Understanding Decompression Sickness

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When you breathe in air as you go deeper, nitrogen will dissolve and enter your body tissue. As you ascend, the nitrogen will leave your body tissue which you expel as you exhale. In the PADI online course and in some videos on YouTube, there is talk about excess nitrogen.

1. When does this excess nitrogen form, while descending or ascending?

2. How does this excess nitrogen form?
The answer to #1 is, it depends. This fits in nicely with your other question about why your tank doesn't shrink - recommend you go back and read that to refresh the physics in your mind. Recall that under water, the regulator will deliver air at whatever the surrounding pressure is, which means that the air pressure in the diver's lungs is the same as the surrounding water pressure. It's still air though, which as others stated above is 78% (roughly 80%) nitrogen.

As @tbone1004 stated above, Dalton's Law of Partial Pressures says that the total pressure of a gas mix is the sum of the partial pressures of the gases in that mix.

Air is a gas mix that's made up of 78% nitrogen, 21% oxygen, and 1% other gases, mainly CO2 and argon. So, in air, 78% of the total pressure is caused by nitrogen, 21% is caused by oxygen, and 1% is caused by those other gases. At 1 atmosphere absolute (sea level), the partial pressure of nitrogen in air is 78% of 1, or 0.78 atmospheres absolute. At 33 feet, the total pressure is 2 atmospheres absolute, so the pressure in the diver's lungs will be the same - 2 atmospheres absolute. Now, the partial pressure of nitrogen in the diver's lungs is 78% of 2 atmospheres absolute, or 1.56 ATA. The percentage of gases in the mix (air) does not change, but the partial pressure does. This increases linearly as the diver goes deeper: at 66 feet/3 ATA, the partial pressure of nitrogen in the lungs is 78% of 3, or 2.34 ATA. At 99 feet, it's 78% of 4 ATA, or 3.12 ATA. If you know the percentage of gas in the mix, you can calculate the partial pressure of that gas in the diver's lungs at any depth.

This is where Henry's Law starts to work. Henry's Law states that the amount of gas that will dissolve in a liquid is directly proportional to the partial pressure of that gas on the liquid. In @OTF 's soda bottle example, the gas/liquid interface is at the surface of the soda, and the force that keeps the CO2 dissolved is the partial pressure of CO2 acting directly on the surface of the soda.

In a diver's body, the gas/liquid interface is in the lungs. There are two very thin layers of tissue between the gas in the lungs and the blood in the body, and gas molecules can pass freely across those layers.

Our bodies are in equilibrium with the surrounding air right now, i.e. because there's 0.78 ATA of nitrogen pressure in our lungs, there's roughly 0.78 ATA of nitrogen dissolved in our bodies. As the pressure increases in the lungs with diving (remember what diving gear does for you), the partial pressures of the gases in the lungs increase, which creates a gradient between the gas pressures in our lungs and the gas that's dissolved in our bodies. The nitrogen will follow that gradient and begin to dissolve into the body via the lungs - first into the blood stream, then into the tissues supplied by the blood. This doesn't happen instantaneously - it's dependent on the pressure gradient and the rate at which the different tissues absorb and release nitrogen. The higher the partial pressure of nitrogen in the lungs (i.e. the deeper the diver goes) and the longer the diver stays down, the more nitrogen will become dissolved in the body via the lungs. So, the first part of question 1 is, on descent and at the bottom, the excess nitrogen is in the lungs. This also speaks to question 2 - how does this excess nitrogen form?

When a diver begins to ascend at the end of a dive, the pressure of gas in the lungs decreases. Critical to understanding decompression (and this is slightly simplified) is that at some point on ascent, the pressure of the dissolved nitrogen in the body will be greater than the partial pressure of nitrogen in the lungs. At that point, the pressure gradient reverses. Instead of going from the lungs into the body, it's going from the body into the lungs. The art and science of decompression involves controlling that pressure gradient so that the diver doesn't develop symptomatic bubbles (i.e. decompression sickness). That's the other half of the answer to question 1 - on ascent, the excess nitrogen is dissolved in the blood and body tissues, and it's our task as divers to release that excess nitrogen safely via our lungs - in a recreational diver's case, by staying within the no-decompression limits.

If the pressure gradient gets too high or if something else goes awry with the diver's decompression, it's like suddenly popping the cap on @OTF 's soda bottle - the pressure inside the bottle goes from high to low very quickly, and the CO2 that was dissolved in the soda forms bubbles. So it is with divers - bubbles of nitrogen can form in the blood and body tissues. Those bubbles can cause symptoms by directly blocking the circulation and by activating the inflammatory pathways in the body. This is decompression sickness.

Best regards,
DDM
 
When the PADI course discusses repetitive dives, they talk about surface interval and how the dive computer tracks your how much time you spend at the surface (after your first dive) and therefore how much theoretical residual nitrogen dissolves out of your body.

During your first dive, if you do any required decompression stops and a safety stop at 15 ft. shouldn't all the dissolved nitrogen leave your body? In other words, after your first dive, you shouldn't even have any residual nitrogen in you right?

If this is true, then why does the course say: "if you scroll your no stop limits from time to time as you relax during a surface interval, you will see no stop limits gradually lengthen as you spend time on the surface."

If your no stop limits are lengthening, that means you do have some residual nitrogen in you after a dive, when you are on the surface.
 
During your first dive, if you do any required decompression stops and a safety stop at 15 ft. shouldn't all the dissolved nitrogen leave your body? In other words, after your first dive, you shouldn't even have any residual nitrogen in you right?
All the dissolved nitrogen is NOT gone from your tissues. You are still off-gassing when you exit the water.
If this is true,
It is not true
If your no stop limits are lengthening, that means you do have some residual nitrogen in you after a dive, when you are on the surface.
Correct.
 
When the PADI course discusses repetitive dives, they talk about surface interval and how the dive computer tracks your how much time you spend at the surface (after your first dive) and therefore how much theoretical residual nitrogen dissolves out of your body.

During your first dive, if you do any required decompression stops and a safety stop at 15 ft. shouldn't all the dissolved nitrogen leave your body? In other words, after your first dive, you shouldn't even have any residual nitrogen in you right?

If this is true, then why does the course say: "if you scroll your no stop limits from time to time as you relax during a surface interval, you will see no stop limits gradually lengthen as you spend time on the surface."

If your no stop limits are lengthening, that means you do have some residual nitrogen in you after a dive, when you are on the surface.
My thoughts:

Well, excess nitrogen takes a while to totally dissipate. You can't fully off-gas underwater, since the inspired N2 pressure is greater than sea level, even if only by a little bit. Yes, you are off-gassing, but to have no residual nitrogen (being at "equilibrium" with the surface), you have to be at sea level. Also, you have to be there for quite a while I believe. The time it takes probably depends on the amount of dives and profiles. There are also parts of you that release nitrogen faster or slower than others. Which means when you surface, you're still going to have residual nitrogen for a bit.

This is why (I believe) your NDLs are shorter after your first dive and generally get longer the more you off-gas, or shorter if you dive more and more. The less residual nitrogen you have, the longer you can stay at depth without incurring decompression stop obligations.

Again, this is just my thoughts on why residual nitrogen takes a while to eliminate, it's not necessarily right.
 
My thoughts:

Well, excess nitrogen takes a while to totally dissipate. You can't fully off-gas underwater, since the inspired N2 pressure is greater than sea level, even if only by a little bit. Yes, you are off-gassing, but to have no residual nitrogen (being at "equilibrium" with the surface), you have to be at sea level.

This was very helpful. This makes sense.
 
Thanks all for a useful discussion, which I'm thinking about in the context of the recent discussion (in the SMB deployment thread) of five-minute safety stops. (Having trained before the safety stop was a thing, my brain still passingly equates it with a pair of suspenders on the weight belt, but I know that's incorrect.) Some of the commenters on the five-minute discussion expressed the view that it makes one a bit less tired after a dive, and equated post-dive tiredness as "subclinical DCS."

So to my question: Given that offgassing after a safety stop continues on the surface, is the rate of offgassing at two ATA (15') sufficiently more gradual that, at the surface, a longer 15' stop reduces residual nitrogen (and thus the risk, whether large or small, of DCS) more than the surface interval? And if so, then would, in theory, a 10-minute stop be even better?

And, further out on the limb, would continuing to breath Nitrox after surfacing be, in theory, better for the bloodstream than switching to 78% nitrogen? There's always some extra, after all . . .
 
Residual nitrogen gassing off on the surface after a dive, this gets me thinking. All the tables and scientific research has determined what a safe level is for a diver to ascend back to the surface and ride that line of an acceptable overload of nitrogen that you can surface with without any ill effects.
Would there be any benefit to go on O2 immediately after surfacing even though you’re not in any deco trouble and well within NDL?
Let’s say you have a free lifetime supply of O2 so cost is removed from the equation. Would there be a benefit and a shortened surface interval, or if not shortened at least an increase in the safety margin? Or does N2 just take what it takes to dissipate from your system regardless?
Would there be any safety precautions like too much O2 in your system for the next dive?
 
Residual nitrogen gassing off on the surface after a dive, this gets me thinking. All the tables and scientific research has determined what a safe level is for a diver to ascend back to the surface and ride that line of an acceptable overload of nitrogen that you can surface with without any ill effects.
Would there be any benefit to go on O2 immediately after surfacing even though you’re not in any deco trouble and well within NDL?
Yeah, it speeds up the rate that you off gas nitrogen. But not dangerously.
Let’s say you have a free lifetime supply of O2 so cost is removed from the equation. Would there be a benefit and a shortened surface interval, or if not shortened at least an increase in the safety margin? Or does N2 just take what it takes to dissipate from your system regardless?
I think breathing pure O2 would shorten the off gassing time on the surface too, but I’m not 100 percent certain. It makes sense though because tech divers use it as a deco gas at 20 feet
Would there be any safety precautions like too much O2 in your system for the next dive?
Nope, your body doesn’t retain excess O2 since it’s a metabolic gas (I think)
 
Would there be any safety precautions like too much O2 in your system for the next dive?
Think back to your Nitrox class where pulmonary oxygen toxicity was said to be a negligible concern for rec diving. You're starting to encroach on that danger, so don't overdo it. The daily limit if breathing oxygen on the surface is 5 hrs.

Since your computer almost certainly won't track surface off-gassing on O2, you wouldn't realize a shorter SI or longer NDL for a given conservatism setting as a practical matter. Even if you could, though, the gains would be minor. For instance, after a 60 ft NDL dive on air (44 min @ GF x/85) and an hour SI, the second NDL time would be 38 minutes. If you breathed O2 for 10 of those 60 minutes, your repetitive NDL time would be 40 mins. Enduring the odd glances of your fellow divers while breathing O2 for the entire SI results in a 53 minute NDL time. But again, I know of no dive computers that would make such a computation.

ETA: those gains neglect any inefficiencies introduced by mucus buildup in the lungs. Our bodies don't actually like breathing O2, and tech divers take breaks during longer O2 deco stops to recover a bit.
 
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