Damn, next time I have an issue underwater, I want to be diving with Billsreef! That answer is so spot-on, I think he could write the chapter in the Nitrox manual that talks about OxTox. Nicely done!
Grunt, yep, you're dead-on. The issue is oxygen toxicity, known in some circles as "OxTox." Bingo!
Billsreef said something really interesting... He said:
Ok, so you've got a guy having obvious and severe problems at 130' while on some unknown blend of Nitrox, could be simple 21% (aka air) or a high enough blend for O2 toxicity to be an issue at that depth.
There's two parts of this sentence that make it really interesting, with reference to the gas breathed known as "nitrox." For those of you who don't know, the term "nitrox" means, literally, "a combination of nitrogen and oxygen," which is what air is... 21% oxygen and 79% nitrogen, save for a few trace gasses like hydrogen, helium, neon, argon, and more. That's what you're breathing right now... Congratulations, you're nitrox-experienced.
Within the dive community, however, the term "nitrox" usually refers to air that's been oxygen-enriched... That is, a breathing gas that's been "souped up" with a higher percentage of oxygen in it than normal air has. Typical "mixes" are 32% and 36%, commonly known in the "recreational" community as "Nitrox 1" and "Nitrox 2." However, to better define an oxygen-enriched breathing gas, many divers with a scientific mindset call it "Enriched Air Nitrox (percentage)," or "EANx" to better define that it's not standard air. Thus, "Nitrox 1" could be better expressed as EAN32 and "Nitrox 2" could be better expressed as EAN36.
There are several advantages to breathing EANx instead of air when diving:
1. Because the diver is breathing gas that has less nitrogen in it (EAN32 would have 68% nitrogen in it rather than 79%), the ongassing of nitrogen is less. Therefore, this extends the diver's NDL. EAN32 has the very convenient properties that it ongasses at 80% of air... In other words, a diver breathing EAN32 at 100 feet uses the same math as a diver breathing air at 80 feet. Thus... More bottom time for a given depth.
2. The diver can alternatively enjoy shorter surface intervals between dives, because he’s ongassed less nitrogen and is therefore in a lower pressure group (this should make sense if you’re familiar with PADI’s tables).
3. My favorite... The diver can use a standard air profile for the depth that they were really diving, but enjoy the additional safety margin that EANx delivers.
4. Since EANx, by default, has less nitrogen in it, a diver is less likely to experience being “narked,†and if he is, the effects of it are less. This can turn into a huge advantage at depth.
5. Many divers report that after a full day of diving EANx, they feel much better than after a day of diving air. This is due to the increased oxygen percentage, the decreased nitrogen loading, and the fact that the diver’s body has not had to fight as many bubbles all day long, which are often considered invaders… Studies show that divers who nitrogen-load themselves consistently have elevated white blood cell counts… In other words, the body is in “defend itself†mode, and mild flu-like symptoms of being tired, groggy and achey can be the result. Using EANx reduces the tendency for the body to defend itself because nitrogen loading is reduced… There’s less bubbles… And a reduction in these symptoms is the result.
The age-old method of creating EANx is to fill an otherwise empty tank with oxygen partially, then topping it off with air. This process is known as "mixing." There are other ways of obtaining EANx as well... However, for the sake of simplicity here we'll consider mixing as the standard method. EAN32, for example, is obtained by filling an empty scuba tank to about 420 psi with oxygen and then topping it off with air.
Enriching breathing gas with oxygen has it's advantages and disadvantages... As with most things scuba, there is a tradeoff. Of course, oxygen is a requirement for life - we need it to survive. However, at pressure (which, to a scuba diver, means depth), oxygen becomes increasingly more
potent. There's not an easier way to explain it other than that. There DOES come a point where you can feed your body too much oxygen... The results of doing so typically result in convulsions and a lack of control of muscle tissues. The medical term for this is "Central Nervous System Toxicity," or CNS Toxicity. Divers call it "Oxygen Toxicity," or OxTox. This is what our diver in the previous scenario was experiencing.
The biggest danger of OxTox is the loss of control. A diver experiencing OxTox could potentially spit out their reg and not be able to retrieve it, or lose control of their buoyancy and not be able to do anything about it. OxTox is considered a life-threatening situation. Any diver experiencing it should immediately ascend - that is, reduce their ambient pressure and the symptoms should subside. Unfortunately, a diver OxToxing will not have the ability to do this on their own. This is what was happening in our dive scenario above.
The higher the percentage of oxygen in a diver's breathing gas, the shallower the diver must limit his depth to in order to remain safe. 100% oxygen is safe to breathe to a depth of 20 feet. EAN32 is safe to breathe to a depth of 120 feet. EAN36 is safe to breathe to a depth of 100 feet. Any deeper with these gasses, and the situation can be considered life-threatening. There is no difference between putting a loaded pistol to your head and diving 36% to a depth of 140 feet.
In fact, if one had to make a choice between the two methods of dying, the bullet to the head would probably be less violent and less painful. People who have experienced OxTox claim that while their bodies were painfully in the throws of spasms, their minds were clear and fully aware of what was happening to them. At least with the bullet it'd be over instantly.
This is why a certification is required before anyone will consider selling you "nitrox." Like most things scuba, there is an element of risk involved, and only the officially informed should be permitted to use it.
In the circles I run in, this is so important that we’ve stopped using the terms “nitrox†or even “EANx†completely. Instead, we simply refer to the gas by the deepest it can be safely used. For example, EAN36 would be “Maximum Operating Depth 100†or MOD100. Pure oxygen would be considered MOD20. EAN32 would be considered MOD120. Our mindset is that we don’t care really what’s in the bottle… What we care about is, “at what point are we putting a loaded gun to our heads?â€Â
Interestingly, air is not somehow exempt from the equation which allows you to figure out at what depth the oxygen content will cause an OxTox… 21% oxygen â€"œ air â€"œ can only be safely breathed down to a depth of about 190 feet. Thus, we don’t call it “air†â€"œ we call it MOD190, or a “190 mix†for short… Even though, technically, we didn’t mix anything to make it.
…Now, since “recreational†divers aren’t taught to go deeper than 140’, nobody bothers to teach this whole OxTox thing until later in a dive education… Called the “Partial Pressure of Oxygen,†or PPO2 for short. However, whether you’re aware of it or not, the science is there… So you may find it interesting that, at some time in your dive career, you were only 50 to 100 feet from death by OxTox.
Taking this one step further, there ARE divers who have dived to depths below 190 feet... Bringing this theory back into play, it can be surmised that a safe gas to dive at that depth would have LESS oxygen in it than air does… What is commonly referred to as a “hypoxic†mix, which can have such oxygen percentages as 18%, 15% and 10%. These are made by diluting EAN32 (or MOD120) with helium, since there is no practical way of removing oxygen from air other than adding another gas… And adding nitrogen would “nark†the diver badly and load him with nitrogen, making decompression very long. Helium is a very lightweight gas which offgasses very easily. A mix of these three gasses (oxygen, helium, and nitrogen/air) is known a “trimix,†but again, we simply refer to it as MOD200, MOD250, and MOD400.
Interestingly, we’ve established that every breathing gas has a MOD… Even air. However, there are some gasses that could have a MINIMUM, too… For example, MOD400 is a required gas at, say, 350 feet… MOD190 (air) would kill the diver at that depth via OxTox. However, on the surface, MOD190 works great… But MOD400 simply doesn’t have enough oxygen in it to support life on the surface… Breathe it, and you’ll fall asleep and eventually die. Thus, a deep diver has one gas that will kill him at depth and one gas that will kill him on the surface… So he’d better know which to breathe, when to switch, and why.
Those who have been following along here may think, “Why not get rid of ALL of the nitrogen in the mix, and just do oxygen and helium, if helium’s so great? The answer is, “some do,†and the mix is called “heliox,†however, the cost of helium can make it a very expensive proposition… And being so light, helium has the odd characteristic of sapping body heat, so heliox is often known to make a diver very cold, very quickly… An uncomfortable situation for most, and downright dangerous for others.
Anyway… Back to Billsreef’s awesome post: Here’s what’s so interesting about the post:
1. Bill knew that this guy was using “some unknown blend of nitrox†because of the green stickers all over the tank. Most scuba shops have made it a point that green “NITROX†stickers or “EANx†stickers be placed all over any tank that could have anything but air in it. In some cases, they will not fill a tank with anything but MOD190 (air) unless these green stickers are in place. The problem with that concept is that the tank MAY typically be filled with EANx, but today is filled with air… There is nothing stopping any “nitrox†diver from putting anything he wants in that tank… Even pure oxygen, trimix or heliox… (Which, interestingly, would be the only situation where there wouldn’t be NITROX in the tank). Additionally, since the green stickers say “NITROX†all over them, there is no definition as to whether the gas in that tank is safe to breathe at that depth. Maybe it is, maybe it isn’t. If it’s MOD190 (air), then sure, he’s fine. If it’s MOD100, he’s way out of his envelope, and we can easily see why he’s OxToxing. The stickers are simply misleading at best, and do nothing to improve the safety of the situation… In fact, they actually hinder it because, with the green stickers in place, a diver is less likely to place pertinent information on the tank, like a label that tells him what MOD the gas in that tank has.
2. Billsreef immediately sought pertinent information about the gas in that tank… In other words, he was looking for a label. This label would have been most helpful if it had told MOD. Yes, that can be figured by having the oxygen percentage on the label, but it’s simply easier to have MOD immediately at your fingertips… Which is why we recommend labeling EVERY tank with a MOD, and ONLY labeling every tank with a MOD. Here’s some examples of NOT pertinent information, that only obscures the pertinent information: Forty-two VIP stickers, “Scuba Bunny†stickers, NITROX stickers, “Divers Do it Deeper†stickers, “Jacques Cousteau’s college roommate†stickers, “Prior Navy SEAL†stickers, fill date stickers, initials of the person who filled the tank stickers, Leisurepro stickers, brand name stickers, DAN stickers, etc. There should be TWO stickers on the tank… One, the
current VIP sticker (placed on the bottom of the tank where it will be protected in it’s depression), and two, the MOD. We label ALL of our tanks with a MOD â€"œ even those we fill with air. It’s as simple as placing a piece of masking tape (we like the blue stuff from Wal-Mart â€"œ it’s not a “Rule 6†violation) and using a marker to write “190†on it. When we're done using the bottle, we pull the tape off and throw it away. Any tank with no sticker tells us that it’s an empty tank.
This system is easy, simple, effective and safe… And we know immediately what gas is safe to breathe at what depth.
…Which is why, when I see a diver with plain tanks and a label with “120†on the back of his tank, I not only know that he’s safe to dive to 100 but not safe to dive to 130… But that he’s a guy who cares and values logical decisions about his life.
