Near Misses with Rebreathers and Carbon-Dioxide

John BantinIt was back in 1993 that rebreather evangalist Peter Readey first introduced me to the pleasures of rebreather diving. I was meant to do his pioneering course with Martha Holmes, a producer/presenter for the BBC, but her employer decided this diving technology was far too hazardous and after the first day she went off to do other things. It was dangerous but in those days I was totally unaware of exactly how dangerous it was.

I learned first on the prototype Prism SCR, later the Draeger Atlantis SCR (even briefly becoming a consultant to Draeger) and then the prototype Inspiration, the first fully closed-circuit rebreather. By the time the Inspiration was available in a full production version, you could say that I was one of the most experienced rebreather divers around. Of course, today there are many rebreather divers who know a lot more about it than me.

I was away in the Maldives with two of the first commercially produced Inspirations and Martin Parker, CEO of the company that made them, when we heard of the first ‘flaker’ back in the UK. An early adopter, he’d passed out but luckily his life was saved by his friends. He’d switched his unit off while waiting to dive, presumably to save battery power, and forgotten to switch it back on again. Not looking at his display and with dropping oxygen levels since the solenoid was unable to open to replenish the breathing loop, he’d been the victim of hypoxia.

There then followed some inexplicably tragic deaths of some high-profile divers using this newly available item of kit. Some divers even passed away whilst swimming and breathing from their units at the surface.

What was happening? The equipment had appeared to perform faultlessly. Many of them appeared to suffer heart attacks while in or underwater.

While out in Egypt a few years earlier with Peter Readey, we’d made it a habit to dry out our scrubber material by unloading it and spreading it out in the sun to dry between dives. I can almost hear the gasps of horror coming from knowledgeable rebreather divers on reading this, but it was the loss of sufficient oxygen from the breathing loop that continually obsessed me. In fact I even sat up in bed one night in my hotel room, breathing off my unit with the oxygen supply turned off in order to experience and be able to recognise the first symptoms of hypoxia. There were no symptoms. It was as dramatic as a bullet in the brain save that the mouthpiece fell from my lips and my unconconcious body was revived with air from the room. I awoke some hours later with the worst hangover imaginable but unbeknown to me, it had been a close-run-thing and I had become a ‘flaker’ in relative safety.

We were also concerned with the effects of two much oxygen. We’d scrutinise our ppO2 gauges as we decended because if you increased the depth too quickly, the level could spike. Equally, if we ascended too quickly it could drop. (It’s sometimes strange to consider you need less ppO2 at depth than you do in the shallows.)

We were so absorbed with the dangers of hypoxia and hyperoxia (that’s caused by too much or too little oxygen in the breathing mix) we completely overlooked the very real dangers of carbon-dioxide poisoning. It was only during a later trip with Peter Readey down in Baja California that I grew to understand the dramatic and usually fatal consequences of failing to remove the CO2 from the gas we breathed.

Using a later prototype Prism closed-circuit rebreather, I proved that, assembling the unit in the darkness of the early hours, you could insert the scrubber canister into the scrubber bucket of the Prism the wrong way up. It was immediately redesigned to make that not possible after the sequence of events I’m going to tell you about.

The scrubber canister is filled with Sofnolime (a typical and commercially available absorbent) that chemically removes the poisonous bi-product of breathing and turns to calcium carbonate in the process. In putting it in inverted I had effectively by-passed the scrubber from the breathing loop. I started to breathe unscrubbed breathing mix as soon as I commenced breathing from the rebreather and long before I got in the water.

You are probably aware that it is the raised level of carbon-dioxide in our lungs that is the trigger that makes us breathe. It is the urge to jettison this poisonous gas. I was always under the illusion that the onset of CO2 poisoning would be preluded by heavy breathing and would form an unmistakable warning sign. This may be the case but the onset of breathlessness is also accompanied by confusion and reduced brain function, something I hadn’t fully appreciated. How do I know this? Wearing the rebreather and pre-breathing it on the deck of the diveboat, I became short of breath. I started to breathe very heavily and assumed it was caused by the air-side weight of my equipment and the hot Mexican sunshine. It got worse and I became focused that all would be well once I was in the water and weightless.

Wrong! I jumped into the sea and descended to thirty feet deep before things got so bad I realised I was fighting for my life. Somehow I managed to fight my way back to the surface where I was dragged back on board the boat. My heart by this time was beating so fast it was nearly climbing out of my chest.

The doctor who annually conducted my British HSE Medical Examination for Divers always reported that I have a very low heart-rate, the blood pressure readings of a teenager and a Harvard step-test result to be envied. It seems I was very fit. That’s lucky because I came the nearest I’ve been to having a heart attack.

I was so exhausted from that briefest of experiences that I was unable to move for a couple of hours and could only lie on the deck of the boat. If I had been any less fit or had to swim further to the surface I might certainly have been fatality caused by another of those mysterious heart-attacks underwater.


Why should you never empty out a half-used canister of scrubber material and repack it as we did when we dried out ours, all those years ago? Water is a by-product of the chemical reaction that occurs between the scrubber chemicals and the CO2. In fact it is said to speed up the reaction so firstly, be aware that it will get damp in use.

Imagine that you have line after line of soldiers in a stack defending the your body from an attacking force. As each molecule of poisonous CO2 comes into contact with a molecule of scrubber material, they both die and inactive carbonate is produced. As each grain of scrubber dies the following molecules of CO2 can pass and are killed by the next active grain of scrubber in the stack. In this way the defensive line of scrubber material passes through the stack with inactive or ‘dead’ granules of carbonate left before it.

The heat generated by the exothermic chemical reaction can be monitored by a temperature-sensing stick nowadays available with some rebreather units. However, it is a guide and should not be regarded as an indication of duration as you might the pressure gauge of a tank.

If you were to empty out the scrubber material mid-use, you would never be able to refill the canister with the same granules in such a way that the original position of each of the granules was retained. You would almost be sure to end up with a route of ‘dead’ granules passing through the stack and this would allow the poisonous CO2 to channel its way through the scrubber with disastrous consequences. How we got away without serious CO2 poisoning all those years ago I’ll never know!

If you use a rebreather, filling the scrubber canister with Softnolime (or similar) is of serious consequence. Follow the manufacturers instructions and do not use a filled canister for a longer duration that recommended in the manual, regardless of whatever else you may believe.

With the growing popularity of rebreathers among divers, all divers, whether they use a rebreather themselves or not, should be aware of the hazards of both hypoxia and carbon-dioxide poisoning.

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4 thoughts on “Near Misses with Rebreathers and Carbon-Dioxide”

  1. Well written personal experience. Sadly, all issues that John learned hard way were well know at time. I hope that newcommers will learn from the books first and gradually increase their knoweleadge through experience without runing risk to be killed by ignorance.

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  2. Before I bought my rebreather, I tried to find as many accident and incident reports as I could find to understand if it was unit or operator caused. In the decade plus, since I got certified in my rebreather, I found, and still hold, that operator error is the cause in virtually every instance. Proper instruction, use, AND maintenance are paramount. Rebreathers don’t and can’t think and will kill you without hesitation.

    All that said, it does not mean that I won’t go and do something stupid and kill myself, I hope I am less likely to.

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  3. Well written John! I worked out years ago after trying a couple of re-breathers that i do not have the temperament to keep them safe for me. There are no sort cuts for re-breather use. Meticulous habits, high intelligence and an anal-retentive instinct are essential. No hope for me!

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  4. This is one of the best articles on the subject of primary rebreather hazards that I’ve ever seen. John puts the valuable information in plain language that most divers can readily grasp and comprehend. All aspiring rebreather divers need to print this and save it for future reference.

    Rebreathers are amazing pieces of equipment that dramatically expand a diver’s universe. But they are not forgiving. Every accident that I have reviewed since rebreathers were introduced into the sport community over 25 years ago were due to “diver error”… not the equipment.

    Be well informed, understand the equipment in detail, study the physiology of gases, and religiously follow the manufacturers’ check lists and guidelines for use.

    When open circuit scuba equipment fails in usage, it is usually immediately obvious. The dangers of rebreathers are multiple and insidious. You’re not going to get a warning.

    Be prepared and thoroughly understand the risks.

    Bret Gilliam

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