Prescription For Safe Freediving Is Simple

It’s simple: Dive in a team, One Up-One Down, be close enough to grab when your buddy surfaces, watch them for no less than 30 seconds, and know how to rescue someone from a blackout.

Everyone thinks they are immune, the rules don’t apply to them, it’s just those other people who push themselves. Have you ever heard of a skydiver saying I’ve been skydiving for 10 years, I”ve never had a problem, I”m going to stop packing this reserve chute. It’s a pain in the butt, and I never use it.  Of course you haven’t heard that because they don’t want to go splat.

Just because you personally have never had a problem doesn’t mean you are diving safe.  I used to say the most dangerous scuba diver is a diver who has a hundred dives and hasn’t had anything go wrong.   What makes you safe is what happens to you when something goes wrong.   In scuba diving the penalty for a mistake is often a trip the to chamber.  In freediving the penalty for a mistake is too severe.  
Dive safe out there, it’s not even that hard.

As a freediving instructor, I can tell you hands down the best way to improve your freediving abilities and become comfortable in a freediving rescue scenario is to take a formal freediving class. You can see a large list of places to take freediving classes from a variety of agencies here.

How To Learn More About Freediving Safety

I’ve been teaching freediving for over ten years, and I hear the following all the time: “Ted, I’d love to take your class, but I can’t get time off work, or permission from the spouse, or someone to watch the kids, or it’s too expensive,” etc.  

After hearing that for so long, I wanted to create a free online course so that anyone could learn the safety information that I teach in my in-person classes. I’ve never liked that the freediving safety knowledge is stuck behind the paywall of a freediving class. If you’re smart enough to understand that learning how to not kill yourself while freediving is important, I want you to have access to that information from a trusted and reliable source at no charge.

Two years ago, I won the Dimitris Kollias award for promoting safe freediving. Within two weeks, I took the check, hired a web guy and two video guys to film at my pool, and launched www.FreedivingSafety.com.

If you are currently freediving and haven’t taken a course, take the time to go through my program. Even if you have taken a course, take the time to go through it. This article is just scratching the surface of what you would learn.

Over ten years ago, I heard about a student who signed up for a freediving class but died from a blackout before the class started. As a result, I’ve had more and more instructors from various agencies suggest their students go through the online program. This way, their students get access to safety information immediately, and then they will learn even more when they show up to the class.

Additional Resources

In My Element: Discovering My Inner Freediver by Michael Menduno. Discusses freediving safety.


Ted Harty offers a host of online freediving courses. Use code techdiver at checkout to receive 20% off any of his courses.

Make your equalizing problems a thing of the past

with Harty’s personal step-by-step method to learn Frenzel equalization used for freediving

Breath Hold Secrets:

Harty’s online program that covers the most common problem that beginning freedivers encounter.

28 Day Freediving Transformation training program:

Harty’s flagship program is the 28-day freediving transformation program. It covers the five most effective training exercises for freediving that you can do in your home. Learn more. 

Harty also offers specific guidance for upcoming freediving instructors wanting to do more, as well as scuba instructors wanting to add freediving teaching to their portfolio.

Ted Harty began his professional underwater career as a Scuba Instructor for PADI, NAUI, and SSI in 2005. In 2008 he took his first freediving class with Performance Freediving International. After that course, he wanted to go freediving instead of scuba diving on his days off, and realized his passion was freediving. In 2009, Ted took PFI’s first official Instructor program, and immediately started working for PFI helping Kirk Krack and Mandy Rae-Kruckshank teach courses all across the USA.

Ted went to his first freediving competition in 2009 as an overweight, out of shape scuba instructor and progressed from 24-27 m/80-90 ft freediver to 54 m/177 ft in three weeks. After the experience he wondered what he could do if he actually started training. Since that time, he’s broken a USA Freediving record in 2011, won three freediving competitions, and was selected to be the captain of the USA Freediving team in 2012; his deepest dive is 85 m/279 ft.

Lately, Ted has been focusing on spreading his message of safe freediving through www.FreeedivingSafety.com, which offers a free online course sharing all of the safety information he teaches in his in-person classes. He can be reached via Facebook, Instagram, Youtube, and Twitter, @ ImmersionFD. Email: tedharty@ImmersionFreediving.com

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