Posted by DWin on January 17, 2002 at 18:35:02:
Diving deep while breathing air is dangerous. The reason? There is too much nitrogen in the mix. What do I mean by this?
The air we breathe on the surface of the earth is a combination of two gases (there are others, but they are present in such small quantities as to be discounted in this discussion). These two gases are nitrogen and oxygen. We metabolize oxygen. It's an oxidizer. Without sufficient oxygen, we die.
The "exhaust" or waste products of metabolizing oxygen in our cells are carbon dioxide (CO2) and water (H20). The CO2 is exhaled from our lungs, along with the nitrogen. If we intake x amount of nitrogen, we exhale the same x amount. We don't use it for anything.
The atmosphere contains roughly 79% nitrogen and 21% oxygen. This is what is referred to when we talk about Fractions of Gases. It's the percentage. The fraction of oxygen on the earth is 21%. It is expressed as F02 = 21%. The fraction of nitrogen (FN2) on the earth is 79%.
Our exhaled breath contains roughly 17% oxygen, 79 percent nitrogen (remember x in, x out) and the rest is carbon dioxide... about 4%. When we dive and surface, what are we primarily off-gasing? Nitrogen.
The problem with nitrogen is that under a relatively low pressure (two atmospheres absolute), it causes a slowing down or decrease in the firing of neurons in our brain. Researchers believe (they aren't sure) that the nitrogen causes a buildup of a fatty deposit on the ends of the neurons. This causes a slowing down of nerve impulses to our brain. We become anesthetized just as if we were breathing laughing gas (nitrox oxide) at the dentists office. The feeling is exactly the same. You're basically becoming mentally retarded the deeper you go. Mental deficiency at depth (impairment) is believed to be the leading causative factor in the deaths of deep air divers as well as drunk drivers. --Hans Petter Roverud adds the following clarification, "Narcosis seems to stem from a reduction of chemical transmission at synapses. Nitrogen "clogs" synapses in some way but I haven't heard of lipids accumulating. Nitrogen is highly soluble in lipids / fat. That's probably why it's so hard to calculate a safe deco from a substantial air exposure. Add to this the slow acquisition / elimination of nitrogen. Helium on the other hand has a low solubility in lipids and is a fast (easy come -- easy go) gas. Thus, helium leaves fast and predictably as soon as you stop breathing trimix." (Thanks Hans!)
I've found through personal experience that amount of sleep and level of fatigue has a direct relationship to impairment due to nitrogen narcosis. I get "narced" very easily when I haven't had enough sleep. I can feel it as early as 90 feet (27.7m) down. Tunnel vision is one of the first symptoms. Also the ability of the eyes to perceive light is greatly reduced, which makes it darker and scarier.
I have gone past 235 feet (72.3m) during a spike dive on air, when I was well rested, but this was IGNORANT. I would NEVER do it again! Please don't do it yourself!
I'll tell you why:
1. A diver is impaired at depth when breathing air.
There's little difference between driving drunk and diving narced. If you have an emergency it takes ages to sort it out. If you have a second problem you will likely be unable to manage at all. Your heart rate will increase, your gas consumption will skyrocket, your C02 levels will jump, you'll likely hyperventilate and you'll probably start to overbreathe your regulator. Now you have a real and very dangerous emergency! You feel that you must surface NOW! How in hell are you going to decompress?
because:
2. You can't sufficiently decompress from a deep dive when you've breathed a gas containing a whopping 79% nitrogen!
Your ascent rate is going to have to be at a snails pace, because you're intaking 79% nitrogen as you go up. To sufficiently decompress in a realistic amount of time, the body needs a pressure gradient between the red blood cells and the lungs or RBC's and cells in the body.
Let me explain it like this. If you were to take two buckets and fill them both with water and place one bucket twenty feet above the other, and then take a hose and siphon the water from the high bucket to the lower bucket. How fast would the water transfer to the lower bucket? Very quickly because of the pressure difference created by the height of the bucket. The same needs to occur between cell walls in our bodies. The pressure gradient is so low, that off-gasing is extremely slow.
If one were to breathe a high concentration of oxygen to assist in decompression, the oxygen would quickly replace the nitrogen, as well as be consumed by your body and metabolized (partially) and turn to CO2 which you exhale. Getting bent on oxygen is a far less threat than getting bent on nitrogen. The body will burn the oxygen (within reason) and the symptoms will often disappear.
3. The decompression obligation when breathing air is so high that it is idiotic to knowingly incur it!
Decompressing from 79% nitrogen (air) is going to take a very long time. Even after you surface, you will feel the effects of what is called, "Sub-clinical decompression illness". You'll be very fatigued and possibly have a headache. This is because you are bent. You're not bent enough to go to the recompression chamber, but you are bent. My dive to 235+ feet (72.3m) for ten minutes (at an altitude of 6000 feet (1846m) required me to decompress for 47 minutes with a total dive time of 63 minutes! This dive required 115 cubic feet (3256 liters) of air. It also requires one to carry enough gas to take care of emergencies such as: crapped out regulator, blown hose or O-ring, entanglement or entrapment or any other problem that takes TIME to sort out. Add five minutes to sort out a problem (while narced? better make it ten!) and you end up running out of gas and options on living real quick. Five more minutes at maximum depth (remember Murphy's law?) will net you 39 more minutes of decompression time, 107 minutes total dive time, and an increase in gas consumption that requires you to have the equivalent of the Goodyear blimp attached to your back. (10 minutes bottom time equals a total dive time of nearly three hours and 233 cubic feet (6597 liters) of breathing gas with nothing left over for emergencies. With the rule of thirds, one would need to carry a minimum of 310 cu. ft. (8778 liters) of gas if you're going to surface with ALL TANKS SUCKED DRY. But breathing air, this still isn't enough.)
If Mr. Murphy hadn't been sleeping on the job, I would have been in a world of hurt.
3. The partial pressure of the oxygen in air at 218 feet (66.4m) is 1.6 atmospheres absolute, which IS THE LIMIT. You can deco. at 1.6 a.t.a.'s, but that's at rest.
At 235 ft. (71.6m) it's 1.622 a.t.a.'s and you're risking it all that you'll not have a seizure because of the high partial pressure of the 02. When we dive, we really need to keep that partial pressure of oxygen above 1.4 a.t.a.'s. 1.6 is the maximum for the decompression portion of the dive (when we are not working).
So, don't dive deep on air. Use the right gas. Anything below 110 feet (33.5m) should be done on trimix. If you can't do it right, don't do it. I've done it wrong in the past because I was ignorant. Now I know better and don't do it that way anymore. That would be stupid. Ignorance can be cured, stupidity goes to the bone. If you can't admit that you've done it wrong in the past, then you'll never learn anything beyond it in the future.
Billy W. adds the following, "And the real horror of deep air - if you survive the narcosis & high pp02, is the systemic physical damage caused by RBCs which stiffen in the presence of high ppN2. The rigid RBCs then block capilliaries and damage the fragile lining of the micro-circulatory system. This is where delicate, finely perfused tissues like the nervous system and the retinas sustain permanent damage."