What we breathe underwater is a major concern for GUE. Due to this concern, GUE does not breathe air under water. Air is not used for deep dives. Helium is introduced shallower than most conventional ideas. Standard gases are used for underwater activities. The basis of these rules is a solid understanding of the interaction between the applicable gas laws: those of Boyle, Dalton and Henry; the characteristics of various gases and human physiology.
The normal functions of breathing and circulating what we breathe throughout the body have developed on the surface. Once we dive into the water, the pressure changes what we breathe and how we breathe it.
Let’s make sure we understand the pressure. The weight of the air around us from “earth” to outer space is measured as one atmosphere (1at) of pressure. To determine the total amount of pressure at any depth, we have to add the pressure at the surface to the pressure exerted by the weight of the water. Every 33 feet of water adds another tie to pressure. For example, at a depth of 99 feet there are 4 ata: one for the weight of the air at the surface and one for every 33 feet. The number of ata or total pressure will help us understand the effects of pressure.
Unlike liquids and solids, gases (air) can be easily compressed by pressure. Pressure brings the molecules of a gas closer (density or think weight) and makes the space it occupies (volume) smaller. For example, imagine a balloon filled with air on the surface. Submerge the balloon to a depth of 33 feet. At that depth, the balloon would “shrink” to half its size at the surface. The air did not escape from the balloon, so there would still be the same amount of air. The same is true of the air in our lungs; at 33 feet, the air would be twice as dense in half the space. This is the concept behind Boyle’s law: volume is inversely proportional to pressure rise and density is proportional to pressure rise.
Back at the surface, our balloon and our lungs fill with air. For the most part, air is made up of oxygen (about 21%) and nitrogen (about 79%). When you add them up, you get 100%. If we only talk about oxygen in relation to air, that is only a part or a partial amount of what constitutes air. In other words, the 1 ata of pressure at the surface is equal to the sum of the partial pressure (pp) of oxygen (0.21) and the partial pressure of nitrogen (0.79).
Now we have to bring Mr. Boyle back to the scene. Let’s take our balloon back to 33 feet. If, as Boyle says, at 33 feet (2 ata) the air is twice as dense, then oxygen and nitrogen each must also be twice as dense. So now we take 2 ata and multiply it by each of the partial pressures: oxygen → 0.21 pp x 2.0 ata = 0.42 pp, and nitrogen → 0.79 pp x 2.0 ata = 1.58 pp; totaling 2.0 ata’s. The math works, there are 2 ties at 33 feet. When breathing air at 33 feet, our lungs have an oxygen partial pressure of 0.42 ata’s and a nitrogen partial pressure of 1.58 ata’s. This is the concept behind Dalton’s law: the sum of the parts forms the whole.
So who gives a balloon? I am not a balloon. You are right! Our body exchanges air between the blood and our lungs. We have lived on the surface for a long time, so blood and tissues have the same amount of dissolved oxygen and nitrogen in them as in our lungs. At 33 feet, the partial pressures of oxygen (0.42 pp) and nitrogen (1.58 pp) in the lungs will eventually force the partial pressures in the blood and tissues to be the same as those of the air in our lungs (equilibrium). This is the concept behind Henry’s Law: the amount of any gas that dissolves in a liquid at a given temperature is directly proportional to the partial pressure of that gas.
However, we forgot something. When we breathe, we not only inhale, we also exhale. So our lungs take in oxygen and nitrogen and remove nitrogen, oxygen, and carbon dioxide. Carbon dioxide is a by-product of our metabolism. Follow the same rules as other gases.
When the partial pressures of oxygen, nitrogen, and, don’t forget, carbon dioxide get high enough, they become toxic (poisonous) and / or narcotic. The onset of narcosis and toxicity is unpredictable; not only from person to person but also from day to day in the same person.
Oxygen partial pressures greater than 1.6 have been shown to cause central nervous system (CNS) problems and lung (lung) problems from prolonged exposure. CNS toxicity targets the nerves of the central nervous system. CNS symptoms include nausea, abnormal vision or hearing, shortness of breath, anxiety, confusion, fatigue, lack of coordination, twitching of the face, lips, or hands, and seizures. Seizures can come on without warning and can lead to air embolism and drowning. Pulmonary oxygen toxicity targets primarily the lungs and causes chest pain and coughing. This can occur after a 24-hour exposure to 0.6 pp O² (eg 60 fsw breathing air).
The results of a study (Meyer-Overton) have been used to predict that the anesthetic potency of a gas is inversely related to its lipid solubility. Gases more soluble in lipids produce narcotic effects in lower concentrations than less soluble gases. Based on lipid solubility, oxygen should be more narcotic than nitrogen.
Nitrogen partial pressures greater than 3.16 (100 ft air equivalent) have been shown to impair a diver’s ability to think clearly and degrade motor skills. This degradation also includes muscle activity associated with respiration.
Carbon dioxide partial pressures falling above or below a very narrow range have been shown to cause narcosis and toxicity. Carbon dioxide toxicity, or hypercapnia, is an abnormally high level of carbon dioxide in body tissues. The average normal range for CO² is considered to be 35 to 45 mmHg (millimeters of mercury). Signs of CO² toxicity are usually evident at PACO² (partial pressure of CO² in alveoli) = 60 mmHg at the upper end and 30 mmHg at the lower end. An increase to 80 mmHg or a decrease to 20 mmHg would be disabling. Normally, your body keeps your arterial CO², almost without exception, within 3 mmHg during rest and exercise, a narrow range. In addition, several studies have shown that carbon dioxide reduces physical and mental capacity in subanesthetic concentrations. Therefore, the accumulation of carbon dioxide should be a concern from both a narcotic and a toxic point of view.
Great ?! I still want to dive. How do I reduce the risks of toxicity and narcosis? We can change the content of what we breathe. If we replace some of the nitrogen in the air with more oxygen, so that at the surface we have 0.32 pp of oxygen and 0.68 pp of nitrogen (this is called Nitrox 32), we try to minimize the potential effects of nitrogen narcosis at depth. Now if we dive 100 feet or 4 ata using Nitrox 32, the partial pressure of nitrogen is 2.72 (4 ata x 0.68 pp of nitrogen). This is below our acceptable maximum of 3.16 pp (approximately 100 feet where the decline in cognitive and motor skills symptomatic of nitrogen narcosis becomes more apparent with a partial pressure of about 3.16 pp.
Yes, but if the pp of oxygen is higher at the surface, then at depth there is more potential for oxygen toxicity due to pressure. (Do you remember Boyle?) Very good. That is why we set the working range for Nitrox 32 to 0 – 100 feet (100 feet or 4ata x 0.32 = 1.28 pp oxygen). This 1.28 pp of oxygen is below our maximum of 1.6 pp where most studies have shown an increased likelihood of experiencing symptoms of oxygen toxicity. Furthermore, by keeping the partial pressure of oxygen low (a maximum of 1.28pp for Nitrox 32) we try to minimize the probability of the occurrence of oxygen narcosis as predicted by Meyer-Overton.
If you want to go deeper, we present you helium. A mixture of 0.30 pp oxygen, 0.30 pp helium, and 0.40 nitrogen (Trimix 30/30) used in the 80 to 120 foot working range maintains oxygen toxicity and narcosis, and nitrogen narcosis in the ranges. acceptable less than 1.6 pp and less than 100 feet; respectively. A mixture of 0.21 pp oxygen, 0.35 pp helium, and 0.44 pp nitrogen with a working range of 120 to 160 feet (Trimix 21/35) keeps oxygen toxicity and narcosis and nitrogen narcosis within the ranges. acceptable.
Okay. What about carbon dioxide? For the most part, changing what we breathe does not affect the amount of carbon dioxide our bodies create. We can make it easier for our bodies to move gas by adding helium. Remember that we are used to the effort it takes to breathe in and out on the surface. With higher gas density (think heavier) it’s harder to breathe, at 99 feet it’s four times harder. (See Table 1 for surface densities and 99 feet.) If our bodies cannot efficiently move carbon dioxide out of the tissues into our lungs and out of our bodies, the levels begin to rise. Several factors increase our production and elimination of CO²; These factors range from respiratory resistance to gas density and fitness level. For example, unsuitable divers can produce approximately twice as much CO² as a suitable diver. Also, the density of the gas can combine with a greater depth to make a gas especially difficult to breathe (due to continuous increases in density). Regardless of the specific reasons for the increased accumulation of CO², the body tries to compensate by increasing the respiratory rate. Very often, this results in rapid but shallow breathing that is not effective in removing CO². Considering that CO² is highly narcotic, this narcosis, along with any narcosis experienced by other gases, can significantly miss the diver. Additionally, the rapid, shallow breathing that can result from attempting to exercise (particularly with heavy gases) can lead to panic and / or CO² toxicity and loss of consciousness. In short, we don’t breathe air because there are less dense, less narcotic, and less toxic alternatives. These alternatives take into account the basic gas laws applied to the properties of gases that interact with human physiology to make diving safer.
References: The Physiology and Medicine of Diving, 4th Ed., By Peter Bennett and David Elliott, et. to the US Navy Diving Manual Doing It Right: The Basics of Better Diving, by Jarod Jablonski Quest Magazine, by GUE NAUI Master Scuba Diver Course Materials
The Basics of DIR, GUE Slideshow