For safety, it’s best to ascend slowly from a dive to reduce the risk of decompression sickness (DCS). Divers should not ascend faster than 29 feet (9 m) per minute — but is it possible to ascend too slowly? How does your ascent profile affect decompression stress? Let’s examine diving ascent methods and decompression stress to find out.
Traditional ascent methods
Within recreational diving, it’s common to begin your dive at the deepest point. During the dive, you’ll gradually ascend to stay comfortably within your NDLs. On a multi-level dive, you’ll spend some time at your deepest depth. Then you’ll ascend to a shallower depth and spend some time there before moving up again to another level. Divers using dive tables were the first to plan multi-level dives, which historically allowed them to maximize already-limited bottom times. Since divers now follow NDLs in real time on dive computers, they tend to gradually ascend in a less predetermined way.
A bit of physiology
Although the dive community considers gradual ascents good practice, they are not very efficient when it comes to off-gassing. But why? Let’s have a quick refresher on how off-gassing works. It occurs when the partial pressure of the nitrogen (N2) in your lungs is lower than the partial pressure of the N2 in your tissues. This creates a diffusion gradient so that the higher concentration of N2 in your tissues will move toward your lungs to equalize the pressure of that gas within your entire body.
Once the nitrogen reaches the lungs, they scrub out and exhale the N2. Since your lungs are at the same ambient pressure as the water at your depth, and because it takes time for N2 to move from your tissues to your lungs, ascent increases the pressure difference. This will increase the rate of off-gassing in a given amount of time.
Ascending too slowly
If we ascend too fast, the N2 cannot leave our tissues quickly enough. Thusly, it will come out of solution while still in the tissues, forming gas bubbles. But if we ascend very slowly, the pressure difference remains quite small. This means that the rate of off-gassing will be slower, so your tissues will retain more N2. It will eventually disappear from your body, but much of that will happen on your surface interval.
This may add to post-dive fatigue, because during a dive — especially during ascent — so-called “silent” bubbles form. Although these bubbles are normal when diving, our bodies consider them foreign invaders and respond by increasing the amount of white blood cells, which will find them and surround them to fight them off. This defense mechanism doesn’t do anything though, because instead of being a nasty virus, these are just bubbles of inert N2 that don’t react with white blood cells.
The body also responds to silent bubbles by producing histamine. This increases the permeability of capillaries so that white blood cells can fight off pathogens more easily. But again, this has no effect on bubbles of N2. What does this all mean in English? Well, a byproduct of a histamine response is inflammation, which can induce fatigue. If we can keep the number and size of silent bubbles to a minimum, perhaps we will feel less tired after a dive, as well as reduce our risk of DCS.
A different approach
Although scientists are still learning about decompression theory, studies have shown that a slow ascent is not always the best way to go, as defined by 3 to 10 feet per minute (1 to 3m/minute).
To illustrate, let’s examine a multi-level dive profile with a few small but significant changes. Once you’ve spent time at the deepest part of the dive, say 100 feet (30 m), make your next level distinctly shallower so that you maximize the diffusion gradient. Perhaps move to 60 to 69 feet (18 to 21 m), but try to do so at the maximum ascent rate of 30 feet (9 m/min). This will have two effects. First, it will minimize any further on-gassing in tissues that on and off-gas more slowly. This will reduce the overall amount of N2 that needs to exit your body on the surface.
Second, you’re creating a larger pressure difference between the partial pressure of the N2 in your lungs and the N2 in your tissues. This is because the N2 in your tissues will not have had much time to move toward the lungs, so it will be comparatively higher than the partial pressure in the lungs by the time you have ascended to 59 feet (18 m). Once you reach the shallower level, stay there and you will off-gas greater amounts of N2 during that time underwater instead of on the boat. This applies for each level that you move up to until you reach your safety stop. Also consider a 5-minute safety stop on every dive for added conservatism.
Technical divers know this kind of ascent profile well, using it during decompression dives from the deepest point in the dive to the next level before moving up to a gas switch. I introduce students to this profile during the deep specialty course. We often find that students to make the transition to a slightly faster ascent, and they are rightly cautious. But over a few dives they learn to ascend closer to 30 feet (9 m/minute), but crucially, in a controlled way.
To be clear, I’m not saying that this is a “safer” way to dive — it’s just a different approach. You must always obey the maximum ascent rate you have learned or the ascent rate that your dive computer will allow. You must also consider that relative pressure change increases as you shallow up, so it’s not necessarily appropriate to continue this kind of ascent once you’re within 33 feet (10 m) of the surface.
Whether this type of ascent decreases fatigue is also subjective. I feel far less fatigue after a dive by using this kind of profile. But although it works for me, it may not work for you. Either way, it is worth remembering that many factors affect decompression stress, and you must consider all of them for every dive. For more detailed information on factors affecting decompression stress, watch this presentation by Dr. Neal Pollock.