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 Closed-circuit Alpine oxygen systems
© 2006 by Tom Holzel 

Part 1. The Issues 

After lying abandoned in the back closet for 43 years, closed-circuit oxygen systems are  making a sudden come-back in Himalayan climbing circles. American and European climbers are having another look at this esoteric breathing method.  Why this sudden interest in a complex technology that has failed so often when  previously used climbing Mt. Everest?  

The current ardor for giving rebreathers a try once again is fueled by several issues.  The first is that rebreathers have always promised on paper to allow a climber to ascend twice as fast as a climber using an open-circuit system.  This promise has never been adequately tested.  

A further issue is that of health.  Breathing “pure” oxygen(1) is much better for sick climbers than breathing air or open-circuit oxygen.  It may  be even more effective than the hyperbaric sacks that have been carried to the mountain.(2) 

Finally, the acclimatization mantra of “climb high, sleep low” is fulfilled to the nines by an oxygen system that “takes the user back to sea level.”  Could sleeping with a closed-circuit system eliminate high altitude deterioration? 




Tom Holzel with his HCR chemical rebreather at  21,000-ft Advanced Base Camp on the north side of Mt. Everest (1986).

Scientists have been recommending “closed-circuit” high altitude oxygen breathing apparatus to climbers since the first one was tested on Mt. Everest in 1938.  But instead of bringing breathing relief, that closed-circuit immediately made its user feel as if he was suffocating.  A small chemical closed-circuit systems was tried by the Swiss in 1952. Designed for sleeping only, it was shared by three freezing climbers on Everest’s SE Ridge who passed one unit around in a storm-bound tent “squatting like Arabs around a narghil.”  

Finally, in 1953, Evans and Bourdillon wearing a closed-circuit system preceded Hillary and Tenzing’s attempt with open-circuit units.  Initially, their climbing speed was exceptional. I theorize that outside air slowly leaked into the system, and by the time they got to the South Summit, they were essentially breathing outside air while carrying 40 dead-weight pounds of oxygen apparatus. (For a highly detailed description of this episode, see:  Windsor, Jeremy S., George W. Rodway, and John Dick. "The use of closed-circuit oxygen in the Himalayas." High Alt. Med. Biol. 6:263–269.) The 1953 closed-circuit apparatus is shown in the Hodder & Stoughton drawing below.


Why this technical fascination with closed-circuit, and why now, when the open-circuit systems have been more or less perfected?  Here is an explanation:(3) 

When breathing normal air, a person is inhaling a gas mixture of nitrogen (78%), oxygen (21%), and a tiny amount (0.03%) of CO2 (plus 1% of other inert gases). 

When working hard, a person ventilates his lungs as high as 100 liters/min of air. This high ventilation must absolutely be sustained–not to get in enough oxygen--but to blow off the exhaust CO2 the body is generating.  Of those 100 liters of inspired air, 21 are pure oxygen.  But working muscles generally require around 2 to 4  l/min of oxygen to produce energy (Let’s use the figure of 3 l/min).  Thus, from those 21 liters of inspired oxygen, the user’s lungs only absorbed three liters. The rest of that inspired oxygen–18 liters-- (and all of the nitrogen, plus the CO2 the body produced) is exhaled to the outside. 

A closed-circuit system takes all the exhaled gases and runs them through a filter that absorbs the CO2.  In the 1938 and 1953, that filter was soda lime (calcium hydroxide).  This permits the user to “rebreathe” the oxygen (now cleansed of CO2) that he already breathed once. (Closed-circuit devices are called “rebreathers.”) Thus, the 18 liters of O2 he exhaled can be supplemented with an additional squirt of 3 liters of oxygen (say from a tank of compressed oxygen), and in the next minute, he would again be running 21 liters of oxygen through his lungs.  But there’s much more to it than that. 

As mentioned, ordinary air at any altitude is comprised of 78% nitrogen–an inert gas that is there only to dilute the oxygen in normal air to a concentration humans find useful.  In order to boost the percentage of oxygen one breathes, instructions for rebreather users entail exhausting one’s lung inventory of air of as much nitrogen as possible.  This is done by breathing in pure oxygen and then exhaling deeply outside the system before breathing in more pure oxygen produced by the rebreather apparatus.  Some rebreathers allow a minute or two of this "purge-breathing." The effect of breathing in pure oxygen and exhaling that lungful outside the system a few times, is to dilute the nitrogen content of the lungs/rebreather system from 78%  to, say, 15%. The result is instead of breathing gas with an oxygen concentration of 21% (ambient air) or 30%(open circuit system)(5) the user is now breathing oxygen at a concentration of 75% to 95%.  Why is this good? 

There is a huge benefit to breathing oxygen in higher concentration.  Oxygen is driven into the lungs by its “partial pressure,” by which is meant its degree of concentration times the ambient atmospheric pressure. At sea level, the atmospheric (i.e., barometric) pressure is 760 mm of mercury. At sea level, 21% times 760mm equals a partial pressure of 160 mm of mercury.  The minimum partial pressure in which the average human can survive, i.e., not become hypoxic,  is around 60mm (but of course, as a genetic trait, this varies a great deal, and you can be hypoxic and still function for some time. But eventually you will die).  See for more on the physiology of breathing air and doing work.

As we climb, the barometric  pressure drops. We can quickly see what happens to the partial pressure of oxygen as the altitude increases:  



mm Hg

Free Air

Open Circuit



Sea Level





Sea Level































Note that in free air at 27,500-ft (blue) the partial pressure of oxygen (between 85 and 68 mm) is about the same for a person using an open-circuit system as it is breathing free air at 20,000-ft (74 mm).  This is a very useful advantage, but, as any climber will tell you, climbing at 20,000-ft with a heavy load is still quite taxing.  And the higher you go, the closer you get to the hypoxic barrier.  But notice the stunning advantage of the closed-circuit system: The partial pressure of breathing closed-circuit oxygen at the summit of Mt. Everest (green) is higher than breathing air at sea level!  Climbers should be able to cruise right up to the summit. But there’s a fly in the ointment–indeed, there is a swarm of flies. 

Fly in the Ointment (FITO) #1.  Rebreathers require a perfect seal between the system and the outside. Any outside air that leaks in it will contain 78% nitrogen.  The nitrogen is recycled by the body  endlessly, and with each inspired leak, will increase its concentration until it reaches equilibrium, i.e., 78% inside the “closed” system.  At that point the climber is breathing the same concentration of oxygen as he would without the entire apparatus!!  (This is what I believe happened to Evans & Bourdillon in 1953.)  

Given the scruffy nature of climbers (with their beards) and the practical difficult of making a custom-fit mask for every climber, obtaining a perfect seal may be easy in the lab, but it is a real chore on The Hill.  It means no beards or, a large full face mask that is tightly fitted to the entire head.  Tight-fit means increased danger of frostbite. Full face mask means goggles that can (and will) fog over and you are blind. You can’t talk.  Putting on a full face mask is a two-handed operation (difficult with gloves, very difficult with mittens), and if custom-fit, means no picking-up a spare rebreathers on the fly. 

FITO #2.  The scrubber operation–the chemical absorption of CO2–requires an elevated temperature to function properly.  Especially if less active calcium hydroxide is used.  Once percolating, it should work fine if the scrubber is adequately insulated, but a cold system may have a difficult time getting up to working temperature, and cause the sensation of acute suffocation within the first few breaths.  The user is getting plenty of oxygen, but the breathing reflex is triggered by one’s CO2 concentration, not the presence or absence of oxygen. With a malfunctioning CO2 scrubber, the CO2 concentration quickly climbs above the intolerable 3% level–and you will be gasping desperately for air, no matter what the oxygen concentration.(6)  

Using the much more chemically active lithium hydroxide is a superior scrubber solution, but that material is even more highly irritating to the lungs, (perhaps even toxic) and should any of it be inspired, would cause the climber acute distress. However this problem seems to have been solved. (MicroPore in Georgia, USA is developing a line of lithium hydroxide CO2 scrubbers.)

FITO #3.  Rebreathers are very wet.  Exhaust breath, which must be filtered of its CO2 content is moist, and the CO2 scrubber reaction may actually produce water. This was not a problem during operation of the HCR, as the reaction is also exothermic (produces heat) which keeps ice from forming. But at typical Everest temperatures, once a canister is exhausted, the wet system will freeze solid in the minutes it takes to change.  Provisions must be made for easily getting rid of the ice without the use of tools, and in a very simple manner with gloves on. 

Thus, many closed-circuit systems work great once–the first time they start-off when they are nice and dry. After one or more uses, the water/ice produced clogs everything and all too often requires a serious strip-down. 

One annoyance has been that the oxygen supply and the scrubber duration were not the same. This means one is always in doubt about whether the scrubber should be changed with the installation of this new O2 bottle, or could wait until the next O2 bottle was opened.  But correct design can result in this disparity being evened out. An ideal system would use an ~800-liter tank for a 4-hr duration. When it expired, both a new scrubber and a new tank would be switched in. (Or, if the new lithium hydroxide scrubbers last for say, 8hrs., a large lever is thrown when the first 4-hr bottle is switched out, showing the user who then puts in the third bottle, that he must also now change the scrubber canister as well.)

FITO #4.  So far, the superior benefits of closed-circuit oxygen over open-circuit systems are all on paper. They have never been convincingly demonstrated in the field.  Proof is still up in thin air.  Finally, superior physiological performance by itself is not enough.  Any oxygen system must also work in the real world: Able to be used more than once, able to be switched among different climbers.  Able to be charged with oxygen easily, preferably with no tools whatever. This testing can only be done in the field on actual expeditions. Climber/scientists must be ready, willing and able to roll the dice.

The Cosmic Issue: How much faster can fully acclimatized climbers climb using a closed-circuit oxygen system than an open-circuit one? And even if the closed-circuit system proves superior in that one metric, how many vertical feet per pound of equipment does that work out to?


Part 2. HCR--A chemical Closed-Circuit System 

With all the provisos of Part 1 of this inquiry in mind,  I built a system designed to overcome the most obvious problems.(7)  I call it the HCR–the Holzel-Chemox(8) Rebreather.  The key details of that system are given here in order to aid current closed-circuit designers in their own quests to devise a practical high-altitude rebreather.  

FITO # 1. A perfect seal.  I immediately gave up on trying to use any kind face-mask seal as being both impractical due to beards, but also unwise because it would prevent users from switching over to different systems if their own broke down or were lost.  

Instead I devised an “oro-nasal” seal.  This is the standard aqua lung bite piece to which had been added a very soft (silicon) rubber nostril pipe.  Protruding out of the mouthpiece was a short forked tube with large conical rims that one pushed into the nostrils.  This created a good seal and permitted normal nose breathing which did much to eliminate the annoyance of having to breath only through the  mouth (or of using a dangerous nose clip to seal the nose). It also proved  be to a boon for sleeping comfortably. A loose mask held the orno-nasal  piece in the mouth and nose while protecting them from the elements. This mask also keeps the user from spitting-out the mouthpiece during sleep.   

FITO #2.  CO2 scrubbing.  I used the Chemox chemical oxygen generation canisters manufactured by Mine Safety Appliances.  They cost about $140 each. The same chemical in these 4-lbs cans–potassium superoxide–KO2--generates oxygen and scrubs the CO2.  When exhausted, you toss them. You never have to figure out or keep track of whether the scrubber is exhausted or you are running out of Oxygen.  (Both situations feel the same–it becomes increasingly difficult to get enough air.)  One could, if desired, add a supplemental lithium hydroxide CO2 scrubber in line with the intake feed to extract a few minutes more of usable (i.e., CO2-free) oxygen out of each canister. Since there is very little extra CO2 (except near the end of the canister’s life) this extra touch-up scrubbing could add minutes of comfortable breathing to each canister, and a single filter could probably last an entire expedition.(9)  

Photo left: The lung-shaped breathing bag
(made by Switlik Parachute company).
The left lobe is rolled up and held fast
with a Velcro strap; the right lob is
unrolled, ready to be emptied of accumulated ice.

FITO #3.  Rebreather wetness.  Because the Chemox system is chemical and not high-pressure, there are no metal valves to freeze or break–which is still an open-circuit problem.(10)  In fact, there are only two metal parts and only one of them–a simple tube--is inside the closed system.  The breathing bag is a lung-shaped sack with two lobes.  These are open at the bottom and rollup to provide a seal.  The lower part of each lobe is held rolled-up by Velcro.  The only two valves are simple flapper valves, also of flexible silicon rubber.  They can be squeezed (“resiliently deformed”) to clear them of ice–which falls into the breathing bag for later removal. 

I tested the HCR system cold-soaked to -25oF at the U.S. Army Cold Regions Research and Engineering Lab in Hanover, NH.  After using-up one canister pedaling furiously on a bicycle ergonometer, I removed it, waited a few minutes, and then inserted a second canister.  The HCR had frozen-up, but by crushing the valves, I was quickly able to free them up and start breathing again.  

The system was also tested briefly on Mt. Everest. In 1986 I took 3 canisters of chemical oxygen up to ABC at 21,000 ft on the North side. I slept one night with a closed-circuit set. WOW! did I get a full night's sleep--for the first time in 67 days on the mountain. I didn't wake up once (instead of every 45-minutes), the oxygen was warm and moist, and the reaction is exothermic, which heated my sleeping bag. When I woke up, it was like coming out from general anesthesia, and I felt like a million bucks.

The next day I rigged David Cheeseman up with a double canister set-up that delivered 4.4l/min of pure oxygen, allowing a 100 l/min ventilation rate with practically no breathing effort. Dave rocketed up the flanks of Changtse right outside of ABC. He was breathing so effortlessly, he stopped for a moment, certain that the system had come open. As soon as he took it off to check, “it was like throwing out anchors,” he remarked.  

Other HCR features

Technically, the HCR is a demand system–the harder you breath, the more oxygen is generated.  But unlike mechanical demand systems, which vary the amount of oxygen you get for each breath, the Chemox canister generates more or less oxygen depending on how much moisture from your breath is blown into the canister.  Breathing harder pushes more moisture into the system and speeds up oxygen production. But the breathing demand reaction time is slow–measured in minutes rather than seconds, especially when slowing down. Slowing the reaction down (by breathing less rapidly) is a drawn-out affair, and oxygen over-produces as the chemical reaction slowly modulates downward.  

Since the entire system is a simple closed loop, there is very little breathing resistance.  This is a great pleasure to users.  You can cough, clear your throat, without upsetting the demand system or disturbing the flow of oxygen.  However, to get maximum benefit when climbing with a single Chemox canister, it is more efficient to take long slow breaths than short choppy ones. When using two Chemox canister in tandem,  any type of breathing is allowed as the canisters can always keep up with your physical output.  A single canister produces a maximum of 2.2l/min–enough for much climbing activity, but not enough for maximum output which is practically around 3-4 l/min. (11)  I was getting about 75% pure oxygen due to nitrogen I could not fully purge--contained within my lungs and blood volume, and the start-up air volume inside the HCR system. (This concentration was measured on a bicycle ergonometer at the U.S. Army Cold Regions Research and Engineering Laboratory (CRREL) in Hanover, NH.)

The canister is carried in an insulating sack which hangs from the neck. The canister is attached to the tubing by being clamped onto the single steel part using a modified laboratory pipe clamp–a C-clamp.  This was screwed closed, but could preferably be an over-lever ski-binding snap-on type.  

David Cheesemond testing a two-canister
HCR system on the flanks of Changtse. In this case
the canisters and breathing bag were slung on his back.

One has a choice of putting the intake tube coaxially inside the exhaust tube, resulting in a single hose going from the canister to the face.  This would be less apt to catch on anything. But it might be more difficult to clear of ice. I used conventional two-hose scuba hoses which were readily available.  But these became very stiff in cold weather. It is preferable to use resiliently deformable urethane rubber. Be mindful of its tear-resistance.



1. While the oxygen breathed is pure, your body never gets 100%. Some varying percentage of nitrogen is unavoidable present.  At sea level, people can breathe 100% oxygen for around 12 hours continuously before deleterious side effects occur, and probably longer at altitude.

2. However (there is always an “however,”) an unconscious climber would not be a good candidate. Because of the need to maintain  a tight seal, if he cannot reflexively obtain outside air by breaking the seal (.e.g., spitting out the mouthpiece), he could suffocate when the rebreather was exhausted of oxygen.

3. An explanation that is necessarily highly simplified. Acclimatized humans breathing pure oxygen at altitude has never been studied (!)

4. The highest measured oxygen uptakes are obtained by elite XC skiers at around 6.5 l/min.

5. The partial pressure of an open system is highly variable. I am estimating it can provide as much as 30%.5 Through physiologic processes, the partial pressure of oxygen in the arterial blood at sea level actually drops to around 100mm Hg. But we will just use the external measures.

6. Imagine with every 100 liters of pure oxygen you ventilated, you sucked in 5 liters of outside air.  Four liters of that outside air would be nitrogen. Breathe another 100 liters and suck in 5 more liters of outside air and now you have 8 liters of nitrogen circulating inside the system—and your 100% oxygen concentration has already dropped to 92%.  The oxygen is replenished, the CO2 is absorbed, but the nitrogen just keeps recirculating, and building up with each breath, reducing the oxygen concentration until it matches that of the outside air. And your oxygen advantage has dropped to zero—but not your extra load!

7. U.S. Patent 4.440,165.

8. .  See also: ../ImportMedia/niosh/mining/pubs/pdfs/ri9650.pdf for a comparison of the conventional Chemox system to other (compressed oxygen) rebreathers.

9. But it might not. It depends on which of the two reactions has slowed the most—the CO2 scrubbing reaction which is different than the oxygen production reaction.

10. When you release any high pressure substance—say CO2 in a fire extinguisher—it causes a radical drop in temperature.  If there is the slightest trace of water in the gas, that water will freeze-up exactly at the point where the pressure is being released--the tiny pin hole of the pressure reduction valve—and clog the system.  This happens all too often in compressed oxygen systems even today and is the fault of the oxygen fill, not the apparatus. A small blow torch would help, but these are rare on the upper slopes of Everest.

11. Do not confuse the oxygen consumption rate of a closed-circuit system with the oxygen feed rate of an open-circuit. While both indicate how much oxygen is flowing, only the rebreather indicates how much oxygen is being utilized by the body.


 Additional climbing oxygen sites:

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