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by Reilly Fogarty
Header photo courtesy of DAN.
The pool of people who explore the ocean depths beyond 122 m/400 ft is small, and the group of people who do it regularly and need to reach those depths quickly is microscopic. This niche application coupled with a significant fire hazard make it easy to understand why exotic gases like hydrogen have escaped both common use and public interest.
Despite the obvious concerns, however, hydrogen has shown some capacity to ameliorate the effects of high pressure nervous syndrome (HPNS) in deep divers, improve function at extreme depth, reduce work of breathing, and present a possible alternative to helium in case of helium reserve depletion. Use of the gas has made it possible to dive deeper, get to depth faster, and stay there longer, but there is a substantial risk-versus-reward calculation to be made before considering its use. Interested in diving deep and dabbling in the cutting edge of diving research? Here’s what we know about using hydrogen as a diving gas.
High Pressure Nervous Syndrome
Records of sojourns into the use of hydrogen as a breathing gas go back to as early as the 18th century with the experiments of Antione Lavoisier, but the use of the gas came to a head during the heyday of deep diving research in the late 20th century. From the Atlantis dives at Duke University to the Hydra Missions and the evolution of the Compagnie Maritime d’Expertises (COMEX) tables, researchers and deep divers quickly found issues as they pushed to explore deeper depths. Racing past 184 m/600 ft, researchers discovered that divers would face several new and potentially deadly phenomena in their push to the bottom. Chief among these was a condition that came to be called high pressure nervous syndrome. The condition was first described as “helium tremors” by Russian researcher G. L. Zal’tsman in 1961 and Peter B. Bennett in 1965 (Zal’tsman’s research was not available outside of the Soviet Union until 1967).
Later this condition would be named and correlated with symptoms including tremors, headache, dizziness, fatigue, myoclonic jerking, muscular weakness, and euphoria. Gastrointestinal complaints, memory and cognitive deficits, psychomotor impairment, nightmares, and somnolence are also possible, and convulsions have been noted in animal models (Kangal, 2019). The mechanism of HPNS has not yet been proven, but there are several working theories.
The first of these involves the compression of lipid components of the cell membranes in the central nervous system (CNS). Some theorize that the compression of these tissues may affect the transmembrane proteins, ion channels, and surface receptors critical to the signaling pathways of the CNS (Talpalar, 2007). Many connect the use of hydrogen to mitigate the effects of HPNS to this model. Some research has shown that anesthetic gases can reduce the effects of HPNS, and this has been proposed to be driven by some pressure reversal effect of narcosis (Kot, 2012). Hydrogen is a narcotic gas, and this may be one component of its ability to reduce the onset or severity of HPNS. This mechanism, the compression of lipid components also appears to be the one that initially gave rise to the use of breathing gases as a way to ameliorate the effects of HPNS, with some groundwork for this foundation laid out by Peter B. Bennett, PhD, as early as 1989 (Bennett, 1989).
Other researchers have focused on neurotransmitters, including gamma-aminobutyric acid (GABA), dopamine, serotonin, acetylcholine, and others. These models have also shown promise, one study showing a GABA increase in the cortex diminishing HPNS signs in baboon models and another showing NMDA antagonists preventing convulsions in rats (Pearce, 1989, 1991). Similar studies have been conducted with a range of neurotransmitters and neuronal calcium ion channels with similar results. In short, there are several potential avenues for the specific mechanism of HPNS, and while the general mechanism is likely a combination of several models, none have yet been definitively proven.
What we know is that there is notable variation in HPNS onset among divers (Bennett, 1989), and onset appears to be the result of a combination of breathing gas, compression rate, and depth. Faster compression, lower nitrogen, or hydrogen content in helium/oxygen breathing gases, and deeper depths have been correlated with more rapid onset and more severe symptoms (Jain, 1994).
The Benefits of Hydrogen
The use of hydrogen as a diving gas doesn’t just stem from its ability to reduce the onset of HPNS—it’s an extraordinarily light gas that’s useful in reducing work of breathing at extreme depth and a potential replacement for helium when worldwide demand has led to a quickly dwindling reserve. One U.S. Navy paper went so far as to propose hydrogen replacement of helium due to helium scarcity leading to a predicted depletion of supply by the year 2000 (Dougherty, 1965). Thankfully, that mark has come and gone due to the discovery of several new helium sources, but it’s not an unrealistic concern when the demand for helium is so high in manufacturing, aerospace, and technology.
The benefits of hydrogen are notable, but the hazards are nothing to balk at. Little research has been done on the decompression or thermal properties of hydrogen in divers, it’s reported to be mildly narcotic, and it’s highly flammable. While oxygen is an oxidizer that can feed a fire, hydrogen is actively flammable—in the presence of sufficient oxygen and a source of ignition, it will combust in dramatic fashion. In practice, this combustibility is managed by reducing both the sources of ignition and the available oxygen. With the lower explosive limit of hydrogen being around 4% by volume, using less than this amount in normoxic environments effectively mitigates the fire risk but does little for deep divers. Instead, extremely hypoxic gases and high concentrations of hydrogen and helium have been used with great success. The COMEX Hydra VIII mission, for example, used a mixture of 49% hydrogen, 50.2% helium, and 0.8% oxygen to take divers to a maximum depth of 536 m/1,752 ft.
The decompression profiles used in these deep saturation dives appear to be effective as well. As early as 1992, COMEX researchers found that teams of divers on a hydrogen mixture at a depth of 210 m/686 ft performed tasks more efficiently, both cognitively and physically, than their counterparts on helium (Offshore-mag.com, 1996). The same experiment resulted in a bubble study that showed “no evidence of bubbles” in the divers following decompression—an exercise in small sample sizes perhaps, but with promising results. (Note: Researchers suspected that “biochemical decompression” might be involved, i.e., a process in which metabolism of H2 by intestinal microbes facilitates decompression—ed.) U.S. Navy research found similar results, indicating that hydrogen increased the capacity for physical effort as a result of a decrease in work of breathing at depth (Dougherty, 1965).
Functionally, the benefits of the gas are hard to dispute; work of breathing is a constantly growing area of concern for dives at all depths, HPNS is a constant concern, and minimizing decompression is a perpetual goal. For divers reaching extreme depths without the ability to perform saturation dives, diving to depths beyond 122m/400 ft is a repetitive gamble with no guarantee of success. Rapid compression combined with limited options for gas mixes result in the need to play a dive profile by “feel” with emergency plans in place to respond to HPNS onset, and more than one diver, likely including Sheck Exley (See: “Examining Early Technical Diving Deaths,” by Michael Menduno, InDepth 2.2) has lost their life to the condition.
The Hazards of Hydrogen
By the same token, hydrogen presents unique hazards that require careful consideration. Unexplored decompression profiles and limited research on long-term effects make the decision to dive with hydrogen difficult, and the significant risk of fire places divers in more danger than is typically acceptable. Add to this the limited applications (either hydrogen content below 4% in normoxic environments or oxygen content below 6% in high-hydrogen environments), and it quickly becomes apparent why hydrogen hasn’t yet hit the mainstream.
The presence of project divers in our community performing near saturation dives with trimix and makeshift in-water habitats skews favor away from hydrogen as well, making it a gas viable only for the deepest dives without the option for saturation. The case for hydrogen isn’t entirely up in smoke, however. Research showing significant decompression benefits or depletion of helium reserves may well push us toward helium’s more flammable cousin, but it’s unlikely you’ll see hydrogen at your favorite fill station any time soon.
- Ozgok Kangal, M.K., & Murphy-Lavoie, H.M., (2019, November 14). Diving, High Pressure Nervous Syndrome. (. In: StatPearls StatPearls Publishing.
- Talpalar, A.E., (2007, Nov 16-30). High pressure neurological syndrome. Rev Neurol., 45(10), 631-6.
- Kot, J., (2012). Extremely deep recreational dives: the risk for carbon dioxide (CO2) retention and high pressure neurological syndrome (HPNS). Int Marit Health, 63(1), 49-55.
- Bennett, P.B., (1989). Physiological limitations to underwater exploration and work. Comp Biochem Physiol A Comp Physiol., 93(1), 295-300.
- Pearce, P.C., Clarke, D., Doré, C.J., Halsey, M.J., Luff, N.P., & Maclean, C.J., (1989. March). Sodium valproate interactions with the HPNS: EEG and behavioral observations. Undersea Biomed Res. 16(2), 99-113.
- Pearce, P.C., Halsey, M.J., MacLean, C.J., Ward, E.M., Webster, M.T., Luff, N.P., Pearson, J., Charlett, A., & Meldrum, B.S., (1991, July). The effects of the competitive NMDA receptor antagonist CPP on the high pressure neurological syndrome in a primate model. Neuropharmacology,30(7), 787-96.
- Jain, K.K. (1994, July). High-pressure neurological syndrome (HPNS). Acta Neurol. Scand.,90(1), 45-50.
- Dougherty, J., (1965). The Use of Hydrogen As An Inert Gas During Diving: Pulmonary Function During Hydrogen-Oxygen Breathing At Pressures Equivalent to 200 Feet of Sea Water.
- Saturation diving tests support claims for hydrogen breathing mix, (1996). Offshore-mag.com.
John Clarke, retired scientific director of the U.S. Navy Experimental Diving Unit, is knowledgeable about hydrogen diving and has used that knowledge both in his blogs as well as in the undersea sci-fi thrillers, the Jason Parker Trilogy , of which he is the author.
Fahlman, A., Lin, W.C., Whitman, W.B., & Kayar, S.R., (2002, November). Modulation of decompression sickness risk in pigs with caffeine during H2 biochemical decompression. JApp. Physio. 93(5).
Kayar, S.R., & Fahlman, A. (2001). Decompression sickness risk reduced by native intestinal flora in pigs after H2 dives. Undersea Hyperb Med., 28(2), 89-97.
Fahlman, A., Tikuisis, P., Himm, J.F., Weathersby, P.K., & Kayar, S,R., (2001, December). On the likelihood of decompression sickness during H2 biochemical decompression in pigs. J Appl Physiol., 91(6), 2720-9.
Reilly Fogarty is a team leader for risk mitigation initiatives at Divers Alert Network (DAN). When not working on safety programs for DAN, he can be found running technical charters and teaching rebreather diving in Gloucester, Mass. Reilly is a USCG licensed captain whose professional background includes surgical and wilderness emergency medicine as well as dive shop management.
Why Do Divers Run Out Of Gas?
Not surprising, the answer is more complicated than simply, they neglected to look at their gauges. Here Aussie diving medical researcher and former editor of DAN’s Annual Diving Report, Peter Buzzacott dives into several deep datasets including DAN’s Incident Reporting System (DIRS) and nearly four decades of cave diving incident data, to tease out some insights on gas emergencies and get a handle on the risks. Don’t stop those S-drills!
by Peter Buzzacott
See companion story for a guestimate of the risk: What is the Risk of Running Out of Gas?
Next year it will be 30 years since I first learned to dive. At the time, I had no idea that diving would occupy such a large part of my life. I distinctly remember kneeling on the sandy bottom end of the Great Barrier Reef, sharing a regulator with my buddy, and seeing sunlight rippling down through crystal clear water. On one of these “confined water” dives we had to swim horizontally for 10 m/30 ft holding our regulators out of our mouths and blowing a steady stream of bubbles. This wasn’t as easy as it sounds and we had to ration our bubbles to make it the whole way. Then, on an open water dive, the instructor took turns holding us with one hand and gripping a rope with the other while we took a breath, took the second stage out of our mouths, and then went for the surface, breathing out all the way. Up, down, up, down, the instructor went, with each student—one at a time.
Most of today’s recreational dive courses do not include buddy breathing, they teach gas sharing with an alternate air source (AAS). Even before COVID-19, the buddy breathing skill had disappeared from most recreational training programs. The controlled emergency swimming ascent (CESA) has also disappeared from some programs.
When I became an instructor, I made many hundreds of these but, now that I think about it, I don’t recall ever seeing anyone actually make one for real after running out of gas. These days everyone dives with two second stage regulators. In technical diving, we even dive with at least two cylinders; so, I wonder, do technical divers run out of gas and, if they do, then why?
What Do The Experts Say?
Some years ago, I asked a panel of 27 diving experts a similar question regarding recreational divers in general.1 The panel consisted of nine diving/hyperbaric doctors who had treated hundreds of injured divers; nine expert dive guides, most of whom were instructors; and nine expert recreational divers who had dived all over the world and written hundreds of feature articles for dive magazines.
At the time, I suspected divers mostly ran out of gas because they didn’t pay attention to their gauge. But, to my surprise, the experts suggested about 20 reasons, such as diving deeper than usual, diving in a current, not wanting to end the dive for their buddy, using a smaller tank than their buddy, being underweighted, and many others, all of which sounded plausible.
I sent the whole list of potential causes back to the group and asked them to rank, in their opinion, the five most likely causes. Then I gave five points to everyone’s most likely potential cause, four points to the second most likely, and so on. I added up all of the points and then ranked all the causes according to the total score. Then I sent this ranked list back to the group for one last review and asked them to consider the “weight of opinion” from the group as a whole, and to reconsider their top five reasons.
As an expert panel, the group moved toward consensus. Just as I’d suspected, failing to monitor the gauge was the number one proposed potential cause of running out of gas, followed by inexperience, overexertion, inadequate training, and poor dive planning. Other than perhaps an unexpected current or underweighting leading to overexertion, the proposed reasons leaned toward human factors rather than the other two types of factors in the classic diving injury causal triad—those being environmental factors and equipment factors (Figure 1).2,3,4
The process I’d followed to gather expert consensus of opinion is called a “Delphi” process, which was originally developed by International Business Machines Corporation (IBM) to make forecasts on matters about which there was considerable uncertainty i.e. where there is little data. Opinions aren’t solid evidence; however, they can point towards a direction worth investigating.
Next, I visited Divers Alert Network(DAN) as an intern and worked on an analysis of diving fatalities within a subset of technical divers—cave divers. More on that later, but while there, I had the opportunity to examine a large dataset of recorded dives from Project Dive Exploration, headed by Drs. Richard Vann and Petar Denoble.
The dataset we had at that time revealed over 50,000 dives recorded by more than 5,000 recreational divers, (including an unknown number of technical divers). We examined these data in two ways. First, to control for environmental and equipment factors, and to focus on demographic (or human) factors, we counted each diver just once and compared those divers who had reported running out of gas, (during any recorded dive in that dataset), with divers who had not run out of gas. Surprisingly (to me), having run out of gas was more common than expected among older females (males were more likely to report other problems, like rapid ascent).
Next, to control for the human factors, we looked at just the dives made by divers who had made both at least one dive where they ran out of gas, and at least one dive where they did not run out of gas. I wanted to know what it was about those dives that might have caused the divers to run out of gas. Well, it turned out the out-of-gas dives were deeper, shorter (probably because they were deeper), often made from a live-aboard or charter boat, and involved a higher perceived workload.5 Hmmm… Perhaps overexertion was a factor after all.
After returning to Western Australia to undertake a PhD researching this, I spent the next few years recording 1,000 recreational dive profiles made by 500 divers. I recorded their start and end pressures, tank size, and noted factors such as current, how they felt their workload was (resting/light, moderate, or severe/exhausting), how many dive experiences they had, and what previous dive training they had completed. For the analysis, dives made by divers who exited with <50 bar/725 psi of pressure (needle in the red zone, n=183) were compared with other dives recorded at the same time at the same dive site (n=510) by divers who exited with >50 bar/725 psi pressure remaining (needle not in the red zone).
Ending a dive low on gas was correlated with younger males with a longer break since their last dive, fewer lifetime dives, at deeper depth, and a higher rate of gas consumption (adjusted to an equivalent surface air consumption (SAC) rate, for comparison between dives made at different depths). Perhaps more tellingly, compared with 1% of the dives with >50 bar/725 psi at the exit, 11% of the low-on-gas divers reported being surprised at the end of the dive by how low their remaining gas pressure was.6 A more detailed analysis of the average workload associated with recreational diving, using this same dataset, identified that higher perceived SAC rate was not associated with sex but was associated with older age, lower dive certification, fewer years of diving, higher perceived workload, and other factors.7
Technically Out of Gas
Returning to the topic of technical diving, a colleague and I re-examined the DAN cave diving fatality reports collection that I had worked with as an intern, and this time we concentrated on the previous 30 years of data: 1985-2015. Dividing it into two equal halves which we referred to as the “early” and “late” groups, reading each report carefully, and using a reliable cave diving fatality factors flow-chart previously developed,5 we classified factors associated with each cave diving fatality and then compared the two groups.
In the late (more recent) group, the proportion of cave divers who were trained in cave diving had significantly improved, perhaps due to increased awareness of the need for proper cave diver training before entering a flooded cave. The majority of the 67 trained cave divers in our dataset were diving with two cylinders on their back (doubles), and the late group was diving further into the cave than the early group. Of the 67 trained cave divers, 41 (62%) had run out of gas. Looking at the five “golden rules” of cave diving, the “rule of thirds” was the most common (n=20) rule that was suspected to have been broken by the trained cave divers: the most lethal.9
So, it would seem that some technical divers do run out of gas, though thankfully that appears rare. We should bear in mind that cave divers may differ from other types of technical divers in their procedures, demography, and equipment; their environment (by definition) certainly differs from that of wreck divers.
Currently, I know of no ongoing research into out-of-gas incidents among technical divers, other than the current Diving Incident Reporting System, hosted by DAN. An analysis of the first 500 reported incidents recently examined every incident—recreational and/or technical—during which the diver ran out of gas.10 The sample (n=38) was divided into two groups: those who made a controlled ascent (e.g. on a buddy’s donated regulator) and those who made rapid ascent (e.g. a bolt to the surface).
Among divers who reported having run out of gas, but survived to report the incident, 57% of the rapid ascents resulted in a reported injury. Among the 24 controlled ascents, just 29% reported an injury.10
Among divers who reported having run out of gas, but survived to report the incident, 57% of the rapid ascents resulted in a reported injury. Among the 24 controlled ascents, just 29% reported an injury.10 This modern finding is in line with the statistics reported 27 years ago by Dr. Chris Acott when he analyzed more than 1,000 diving incident reports. Examining 189 out-of-gas incident reports, Dr. Acott found 89 made a rapid ascent, and 58% of those reported an injury. Among the 79 controlled ascents, only 6% reported an injury.11
Table 1 shows the total number of dive incidents in each category, after adding both studies together. It seems to me that, while we have moved on from buddy-breathing and the controlled emergency swimming ascent, in the last 30 years the problem of running out of gas has not gone away.
|No Injury |
|Non-rapid ascent||91 (88)||12 (12)||103 (50)|
|Rapid ascent||43 (42)||60 (58)||103 (50)|
|Total||134 (65)||72 (35)||206 (100)|
In conclusion, the evidence confirms what we all know: running out of gas is associated with diving injuries and fatalities. It appears that the level of correlation of demography information (like age and sex) with out-of-gas incidents may depend upon the study design, the pool of divers studied, and/or the specific potential causes of running out of gas being investigated. For example, in one study, older females were more likely to self-report out of gas problems; in another study, young males’ remaining gas was measured and observed to be low. In yet another study, SAC rate increased when perceived workload increased, regardless of sex.
Therefore, I’d suggest it is prudent to consider everyone potentially at risk of running out of gas and, in order to mitigate this risk, both recreational and technical divers should be proficient in gas planning and monitoring their remaining gas, regardless of age and/or sex.
[Ed.note—Most agencies today require some level of proficiency in managing emergency out of gas scenarios. For example, GUE requires divers at all levels to train regularly for this eventuality. This training also emphasizes gas management strategies like “minimum gas reserves” and the related “one third” rule to ensure divers always have enough supply to share gas aka buddy breathe from any point in the dive, and all the way to the surface. Violation of these strategies risks insufficient gas in all environments.]
The influence of workload is interesting, and technical divers who perceive an elevated workload may well remember that this has been associated with both higher rates of gas consumption and unexpectedly running low on gas. So, when detecting a current or perceiving an elevated workload, I recommend keeping a closer-than-usual eye on the remaining gas and, if a current is suspected before the dive, then plan for an elevated SAC rate.
The influence of training/certification consistently appears to be associated with the risk of running out of gas, as does having made fewer lifetime dives. Highly trained and experienced divers might bear this in mind when diving with buddies who are newer to our sport. Offer them opportunities to gain experience and recommend additional training when they are ready. We were all inexperienced once.
Technology has improved in recent years; for example, tank pressure transponders are more reliable today than ever before. It is possible that in the future these resources, coupled with audible alarms, may prove to be highly effective at preventing technical divers from running out of gas. Until we know how effective such alarms are at preventing out-of-gas dives, our best course of action is to dive within the limits of our training and experience, and to keep an eye on our remaining gas.
See companion story for an estimate of the risk: What is the Risk of Running Out of Gas?
Do you think that it could it happen to you?
1. Buzzacott P, Rosenberg M, Pikora T. Using a Delphi technique to rank potential causes of scuba diving incidents. Diving and Hyperbaric Medicine. 2009;39(1):29-32.
2.Buzzacott P, Rosenberg M, Pikora T. Using a Delphi technique to rank potential causes of scuba diving incidents. Diving and Hyperbaric Medicine. 2009;39(1):29-32.
3. Buzzacott P, Rosenberg M, Pikora T. Using a Delphi technique to rank potential causes of scuba diving incidents. Diving and Hyperbaric Medicine. 2009;39(1):29-32.
4. Buzzacott P, Rosenberg M, Pikora T. Using a Delphi technique to rank potential causes of scuba diving incidents. Diving and Hyperbaric Medicine. 2009;39(1):29-32.
5. Buzzacott P, Denoble P, Dunford R, Vann R. Dive problems and risk factors for diving morbidity. Diving and Hyperbaric Medicine. 2009;39(4):205-9.
6. Buzzacott P, Rosenberg M, Heyworth J, Pikora T. Risk factors for running low on gas in recreational divers in Western Australia. Diving and Hyperbaric Medicine. 2011;41(2):85-9.
7. Buzzacott P, Pollock NW, Rosenberg M. Exercise intensity inferred from air consumption during recreational scuba diving. Diving and Hyperbaric Medicine. 2014;44(2):74-8.
8. Buzzacott P, Zeigler E, Denoble P, Vann R. American cave diving fatalities 1969-2007. International Journal of Aquatic Research and Education. 2009;3:162-77.
9. Potts L, Buzzacott P, Denoble P. Thirty years of American cave diving fatalities. Diving and Hyperbaric Medicine. 2016;46(3):150-4.
10. Buzzacott P, Bennett C, Denoble P, Gunderson J. The Diving Incident Reporting System. In: Denoble P, editor. DAN Annual Diving Report 2019 Edition: A Report on 2017 Diving Fatalities, Injuries, and Incidents. Durham (NC): Divers Alert Network; 2020. p. 49-67.
11. Acott C. Diving incidents – Errors divers make. Safe Limits: An international dive symposium; 1994; Cairns: Division of Workplace Health and Safety.
12. Buzzacott P, Schiller D, Crain J, Denoble PJ. (2018). Epidemiology of morbidity and mortality in US and Canadian recreational scuba diving. Public Health 155: 62-68.
13. Buzzacott P. (editor) (2016). DAN Annual Diving Report 2016 Edition: A report on 2014 data on diving fatalities, injuries, and incidents. Durham, NC, Divers Alert Network
14. Buzzacott P (editor) (2017). DAN Annual Diving Report 2017 Edition: A Report on 2015 Diving Fatalities, Injuries, and Incidents. Durham (NC), Divers Alert Network.
15. Buzzacott P and Denoble PJ. (editors) (2018). DAN Annual Diving Report 2018 Edition: A report on 2016 data on diving fatalities, injuries, and incidents. Durham, NC, Divers Alert Network
16. Denoble PJ. (editor) (2019). DAN Annual Diving Report 2019 Edition: A Report on 2017 Diving Fatalities, Injuries, and Incidents. Durham (NC), Divers Alert Network.
You can add a diving incident to the DAN database by name or anonymously here: Diving Incident Reporting System (DIRS).
Dr. Peter Buzzacott MPH, PhD, FUHM, is a former PADI Master Instructor and TDI Advanced Nitrox/Decompression Procedures instructor, having issued >500 diver certifications. Today he is an active cave diver, holding various advanced cave diver certifications including advanced (hypoxic) trimix diver, and he is gradually gaining experience with CCR diving. To finance this, he conducts research into diving injuries and decompression/bubble modeling at Curtin University in Perth, Western Australia.
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Why Do Divers Run Out Of Gas?
Not surprising, the answer is more complicated than simply, they neglected to look at their gauges. Here Aussie diving medical...