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Off the Deep End? What You Should Know About Pool Chemistry

Now that pools are re-opening for swimming and training, we thought it fitting for us to take a bit of a deep dive into pool chemistry. Fortunately, Reilly Fogarty’s got the scoop on what’s going on in your favorite pool.

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By Reilly Fogarty
Header image by GUE instructor Steve Millington, http://socalscubadiving.com.

When divers dream of adventure, it’s not often that their mind wanders to images of the shallow end of the local YMCA pool. Adventures may not often take place in waist-deep water, but a chlorinated escape from our terrestrial confines can do a great deal for our health and safety. Between fitness, training, and cooling off on a hot summer day, you’ll spend a lot of your life in a pool, so you should know what you’re swimming in. Here’s the scoop on what’s going on in your favorite pool.

Swimming in History

Pools have been used for recreation, religion, and fitness for longer than history has been recorded. The first man-made swimming pool is thought to be the Great Bath of Mohenjo-Daro, a thirty-by-twenty-three foot bath created sometime in the 3rd century BCE. This structure was predated by a pair of religious pools located in Sri Lanka built nearly a century before. The first heated pool is credited to Gaius Maecenas, who may have been the architect of a bath heated by fire pits in Rome around the first century BCE. It’s not hard to see why ancient societies wanted the pools—from religious ceremonies to block parties, the uses for pools haven’t changed dramatically over the centuries.

Pools have been around for more than 2000 years. Photo by Saqib Qayyum / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

From these earliest pools sprouted dozens of similar structures across the globe. The logistics of building and maintaining the pools limited their use to only the most affluent until their popularity exploded during the mid-19th century with the rapid advent of new building technologies. Six indoor pools were built in London in 1837, and then the creation of the modern Olympic Games in 1896 rapidly spread public demand for public pools. 

Leaps in technology in the 20th century brought chlorination and filtration systems to pool design, and made pools easier to both build and maintain. The brick and tar construction of early history gave way to a flexible alternative, gunite, and soon after above-ground pool kits hit the market. Once the cost to build a pool dropped to levels attainable by common folk, they came to American backyards in droves. How to keep all those pools clean, however, was another issue. 

Pathogens & Pool Noodles

Once upon a time, the only way to clean a pool was to drain it and refill it regularly. Pools were often built on downward slopes to help drain them, and the water was cycled frequently. In the late 19th century people began to worry about large bodies of freshwater becoming disease ridden.

The first attempt to sterilize a pool in the U.S. using chlorine was at Brown University in 1910. The 75,000-gallon/284 kiloliters Colgate Hoyt Pool was chlorinated by graduate student John Wymond Miller Bunker, who used a bleaching powder, hypochlorite of lime (calcium hypochlorite), which had been recently discovered as a method to treat drinking water, at a concentration of 0.5 ppm. The pool remained sterile for four days. Bleaching powder, including both calcium hypochlorite and sodium hypochlorite (both a form of chlorine) instantly became the standard in pool sanitation, and spread across the world. 

Laws dictating pool sanitation appeared, and soon after diatomaceous earth filters hit the market. The filters use powdered rock to capture particles in the water and are frequently combined with skimmers, devices that filter larger objects from the surface of the pool through a mechanism similar to a storm drain. Pool use continued to increase in popularity and owners dabbled in a number of purification systems, from ultraviolet light, to ozone gas, to the chlorine and salt chlorinator systems most pools use today. 

Chlorine can cause irritation of the eyes and airways. Photo by Ted Harty, www.freedivingsafety.com 

Purification systems aren’t without their flaws. Put too much chlorine in a pool and you risk irritating your eyes and airway, causing rashes, breathing difficulties or even chemically burning the fine hairs off your body. More commonly, the combination of chlorine with the ammonia found in urine can create compounds called chloramines, or cyanogen chlorides. Chloramines cause the typical “over-chlorinated pool smell” we associate with hotel pools, and can cause skin and eye irritation, as well as exacerbate allergies or asthma. Cyanogen chloride can interfere with the body’s ability to use oxygen and can be fatal—thankfully it’s volatile and rarely forms in dangerous concentrations and degrades quickly when it does. 

Pee isn’t the biggest concern for pool hygiene, despite the fact that swimmers leave, on average, about a shot glass worth of urine every time they jump in.

Pee isn’t the biggest concern for pool hygiene, despite the fact that swimmers leave, on average, about a shot glass worth of urine every time they jump in. [Ed.note: Fitness and competitive swimmers urinate in the pool!] We tolerate the hazard and complications of chlorination because of the microbial risks associated with large numbers of people effectively bathing together. The World Health organization (WHO) points to Shigella and Escherichia coli O157 as bacteria of particular concern for swimmers. 

Competitive swimmers urinate in pools. Photo from Abbie Fish, swimlikeafish.org

Bacterial outbreaks are relatively rare among pool use but these bacteria, as well as a host of viruses, protozoa and fungi can be passed from swimmer to swimmer with relative ease. Both bacteria cause vomiting fever and diarrhea, although E. Coli O157 can cause hemorrhagic colitis and haemolytic uraemic syndrome (HUS) in severe cases. Giardia and Cryptosporidium are two protozoa that also pose a risk to swimmers, both being carried with fecal material. Both are highly resistant to disinfectants, are very infectious, and are shed in high densities by those infected. Diarrhea, cramping, vomiting and fever are common symptoms of both. Adenovirus, hepatitis A, norovirus and echovirus round out the list of common contagions in pool water, each with their own unique symptoms. 

There are a number of less common viruses and bacteria that can pose a risk to swimmers, but it’s worth noting that very few instances of group infection can be traced back to pool water. For the most part, modern pools are quite safe, and a combination of sterilization (to kill pathogens) and filtration (to control fecal release and other contaminants) can effectively keep a pool safe. 

The Mystery of Chlorine

Interestingly, the mechanism of chlorine sterilization is not fully understood. Research from the mid-20th century seemed to show that chlorine would react with some biomolecules as a result of it’s division into hypochlorite and hypochlorous acid in water. Later work indicated that chlorine likely reacted with a variety of bacterial targets and specific nucleic enzymes and membrane lipids – this was called the “multiple hit” theory, as explained in this 1998 Scientific American article titled, “How does chlorine added to drinking water kill bacteria and other harmful organisms? Why doesn’t it harm us?

Chlorine can eliminate a wide range of contamination factors. Photo by kappykeepers.com

More recent work suggests that chlorine specifically attacks cell walls by altering them physically and chemically, killing microorganisms by interrupting cell functions. Mechanically this theory involves a few steps. First chlorine disrupts the structure of the cell wall. This allows components of the cell that are critical to its function to escape, which causes a chain reaction of function termination, and eventually cell termination.

What this means effectively is that chlorine can kill a wide range of pathogens in relatively low doses. The concentrations used in public pools and water supplies are carefully monitored and designed to be small enough that ingestion of a normal amount allows only enough chlorine into the intestinal tract as can be neutralized by the action of the digestive system. That’s not to say that chlorine isn’t toxic – it can be extremely dangerous and must be handled with care – but like many poisons the dose determines the lethality. Because the concentrations used in pools are so low, the amounts that humans are likely to ingest are not harmful. 

At low concentrations chlorine in the body can be neutralized by harmlessly reacting with food in our stomachs, material in our intestinal tracts, or by the acidic environment of the stomach.

At low concentrations chlorine in the body can be neutralized by harmlessly reacting with food in our stomachs, material in our intestinal tracts, or by the acidic environment of the stomach. The Environmental Protection Agency (EPA) works closely with water utilities and environmental groups to reassess safe chlorine levels in drinking water and pools on an regular basis, and these guidelines along with those from the CDC should be used to determine what chlorine concentrations are safe for normal use.



Bug Count

The Centers for Disease Control  do provide some recommendations for specific chlorine and levels for pool use. Free chlorine in a concentration of a minimum of 1 part per million (ppm) in a pool, or 3 ppm in a hot tub, and a pH of 7.2-7.8 provides a safe concentration for swimmers and should kill most bacteria within a few minutes. Because bacteria levels are so difficult to measure in real time, testing is expensive, and equipment is scarce, regulations focus on mandating minimum free chlorine levels that are based on the environment rather than changing sanitation regulations that are based on bacterial load. This works on the assumption that known chlorine concentrations will kill common bacteria in a reasonably effective manner, and free chlorine indicates a sanitized body of water with a margin of safety. 

Something that might be confusing is the common chlorine smell found around high-traffic pools. This is actually caused by chloramines, the byproduct of a reaction of chlorine and urine, and can give off a strong odor and irritate the eyes, skin and airway. While the smell would seem to indicate that there is too much chlorine in the water, the opposite is actually true—eliminating the smell requires the superchlorination of the pool. Superchlorination, or “shocking” oxidizes the chloramines and leaves only free chlorine by flooding the body of water with chlorine levels five to ten times the normal concentration. Bathing during superchlorination is ill-advised, but the process should be done once a month in most cases, or once a week in hot weather. 

The risk posed by fecal contamination is much greater than general bacterial shedding, and diarrheal contamination is significantly higher-risk than a formed fecal incident. Both types of contamination require a fairly rapid response to minimize infection risk, effectively removing swimmers, isolating the hazardous material and superchlorinating the pool to disinfect it. The primary concerns with fecal contamination are Giardia and Cryptosporidium. While Giardia can be eliminated in as little as 20 minutes through superchlorination, Cryptosporidium is chlorine-resistant and can take as long as 25.5 hours to be safely removed. 

Treating Pools

Methods to treat pools have changed over the centuries, but only chlorine and a few similar chemicals have proven really effective. From alternatives like ultraviolet purification, to ozone, to constantly moving water, chlorine alternatives have failed for centuries and left us with traditional chlorine, bromine, and cyanuric acid. 

Chlorine used as free chlorine is fairly straightforward to use—it’s added to the pool and kills microbes. The disinfectant can be added as a liquid, tablet, stick, or granular powder. These products are typically a sodium, lithium or calcium base bonded to chlorine to stabilize the product and prevent dangerous accidental exposures. When dissolved in water the bonds between the chlorine and it’s stabilizing compound break and the free chlorine is released. This free chlorine is actually not the compound that disinfects the pool, but it must be broken down one more step to hypochlorous acid through dissolution in water. We can estimate hypochlorous acid concentration fairly accurately through the known reaction with water, so it’s easier to deal with these chemicals as “chlorine” in broad terms. 

There is a bit of an art to keeping chlorine levels in check, as too little chlorine will allow bacteria to grow and too much will cause skin and mucous membrane irritation, but chlorine sanitization is more labor than rocket science. The average public pool should have somewhere between 3 and 5 parts per million of free chlorine, while jacuzzis may require up to 10 parts per million, due to the hot environment providing an incubator for bacteria. 

Salt water pools are now common as well, but these too rely on chlorine. In a salt water pool a salt cell or generator breaks down the components of salt water via electrolysis. This reaction results in the formation of chlorine in basic and acidic analogs as sodium hypochlorite and hypochlorous acid, and these are used to sanitize the pool. Salt water pools can be a nice alternative to traditional chlorine pools, but they don’t feel like the ocean, since most residential salt systems require salt levels around 4200 parts per million, while the ocean has an average salt concentration of about 35,000 parts per million. 

Bromine is a relatively recent alternative to chlorine. It is similar in structure and behavior to chlorine but less pH sensitive, and it’s reaction in water leaves bromide salts in solution which can be recycled. The downside to bromine, however, is that it’s very unstable in sunlight. Chlorine will degrade in sunlight somewhat, but Bromine quickly becomes ineffective in direct light. This means that it can be used to sanitize indoor pools but won’t do much good if used outdoors. Concentration levels for most pools are similar to chlorine, as are the side effects and signs of overuse. 

Cyanuric acid is the solution to the instability of chlorine in strong ultraviolet light. The acid can be added to a pool to stabilize the chlorine in solution. It does this by binding to the sodium hypochlorite ions released by the chlorine after reaction with pool water, and shielding them from UV rays. This allows free chlorine to be effective approximately three times as long as it would otherwise be. Because Cyanuric Acid binds to active sites on the hypochlorite ions, it can decrease the active sites available for reactions with target pathogens, so levels that are too high will reduce chlorine’s effectiveness and may require fresh water dilution. 

Watersports anyone?

If there’s one thing divers are good at, it’s producing astonishing amounts of urine as soon as they put on a wetsuit. Unfortunately for us, neither dive equipment or urine reacts well with chlorine. There are no color-changing indicators to show who pees in a pool right now, but pee does react with chlorine to produce chloramines. 

Chlorine and urine, including their joint reaction, can be harmful to dive gear. Photo by GUE instructor Steve Millington, http://socalscubadiving.com.

This is a two-part concern for us, and serious enough that the CDC has to send out warnings every year. Pool urination simultaneously removes free chlorine from the pool, decreasing the pool’s ability to self-sanitize, and creates a chemical irritant called chloramines. The byproduct of the reaction of chlorine with the amines in urine, chloramines cause respiratory irritation, skin rashes and can irritate the eyes and mucus membranes. They also produce the smell we typically associate with over-chlorinated pools. 

As if that wasn’t enough, chlorine also degrades rubber like that’s it’s job. Black harnesses will fade to brown, o-rings and wing bladders will degrade, and regulators will need shortened maintenance intervals. Want to save your gear, your pool and your skin? Pee before you dive, rinse your gear well and keep that pool chlorinated. 

Want to save your gear, your pool and your skin? Pee before you dive, rinse your gear well and keep that pool chlorinated. 

Additional Resources:


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.

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Learning from Others’ Mistakes: The Power of Context-Rich “Second” Stories

Proper storytelling is a key to learning from the mistakes of others. Human Factors consultant and educator Gareth Lock explains the power of context-rich stories to inform and help us to develop the non-technical skills needed to make better decisions, communicate more clearly, and lead/teach more effectively.

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by Gareth Lock
Header image courtesy of Gareth Lock
. Divers from Red Sea Explorers’ examining a magnificent gorgonian coral.

Diving can be a fun, sociable, and peaceful activity; it can be challenging and technically difficult; and it can be a way of escaping the hustle and bustle of modern life. Sometimes new wrecks are discovered, caves have new line laid in them, new encounters with wildlife are experienced, and in many cases, courses are completed where both instructors and students have learned something new. 

However, it can also be scary, harrowing and frightening if things don’t go to plan or if the plan was flawed in the first place. 

Fortunately, the majority of dives which take place are the former and we consider the outcomes to be positive. If we think about it, the goal for every dive should be to surface, having had an enjoyable time, with gas reserves intact and no-one feeling physically or emotionally injured. But how do we achieve this goal considering the inherent risks we face while diving? 

The easy answer would be to have effective training, to have the correct equipment, and to have and apply the right mindset. These three things together then lead to safe diving practices. You could say that the majority of safe diving practices and safely designed and configured equipment comes from feedback following accidents, incidents, and near misses. You only have to look at the work which the late, famed cave explorer Sheck Exley did in terms of cave diving fatalities and his “Blueprint for Survival” to see how procedures and equipment have evolved.

What do we learn?

There are accident and incident reports available to us. What do we learn from them? Bearing in mind that the majority of reports which divers see are either in social media or summarised in reports like the Divers Alert Network Annual Incident Report or the BS-AC Annual Incident Report.

For example, the following incident reports are written in a style similar to those you would find on social media or in an organization’s incident report.   

An inexperienced diver entered the water to provide support for a guided dive to 24m. They got separated from their buddy, made a rapid ascent to the surface after nearly running out of gas. They were recovered on the boat without any symptoms of DCS being present.

A diver on the final dive of a rebreather training course entered the water from a dive boat. The diver swam to the side of the boat to receive their bailout cylinder to clip on. While sorting their gear out alongside the boat, they appeared to go unconscious and descend below the surface. The diver was recovered from 38 m/124 ft and despite CPR and first aid being applied, they were pronounced dead on arrival at the hospital ER. On inspection, the oxygen cylinder on their rebreather was found to be turned off and the controller logs showed that the pO2 had dropped to 0.05 while they were on the surface.

How much learning do you get from these reports? What emotions did you feel while reading them? What did you think was the primary cause of each of these events? If you were to choose two or three words to describe the causes, what would they be? 

Human error? Complacency? Inexperience? Rushing? Not paying attention? Overconfidence? Naivety? Arrogance? Stupidity? Who was it? Where was the instructor? Were they certified? Which agency? Were they qualified?

All of these are normal responses, and they make up the first story.

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Photo courtesy of 123rf Image Library

The First Story

The first story is the narrative we hear, and we start to make immediate judgments on. We can’t help making judgments, even when we try not to. We make judgments because we compare the stories we’ve just read or heard to our own previous experiences. We match patterns to what we ‘know’ and then fill in the gaps with what we think happened, all the time thinking about whether it was the ‘right thing’ to do based on our own experiences.

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Photo by Gareth Lock.

This ‘filling in gaps’ is normal human behavior. Because our brains are constantly trying to make sense of the situation when we don’t have enough information about a scene or a situation, we reflect on what we’ve seen, read, and heard in the past and then make a best guess or closest fit. During this process, we will be subject to a number of biases, and one of the strongest at this stage is called confirmation bias. This is where we think we know the answer to the question, then as we read or hear something in the story that aligns with our reasoning, we stop looking any further because we have confirmed our suspicions.

In many cases, we carry on and don’t think anything of the learning opportunities presented because we know what happened, we know that ‘we wouldn’t do that’ because we would have spotted the issue before it became critical. We often make use of counterfactuals (could have, should have, and would have) to describe how the incident could have been prevented.



Unfortunately, this means that often we don’t learn. There is a difference between a lesson identified and a lesson learned—a lesson learned is where we make a conscious decision to accept how we do things based on the conditions and outcomes, or we actually put something in place which is different than what was there before and see how effective it is to resolve the problem encountered. 

If we are to make improvements, we need to look at the errors, mistakes, and deviations that were made. However, we must recognize that errors are outcomes, not causes of adverse events. If we want to stop an adverse event from occurring, we need to look closer at the conditions which led to the error occurring i.e., the error-producing conditions. 

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Extracted from INPO/DOE Human Performance Improvement Handbook Vol 1 – The Human Diver.

The easiest way to look for error-producing conditions in an event that has already happened is to get those involved to tell context-rich stories. This becomes the second story.

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Photo courtesy of 123rf Image Library.

The Second Story

Second stories look much deeper than what we first hear. They look at the context, the local rationality, the conditions, especially those conditions which might lead to errors. Ultimately, they expose the inherent weakness and gaps in any system, where the system includes people, paperwork, equipment, relationships, the environment and their interactions. 

Second stories also highlight how divers and instructors are constantly adapting and changing their behaviors/actions to deal with the dynamic nature of diving. They describe ‘normal work’. This adaptation could be moving dive sites, increasing or reducing the time for a course, the order in which skills are taught or the amount of gas used/planned for a dive. Second stories describe the difference between ‘Work as Imagined’, which is what is written down, what is expected to happen, and against which compliance is assessed, and ‘Work as Done’ which is what actually happens in the real world and takes into account the pressures, drivers, and constraints which are faced by those on the dive or the course.

The easiest way to see what a second story looks like is to tell it, and the following account is the same recreational event as above but told as a second story. 

An Advanced Open Water (AOW) diver with around 50 dives was acting as an ‘assistant’ to the instructor and dive-centre owner on a guided dive with five Open Water (OW) divers and recent graduates from the school they themselves had learned at. The AOW diver felt a social obligation to help the Open Water Scuba Instructor (OWSI) who was leading the dive, because the OWSI had done so much to help her conquer her fear of mask-clearing during her own training. However, she was also wary that, over time, her role had moved from being a diver on the trip to being almost the divemaster by helping other divers out, which she wasn’t trained to do. In addition, the instructor regularly asked her, at the last minute, to help out and change teams to ensure the ‘experience’ dives happened.

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Photo by Gareth Lock.

On this particular occasion, the AOW diver was buddied with a low-skilled OW diver who acted arrogantly and did not communicate well. In fact, she didn’t believe that three of the five on this trip should have received their OW certificates, given their poor in-water skills. As they approached the dive site, the visibility could be seen to be poor from the boat and the surface conditions weren’t great. The instructor said to the AOW diver, “Don’t lose the divers. I want you at the back shepherding them.”

They entered the water and descended to 24 m/78 ft and made their way in the poor visibility. On two occasions, the OW buddy had to be brought back down by the AOW diver as they ascended out of control. At one point, the OW diver turned around quickly and accidently knocked the AOW diver into the reef. Unfortunately, the AOW diver became entangled in some line there, and the OW diver swam off oblivious to the entanglement. When the five divers and instructor reached the shot-line ready to ascend, the instructor realized the AOW diver was missing. The instructor couldn’t trust the five divers to ascend on their own and didn’t have enough time to wait at the bottom and conduct a search, so the six ascended. On the surface, the buddied OW diver said that the AOW diver had swum off looking at fish in a certain area.

In the meantime, the AOW diver had managed to free herself; but in her panic, while stuck on the bottom, she breathed her gas down to almost zero and had to do a rapid ascent. She surfaced, feeling very scared and sick with panic, just as the instructor was speaking to the other six on the surface. On seeing the AOW diver break the surface, the instructor swam to her but turned and shouted at the other divers, admonishing them for abandoning their buddy on the bottom. The AOW diver felt very alone and wanted to give up diving as she was not given the opportunity to tell her side of the story.

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Photo by Gareth Lock.

Observations on potential contributory factors and error-producing conditions:

  • Deviation of standards on the part of the instructor/dive-center owner taking OW divers to 24 m/78 ft, maybe driven because of the need to generate revenue and offer something unique.
  • Authority gradient between the instructor and AOW diver meant that the AOW diver felt they couldn’t end the dive before they even got in the water or once in the water.
  • Inferred peer pressure to help out when they weren’t qualified or experienced enough to act in a supervisory role.
  • Poor technical skills on the part of the OW divers and the AOW limited their situation awareness to be aware of hazards and risks.
  • Limited awareness on the part of the instructor regarding the location of all the divers during the dive.
  • Positive note – good decision on the part of the instructor to ascend with the five OW divers in poor conditions and not keep them on the bottom or get them to ascend on their own.

A full account of the second event can be found here where you can also download a guide which contains more detail than the video covers and also gives you details on how to run a learning event at your dive center or in your own classes.

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We can see that the learning opportunities have increased in the second stories. They allow certain issues to be identified like time pressures, financial pressures, peer-pressure, authority gradient, teamwork, leadership, decision-making and situation awareness. These aspects are rarely captured or recounted in the narratives we see online or in incident reports. There are a number of reasons: 

  • They are often considered ‘common sense’, 
  • Our brains are constantly looking for simple answers to complicated or complex problems, and one of the easiest ways to do this is to find an individual or piece of equipment to ‘blame’ rather than look wider.
  • Those involved don’t consider these factors to be important so they don’t write them down.
  • Those involved don’t know about these error-producing conditions or human factors so they don’t know to include them.
  • There is no formalised and structured investigation process for diving incidents by diving organisations to facilitate the capture, analysis and sharing of second stories.

Telling second stories isn’t enough to create learning though. We have to work out how to change our own behaviors, and that is where the free materials and courses which The Human Diver provides come in. They help develop these non-technical skills in divers, instructors, instructor trainers, and dive center managers/owners to help them make better decisions, communicate more clearly and lead/teach more effectively. Ultimately, it is about having more fun on the dive, and ending each dive with the goal described at the start of this article intact and creating learning in the process.


Since 2011, Gareth has been on a mission to take the human factors and crew resource management lessons learned from his 25 year military aviation career and apply it to diving. In 2016, he formed The Human Diver with the goal to bring human factors, non-technical skills and a Just Culture to the diving industry via a number of different online and face-to-face programmes. Since then, he has trained more than 350 divers from across the globe in face-to-face programmes and nearly 1500 people are subscribed to his online micro-class. In March 2019, he published ‘Under Pressure: Diving Deeper with Human Factors’ which has sold more than 4000 copies and on 20 May 2020, the documentary ‘If Only…’ was released which tells the story of a tragic diving accident through the lens of human factors and a Just Culture. He has presented around the globe at dive shows and conferences to share his passion and knowledge. He has also acted as a subject matter expert on a number of military diving incidents and accidents focusing on the role of human factors.

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