What we breathe underwater is a significant concern for Gue. Because of this concern, Gue does not breathe air underwater. Air is not used for deep diving. Helium is introduced shallower than most accepted thinking. Accepted gases are used for underwater activities. The foundation of these rules is a sound insight of the interaction among the applicable gas laws: Boyle's, Dalton's and Henry's; the characteristics of discrete gases and human physiology.
The normal functions of breathing and circulating what we breathe throughout the body have developed at the surface. Once we submerge in water, pressure changes what we breathe and how we breathe it.
Henry S Camera
Let's make sure we understand pressure. The weight of the air surrounding us from "land" to outer-space is measured as one atmosphere (1ata) of pressure. In order to decree the total whole of pressure at any depth we have to add the pressure at the face to the pressure exerted by the weight of water. Every 33 feet of water adds other ata to the pressure. For example, at a depth of 99 feet there are 4 ata's: one for the weight of air at the face and one for each 33 feet. The whole of ata's or total pressure will help us understand the effects of pressure.
Unlike liquids and solids, gases (air) can be positively compressed by pressure. Pressure moves molecules of a gas closer together (density or think of weight) and makes the space it occupies (volume) smaller. For example, picture a balloon filled with air at 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 didn't fly from the balloon, so there still would be the same whole of air. The same goes for the air in our lungs; at 33 feet the air would be twice as dense in half the space. This is the conception behind Boyle's law: volume is inversely proportional to the increase in pressure and density is proportional to the increase in pressure.
Back at the face our balloon and our lungs are filled with air. For the most part air is made up of oxygen (about 21%) and nitrogen (about 79%). When you add these up you get 100%. If we only talk about oxygen in relation to air, that is only a quantum of, or a partial whole of what makes up air. Stated other way the 1 ata of pressure at the face equals the sum of the partial pressure (pp) of oxygen (0.21) and the partial pressure of nitrogen (0.79).
Now we need to bring Mr. Boyle back into the picture. Let's take our balloon back to 33 feet. If, as Boyle says, at 33 feet (2 ata's) the air is twice as dense then the oxygen and nitrogen, each, must also be twice as dense. So now we take 2 ata's and multiply it by each 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 ata's at 33 feet. 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 conception behind Dalton's law: the sum of the parts make up the whole.
So who cares about a balloon, I'm not a balloon. Your right! Our body exchanges air in the middle of blood and our lungs. We've lived at the face for a long time so the blood and tissues have the same whole of oxygen and nitrogen dissolved in them as is 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 the air in our lungs (equilibrium). This is the conception behind Henry's law: The whole of any given gas that will dissolve in a liquid at a given climatic characteristic is directly proportional to the partial pressure of that gas.
We forgot something though. When we breathe we not only inhale we also exhale. So our lungs are taking in oxygen and nitrogen and getting rid of nitrogen, oxygen, and carbon dioxide. Carbon dioxide is a by-product of our metabolism. It follows the same rules as other gases.
When oxygen, nitrogen, and, don't forget, carbon dioxide partial pressures get high enough they come to be toxic (poisonous) and/or narcotic. The onset of narcosis and toxicity is unpredictable; not only from someone to someone but also day to day in the same person.
Oxygen partial pressures in excess of 1.6 have been shown to cause Central Nervous law (Cns) problems and with longer exposure lung (pulmonary) problems. Cns toxicity targets your central nervous law nerves. The symptoms of Cns consist of nausea, abnormal vision or hearing, breathing difficulty, anxiety, confusion, fatigue, incoordination, twitching of your face, lips, or hands, and convulsions. Convulsions can appear without warning, and can lead to air embolism and drowning. Pulmonary oxygen toxicity primarily targets your lungs, producing chest pain and coughing. This can occur after a 24-hour exposure to pp O² of 0.6 pp (e.g., 60 fsw breathing air).
The results of one study (Meyer-Overton) have been used to predict that the anesthetic potency of a gas is inversely associated to its lipid solubility. The more lipid soluble gases furnish narcotic effects at lower concentrations than less soluble gases. Based on lipid solubility oxygen should be more narcotic than nitrogen.
Nitrogen partial pressures in excess of 3.16 (equivalent to air at 100 feet) have been shown to impair a diver's potential to think clearly and degrades motor skills. This degradation also includes the muscular action associated with breathing.
Partial pressures of carbon dioxide that fall 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 the body tissues. The median normal range of Co² is carefully to be 35-45 mmHg (millimeters of mercury). Signs of Co² toxicity are regularly clear at Paco² (partial pressure of Co² in the alveoli) = 60 mmHg on the high end and 30 mmHg on the low end. A rise to 80 mmHg or decrease to 20 mmHg would be incapacitating. Normally, your body keeps your arterial Co², almost without exception, within 3 mmHg while both rest and exercise, a narrow range. Also, some studies have shown that carbon dioxide reduces mental and corporeal capacity at sub-anesthetic concentrations. Therefore, the build up of carbon dioxide should be a concern from both a narcotic and toxic standpoint.
Great?! I still want to dive. How do I reduce the risks of toxicity and narcosis? We can change the article of what we breathe. If we replace some of the nitrogen in air with more oxygen, so that at the face we have 0.32 pp of oxygen and 0.68 pp of nitrogen (This is called Nitrox 32), we try to minimize the inherent effects of nitrogen narcosis at depth. Now if we dive to 100 feet or 4 ata's using Nitrox 32, the partial pressure of nitrogen is 2.72 (4 ata x 0.68 pp of nitrogen). This is below our Accepted maximum of 3.16 pp (approximately 100 feet where the diminished cognitive and motor skills symptomatic of nitrogen narcosis come to be more clear with a partial pressure of colse to 3.16 pp.
Yeah, but if the oxygen pp is higher at the surface, then at depth there is more inherent for oxygen toxicity because of pressure! (remember Boyle?) Very good. That's why we compose the working range for Nitrox 32 as 0 - 100 feet (100 feet or 4ata x 0.32 = 1.28 pp of oxygen). This 1.28 pp of oxygen is below our 1.6 pp maximum where most studies have shown an increase in the probability of experiencing symptoms of oxygen toxicity. Also, by retention the oxygen partial pressure low (a max of 1.28pp for Nitrox 32) we effort to minimize the probability of the incidence of oxygen narcosis as expected by Meyer-Overton.
If you want to go deeper, we introduce helium. A composition of 0.30 pp of oxygen, 0.30 pp of helium and 0.40 of nitrogen (Trimix 30/30) used in the working range of 80 - 120 feet keeps oxygen toxicity and narcosis, and nitrogen narcosis in the Accepted ranges of less than 1.6 pp and less than 100 feet; respectively. A composition of 0.21 pp of oxygen, 0.35 pp of helium and 0.44 pp of nitrogen with a working range of 120 -160 feet (Trimix 21/35) keeps oxygen toxicity and narcosis, and nitrogen narcosis within the Accepted ranges.
Ok. What about carbon dioxide? For the most part changing what we breathe doesn't work on the whole of carbon dioxide our bodies create. We can make it easier for our bodies to move the gas colse to by adding helium. Remember we are used to the effort required to inhale and exhale at the surface. With a higher density of gas (think heavier) it is more difficult to breathe, at 99 feet it is four times as difficult. (See table 1 for densities at the face and 99 feet.) If our bodies can not efficiently move carbon dioxide from tissues to our lungs and out of our bodies the levels begin to rise. some factors increase our yield and elimination of Co²; these factors range from breathing resistance to gas density and fitness. For example, unfit divers may furnish about twice as much Co² as that of a fit diver. In addition, gas density can incorporate with increased depth to make a gas especially hard to breathe (due to continued increases in density). Regardless of the definite reasons for increased Co² accumulation the body attempts to compensate by expanding the breathing rate. Very often this results in rapid but shallow breathing which is not sufficient at removing Co². Considering that Co² is very narcotic, this narcosis together with any narcosis experienced from other gasses can significantly impair the diver. In addition, the rapid shallow breathing that might result from trying to exert (particularly with dense gasses) can lead one into panic and/or Co² toxicity and unconsciousness. In summary we don't breathe air because there are less dense, less narcotic and less toxic alternatives. These alternatives take into account basic gas laws applied to gas properties interacting with human physiology to make diving safer.
References: The physiology and medicine of diving, 4th Ed., by Peter Bennett & David Elliott, et. Al Us Navy Diving manual Doing it Right: The Fundamentals of better Diving, by Jarod Jablonski Quest Magazine, by Gue Naui expert Scuba Diver policy Materials
The Fundamentals of Dir, slide presentation by Gue
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