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How To Calculate the Risk Of Pulmonary Oxygen Toxicity

Most tech divers track their oxygen exposure on big and or long dives via computer using methods, such as REPEX OTUs, developed in the 1980s. The consensus among researchers, however, is that these methods aren’t accurate. Enter retired Israeli hyperbaric physiologist, Ran Arieli, who offers a new data-driven method for computing your risk of pulmonary oxygen toxicity.

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By Ran Arieli
Header image by Sean Romanowski

Hyperbaric oxygen (HBO) is an intrinsic facet of diving. However the risk of pulmonary oxygen toxicity (POT) has become a prominent issue due to the expansion of diving techniques, which include oxygen-enriched gas mixtures and technical diving. But there is still no satisfactory, practicable method of calculating the cumulative risk of oxygen toxicity during an HBO exposure. 

The concept of the Unit Pulmonary Toxic Dose (UPTD), which is based on a modification of the rectangular hyperbola, was proposed in response to a request for oxygen exposure limits based on a very small amount of research data: a point at 4 bar and the absence of known injury at 0.5 bar (Lambertsen, personal communication). However, this was merely descriptive, and not based on any physical-chemical-physiological mechanism. The NOAA REPEX method, originally developed by R.W.”Bill” Hamilton in the 1980s, is based on a simple linear assumption without sufficient research validation. It is well accepted that both of these methods are inaccurate.

Because any chemical reaction, including the production of reactive oxygen species (ROS) and reactive nitrogen species (RNS), can be described by a polynomic expression, we chose the power law approach. Having incomplete knowledge of the reaction, we assumed that the rate of development of oxygen toxicity is related to the highest power of the PO2. When the various oxygen toxicity parameters such as a decrease in lung capacity, reduced hypoxic ventilatory drive, changes in skin conduction, or increased thickness of the alveolar wall, among others, are modeled as a function of exposure time, the result can best be expressed as a quadratic equation.

The rate of production of hydrogen peroxide (a precursor of ROS and RNS) is also related to the square of time, which can explain this time relationship. The power equation was shown to have good predictive capability.1,2  

Deriving The Power Equation

From the above considerations, it follows that the development of oxygen toxicity should be related to the square of the exposure time (t) and to some power of PO2 (PO2c).

Initially, we derived the power equation for the loss of vital capacity (VC), with the addition of a parameter to adjust for the units:

%ΔVC = 0.0082 × t2 × (PO2)4.57                       

The predictive capacity of the power equation compared with the UPTD concept is shown in the following figure. At a PO2 above 1 bar, the UPTD concept fails.

Figure 1. Prediction by two models of the reduction in vital capacity of the lung at four oxygen pressures as a function of time: the NMRI modified pulmonary toxicity dose (blue lines), and our POT index (red lines).

It has been found that the recovery of VC (at a PO2 < 0.48 bar) has the form of an exponential expression, where the time constant increases linearly with the oxygen pressure of the previous exposure, as seen in the following figure. 

Figure 2. Time constant (τ) for the recovery of human VC as a function of pre-recovery PO2 exposure. The line represents the linear regression solution.

It was demonstrated that the pulmonary pathology is different at high and low PO2, that is, they represent distinct pathologies. With exposure to an increased PO2, central (cerebral) effects on the lung are greater than the local pulmonary effects of HBO. Thus, exponential recovery of pulmonary oxygen toxicity took the form:

ΔVCtr% = ΔVCe% × e – [- 0.42 + 0.384 × (PO2)ex] × tr

where tr is the recovery time in hours, ΔVCtr is the value after the recovery time, ΔVCe is the value following the previous hyperbaric oxygen exposure, and (PO2)ex is the previous exposure to hyperbaric oxygen in bar. The rate of recovery depends on the PO2 which caused the insult, and occurs with exposure to a PO2 > 1.1 bar.

A recently published study proposed other parameters to replace the changes in VC as an indicator of POT: incidence of symptoms (inspiratory burning, cough, chest tightness and dyspnea) and a change in pulmonary physiological parameters (FVC, FEV25-75 , FEV1  and DLCO). Because the units of the POT index [t2 × (PO2)4.57] are squared for time and the powered PO2, this index can also accommodate estimates which employ the other parameters. The incidence of POT in 16 different HBO exposures conducted at the U.S. Navy Experimental Diving Unit (NEDU) is plotted in the next figure as a function of the calculated POT index.

Figure 3. Incidence of POT plotted as a function of the POT index calculated for each of the 16 different exposures. The regression line is also shown.

Thus, the POT index can be used to predict the incidence of POT:

Incidence (%) = 1.85 + 0.171 × POT index   (1)                   

Calculations

For the accumulation of toxicity at a PO2 above 0.6 bar use Eq. 2:

For a number of periods (n) of continuous hyperoxic exposure, each for a different length of time and at a different PO2, the calculation should take the form of Eq. 3.

During recovery at oxygen pressures below 0.50 bar, Eq. 4 is used.

POT index trPOT indexe  × e – [- 0.42 + 0.384 × (PO2)ex] × tr  (4)

where tr is the recovery time in hours, POT indextr  is the value after the recovery time, POT indexe is the value following the previous hyperbaric oxygen exposure, and (PO2)ex is the PO2 in the previous exposure in bar.

When there is a recovery period in between the hyperoxic exposures, the POT index at the end of recovery should be calculated from Eq. 4. The time required to obtain the same POT index for the next PO2 (PO2nx) in the following hyperoxic exposure will then be derived by rearranging Eq. 2 thus:

t* = [POT index / (PO2nx)4.57)]0.5. (5)

This calculated time t* should be added to the time of the coming hyperoxic period, as if the whole exposure started at this PO2. Thus:

POT index = (t*+tnx)2 × (PO2nx)4.57 (6)

The U.S. Navy recommends oxygen exposure limits that will result in a 2% change in VC, the maximum permissible exposure being expected to produce a 10% decrease. Thus, inserting ΔVC = 2% or ΔVC = 10% into the power equation will set the PO2 and time limits. For these two values of ΔVC, the POT index should not exceed 244 and 1,220, respectively, both at a constant pressure and for a complex exposure. We propose that the POT index be used to replace the UPTD or REPEX methods. 

In summary, one may either employ the POT index limits of 244 (mild) to 1220 (exceptional), or determine the appropriate chosen risk from the incidence equation: Incidence (%) = 1.85 + 0.171 × POT index

“In summary, one may either employ the POT index limits of 244 (mild) to 1220 (exceptional), or determine the appropriate chosen risk from the incidence equation: Incidence (%) = 1.85 + 0.171 × POT index.”

Saturation Dives

Ed. note: Saturation diving has become a main modality for commercial diving (see: “Anatomy Of A Commercial Mixed Gas Dive”). Though it is not directly relevant for technical dives, it is remarkable that Arieli’s model spans the gamut from bounce dives to saturation. Mind those PO2s!

In principle, no threshold was incorporated in the power expression, which operates when ROS and RNS production overpowers the antioxidant activity that induces recovery. It was suggested that in prolonged exposures with a relatively low PO2, for example in saturation diving with a PO2 of 0.45–0.6 bar, a recovery process for POT accompanies the development of toxicity to attenuate but not entirely eliminate the toxic outcome.3

In one report of an experimental chamber saturation dive lasting 261 hours with a PO2 of 0.5‒0.6 bar, 2 of the 8 subjects (25%) developed POT. The POT index for 25% amounts to 136 (from Eq. 1). To adjust for these two opposing effects of cumulative toxicity and the recovery process, the following equation may be used:

POT index = t2 × PO24.57 × e-0.0135 × t (7)                                       

where t is the exposure time to a toxic level of hyperoxia in h. 

Figure 4. POT index calculated for the 261 hr. exposure to a PO2 of 0.55 bar for both cumulative toxicity and recovery which take place throughout the exposure (Eq. 7). The POT index reaches 136 at the end of the exposure, which is consistent with a POT incidence of 25%. 

Evidently, eight dives are an insufficient sample. However, after the publication of reference #3, I obtained a further set of eight saturation dives. These divers dived for 4 days at a PO2 of 0.6 bar. Half of them suffered POT. The calculated percentage using Eq. 7 and Eq. 1 yielded 43.6% – rather close to the 50%. I would therefore recommend the use of Eq. 7 and Eq. 1 for long saturation dives with a PO2 close to the lower range of toxicity and above 0.48 bar.

References

1. Arieli R, Yalov A, Goldenshluger A. Modeling pulmonary and CNS O2 toxicity and estimation of parameters for humans. J Appl Physiol. 2002;92:248‒56. doi: 10.1152/japplphysiol.00434.2001. PMID: 11744667.

2. Arieli R. Calculated risk of pulmonary and central nervous system oxygen toxicity: a toxicity index derived from the power equation. Diving Hyperb Med. 49: 154-160, 2019. doi: 10.28920/dhm49.3.154-160. PMID: 31523789

3. Arieli R. Pulmonary oxygen toxicity in saturation dives with PO2 close to the lower end of the toxic range – a quantitative approach. Respir Physiol Neurobiol 268: 103243, 2019. doi: 10.1016/j.resp.2019.05.017. PMID: 31158523.

Additional Resources:

Respiratory physiologist Barbara Shykoff, US Navy Experimental Diving Unit (NEDU), has also developed a model for estimating risk of pulmonary toxicity (2018): Calculator For Estimating The Risk of Pulmonary Toxicity 

Shearwater Research: Why UPTD Calculations Should Not Be Used by Barbara Shykoff, 2017

Shearwater Research: Oxygen Toxicity Calculations by Erik C. Baker (2012). Explains earlier UPTD and REPEX calculations.

Tolerating Oxygen Exposure by RW Bill Hamilton, 1997

RW Bill Hamilton’s Original REPEX paper: Tolerating Exposure To High Oxygen Levels: Repex And Other Methods by RW Hamilton, 1989

An early 1985 review of the UPTD Model: Predicting Pulmonary O2 Toxicity: A New Look at the Unit Pulmonary Toxicity Dose by AL Harabin, L.D. Homer, PK Weathersby and ET Flynn 

Ed. note: We plan to run an article discussing and comparing these various methods for calculating the risk of pulmonary oxygen toxicity in a coming issue of InDepth, including some practical tips for calculating the risks of your own dives.


Dr. Arieli is the retired Head of the Hyperbaric Physiology Research Unit at the Israel Naval Medical Institute. He obtained his Ph.D. from Tel-Aviv University, completing a post-doctorate at Buffalo, The State University of New York.  He lectured in respiration physiology at the Technion Faculty of Medicine in Haifa. His main topics of research are respiratory physiology, integrative physiology, oxygen toxicity, and decompression physiology. Dr. Arieli has investigated the environmental factors which affect oxygen toxicity, proposing algorithms for the prediction of pulmonary and central nervous system oxygen toxicity. In his research into decompression physiology, Dr. Arieli has presented a new mechanism underlying bubble formation on decompression. Dr. Arieli has published 128 research papers, and continues to pursue his research at the Israel Naval Medical Institute in Haifa and the Western Galilee Medical Center in Nahariya, Israel.

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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.

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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.

TRAINING:

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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