Rather an unusual topic for a blog dealing with hyperbaric oxygen therapy, but one of fairly significant importance when it comes to diving breathing gas and hyperbaric chamber internal atmospheres.
Divers and supervisors work with gas physics daily and of all the gasses, CO2 is among a few of the most important ones to have a solid understanding of. Divers aren’t scientists, or atmospheric specialists, but we do have a solid understanding of CO2. Kind of the “mechanics” of gas science for lack of a better description. It is imperative that anyone in charge of a chamber or other confined and potentially un-ventilated space have a good academic grasp of at least some gas physics.
With this in mind today’s science bit discusses carbon dioxide (CO2). Seemingly a topic of great interest in the media and political spectrums this article isnt intended to divide any audiences but rather present an impartial and academic view of the gas so many are so afraid of, and one which chamber or diving supervisors perpetually hold in the back of their minds owing to the many situations in diving where this gas becomes extremely important to know about such as diving in helmets, band masks, chambers and bells as well as any other scenario which may present arge “dead air” spaces. After all it’s the one that is readily changeable in the hyperbaric environment including diving, as we all produce at least a quarter of a litre of it per minute at rest. More when we are working hard. It takes little more than primary school mathematics and high school physics to develop an understanding of the role of CO2 in the air we breathe and how it behaves.
If there is one gas a diver really knows its CO2 .
Carbon dioxide (chemical formula CO2)
CO2 is a colourless gas, about 60% higher in density than dry air. It consists of a carbon atom covalently bonded to two oxygen atoms. Occurring naturally in the atmosphere as a trace gas the current concentration is 0.04% (approximately 400 ppm) by volume. Pre Industrial levels of 280 ppm indicate a definite rise following the industrial revolution. A rise of approximately 120 ppm or 0.012% concentration in the atmosphere. Approximately 12 one thousandths of a percent. (Depending in which sources one subscribes to this may as high as 140 ppm or 0.014%). As a technician rather than a scientist, it’s beyond my scope to determine if this amount is scentifically or statistically significant so I’ll let the reader decide on that score.
Natural sources include volcanoes, hot springs and geysers. it is stored in carbonate rocks and is freed by dissolution in water. It occurs naturally in most water. In fact, ocean water is the largest CO2 sink and absorbs excess CO2 in the atmosphere. In normal concentration it is odourless but in higher concentration has a sharp acid smell.
As previously discussed under the gas laws, Henry’s and Dalton’s Law, it is explained that as concentration rises in the atmosphere, CO2 dissolves into the ocean proportionately with its partial pressure and vice versa. As concentration drops in the atmosphere so it out-gasses back into gaseous form and a balance is maintained in the atmosphere in this way. Reducing atmospheric CO2 will cause the ocean and geological structures to release some of its stored gas in accordance with Henry’s Law and any differential that may exist. The gas will always seek equilibrium where a pressure differential exists. Gas “pressure” in the gas phase and gas “tension” in the liquid phase mean the same thing essentially. They refer to the gas pressure either in the gas or the liquid as the case may be. Respiration in mammals and gas exchange in the lungs depends on this differential between gas pressure in the air and blood gas tension. Without it, oxygen transport would not occur. Neither would the mammal breathing response in the absence of an elevated CO2 tension differential.
Atmospheric CO2 is the primary source of carbon for carbon-based life on earth. (Us, among others). Its concentration in the pre-industrial atmosphere since the late Precambrian has been regulated by photosynthetic organisms and geological phenomena. Cyanobacteria, algae and plants use light energy to photosynthesise carbohydrate from CO2 and water, with oxygen as the “waste” product. This only occurs in the daylight however when photosynthesis is possible. In the dark, at night, plants inspire oxygen and expire CO2 just as animals do.
CO2 is a byproduct of metabolism and is therefore produced by all aerobic organisms when they metabolize carbohydrate and lipids to produce energy by respiration. The importance of this will be further discussed under physiology. Oxidative metabolism is one of the fundamental principles of hyperbaric oxygenation and its mechanisms. It returns to water via the gills of fish, and the air via the lungs in air breathing animals including humans.
Photo by Engin Akyurt on Pexels.com 
Photo by Lone Jensen on Pexels.com
Decay of organic material also causes the formation of CO2 . Such as in processes of bread, beer and wine making. It is also produced by the combustion of wood and other fossil fuels. It is an unwanted by-product in many large-scale processes such as the production of acrylic acid.
As a versatile industrial material, it is used as an inert gas in welding and fire extinguishers, as a pressurizing gas in air guns and oil recovery, as a chemical feedstock and as a supercritical fluid solvent in decaffeination of coffee and supercritical drying. It is added to drinking water and carbonated beverages including beer and sparkling wine to add effervescence thanks to the above mentioned Henry’s Law. The frozen solid form of CO2, known as dry ice is used as a refrigerant and as an abrasive in dry-ice blasting.
Of the greenhouse gasses, CO2 is the most long lived. It is not necessarily the gas with the highest heat retention capacity though. In fact, it isn’t even close, with Sulfur Hexafluoride reportedly being 23’900 times the greenhouse gas that CO2 is. Following the rise in concentrations since the industrial revolution it is said to have contributed to global warming as it is more insulative than air and has a higher energy carrying capacity. It also contributes to acidification of the oceans as it is readily soluble in water and as mentioned previously the ocean serves as a carbon dioxide sink absorbing excess atmospheric CO2. The total rise in concentration since pre-industrial times is 120 to 140 parts per million, or 12 to 14 one thousandths of a percent as calculated above. This could be said to be the total human industrial impact on concentration increase in the last 150 years.
Breathing Reflex
The human desire to breathe is driven by CO2 levels in the blood. Contrary to popular belief, it is not driven by lower oxygen levels. As the body metabolises oxygen it produces CO2. This results in a rise in gas tension (pressure) in the bloodstream. Receptors in the brain detect this and the breathing response is signalled. Even with sufficient oxygen in the air one breathes, increased CO2 blood gas tension will trigger the breathing response. Very high concentrations can become problematic leading to an individual feeling like they cant get enough air. Very low levels may result in the breathing trigger not being signalled.
This is why free divers hyperventilate before submerging. Hyperventilation artificially flushes and lowers CO2 from the blood, lowering it’s gas tension and with it the desire to breathe. Do not try this at home though. Proper training is advised and many free divers die following a condition known as “shallow water blackout”. A condition in which, when the diver ascends from depth, and the the now more severely depleted oxygen levels partial pressure drops below that required for consciousness, blacks out.
An interesting experiment demonstrating this (within sensibility and reason) is to hold one’s breath for as long as possible until the desire to breathe becomes pressing. At this point breath out only and pause before breathing in again. The action of breathing out lowers CO2 tension briefly, very briefly relieving some of the urge to breathe. Free divers make use of this technique to stay submerged for longer by breathing out once CO2 tension begins to rise as well as pre-dive hyperventilation.
CO2 levels which are too low may compromise the breathing response in humans and other mammals. The result should be obvious. Lower partial pressures of CO2 in the air we breathe lowers the tension of the gas in the blood in the same way hyperventilation does and can result in a failed breathing response leading to unconsciousness.
Toxicity
CO2 is an asphyxiant. It is however not classified as toxic in accordance with Globally Harmonised System of Classification and Labelling of Chemicals standards of United Nations Economic Commission for Europe by using OECD guidelines for the testing of chemicals.
Concentrations of up to 1% (10’000 ppm) can lead to drowsiness in some and can lead to a stuffy air feeling in the lungs but would be unnoticed by most. It is generally considered that in concentrations up to 3% no significant toxic effect occurs. At 5% (50’000ppm), individuals may become considerably unwell with headaches, nausea, hypercapnia etc resulting from increased gas tension in the bloodstream. This Can lead to a sense of not being able to get enough air. Concentrations of 7 to 10% (70’000 to 100’000 ppm) may cause suffocation even in the presence of sufficient oxygen. It is generally considered lethal in short order in concentration of 10% and higher.
Toxicity manifest as dizziness, headache, visual and hearing dysfunction, unconsciousness within anything from minutes to an hour, and death from asphyxiation.
Concentration is of particular importance in the hyperbaric environment since, with increased pressure, comes increased concentration owing to higher partial pressure. This loosely means that what may be tolerable at normal pressure may well not be under increased pressure and CO2 monitoring is crucial. The environment is also relatively confined allowing for fairly rapid build-up in concentrations.This is the reason chambers are flush ventilated regularly and in some chambers CO2 analysers are present. Other chambers may have soda lime-based scrubbers which absorb CO2. Also known as carbon sinks, these devices absorb excess CO2 from the atmosphere.
Carbon dioxide concentration in air (averaged between sea-level and 10 kPa level, i.e., about 30 km (19 mi) altitude varies between 0.036% (360 ppm) and 0.041% (410 ppm), depending on geographical location.
CO2 is heavier than air and as such will settle at low points. Where it seeps from the ground in higher concentrations and remains un-dispersed by wind, it can collect in pockets and lead to the mass death of animals in this zone. This has been observed by massive natural release of the gas surrounding lakes, geothermically active areas and volcanoes. Birds of prey venturing into this zone for a free meal are also killed as they suffocate. It is not unheard of for children to die while adults with them live. Adults then bending down into that zone can also succumb.
Many chambers are fitted with air circulating scrubbers which remove CO2 from the atmosphere by filtering air through lime-based soda sorb canisters. Many chambers do not though, and a good flush ventilation technique is required to maintain safe levels of CO2.
Adaptation to increased concentrations of CO2 occurs in humans, including modified breathing and kidney bicarbonate production. This means that repeated exposure to increased concentration develops a certain resistance to toxicity. This occurs In order to balance the effects of blood acidification (acidosis).
Several studies suggested that 2% (20’000 ppm) inspired concentrations could be used for closed air spaces (e.g. a submarine or hyperbaric chamber) since the adaptation is physiological and reversible, as decrement in performance or in normal physical activity does not happen at this level of exposure for five days. Yet, other studies show a decrease in cognitive function even at much lower levels. Also, with ongoing respiratory acidosis, adaptation or compensatory mechanisms will be unable to reverse such condition.
Submarines and other closed air spaces are generally maintained at atmospheric pressure however hyperbaric chambers venture well beyond atmospheric pressure and for this reason, regardless of what the above studies suggest, concentration should be kept even lower. The opposite recommendation is valid since increased pressure has the same effect as increased concentration.
Poor ventilation is one of the main causes of excessive CO2 concentrations in closed spaces. Carbon dioxide differential, above outdoor concentrations, at steady state conditions, (when the occupancy and ventilation systems operation are sufficiently long enough that CO2 concentration has stabilized), are sometimes used to estimate ventilation rates per person. Higher CO2 concentrations are associated with occupant health, comfort and performance degradation. Rate of increase of CO2 in an unventilated closed space can be calculated and shall be discussed further.
Miners, who are particularly vulnerable to gas exposure due to an insufficient ventilation, referred to mixtures of carbon dioxide and nitrogen as “blackdamp,” “choke damp” or “stythe.” Before more effective technologies were developed, miners would frequently monitor for dangerous levels of blackdamp and other gases in mine shafts by bringing a caged canary with them as they worked. The canary is more sensitive to asphyxiant gases than humans, and as it became unconscious would stop singing and fall off its perch. The Davy lamp could also detect high levels of blackdamp (which sinks, and collects near the floor) by burning less brightly, while methane, another suffocating gas and explosion risk, would make the lamp burn more brightly.
Other uses of carbon dioxide include:
Foods: products such as bicarbonate, silicate, carboxylic acid derivatives and carbonates are produced using CO2.
Beverages: Carbonated beverages are made fizzy by dissolving CO2 into the liquid courtesy of Henry’s Law. Other beverages include beer and wine making which produces CO2 when bacteria in yeast break down sugars to form alcohol.
Stunning Animals: CO2 is used in the humane stunning of animals prior to slaughter. As discussed, unconsciousness can be immediate in very high to pure concentrations. This is a painless method of rendering an animal immediately unconscious. A shortage of CO2 in the UK in 2018 brought meat production to a standstill.
Inert gas: CO2 is one of the most common gasses used for compressed gas systems such as portable pneumatic tools. Because of its non-flammable nature it is a relative safe gas to use in high pressures. It is a commonly used gas in welding as the gas protects the arc during welding facilitating better quality welds. It is also used in drying of foods, decaffeination of coffee beans and as a propellant in paintball guns.
Fire Extinguisher: Because it is non-flammable, CO2 is a superior fire extinguisher gas as it displaces the oxygen required to support combustion. Pressurised CO2 is also very cold and removes heat from a fire successfully thus removing 2 of the 3 required factors for ignition.
CO2 fire extinguishers are not to be used inside a chamber though since as little as 2 relatively small extinguishers could raise concentration in the chamber to the aforementioned 10% in relatively short periods of time creating a dangerous situation. Specialist hyperbaric fire extinguishers are further discussed under design and layout and allied equipment.
CO2 can also be used as a solvent, in biological and agricultural application, medical and pharmacological uses such as treatment of apnoea and the balancing of O2/CO2 in the blood, Oil recovery, bio transformation into fuel and as a refrigerant among other uses.
The limit for contamination in diving and breathing air (ie chamber air) applications is 500 ppm. It is this low because of the effect of increased pressure. In line filters provide for removal of ambient CO2 in the compression stage.
This article is followed by CARBON DIOXIDE MANAGEMENT IN CONFINED ATMOSPHERES .
©Hayden Dunstan
cumbria-hyperbaric.org 
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