All divers are introduced to the concept of ‘decompression injuries’ in their basic training. However, how well explained they are and how fully the divers actually understand them, and more importantly how they relate to the safety procedures we are all taught, varies considerably. I was lucky in that I learnt to dive as part of a conservation diving trip to a remote Fijian island and, due to the considerable distance to any sort of advanced medical treatment let alone a decompression chamber, we were thoroughly schooled in the nature of different diving injuries, how to avoid them, and the symptoms. We were even lucky enough at one point to get a lecture from a doctor specialising in diving-related medicine who was visiting the project.
Anyway, my point is that I suspect a lot of divers, even some quite experienced ones, may not fully understand what ‘the bends’ are, why you don’t hold your breath underwater, what a safety stop is for, and so on. Hopefully they all wisely follow the procedures, but having a better understanding of the reasons behind it is important too, particularly as you progress to deeper dives or diving on hyperoxic or hypoxic gases.
So, here is a brief and simple, but hopefully not oversimplified, overview of the diving injuries caused primarily by breathing gasses and pressure changes – those caused by marine life, coral, pre-existing heart conditions, or drowning are outside the scope.
Some fundamental concepts
To begin with you need to understand a couple of fundamental concepts about how air (and other gasses) behave under pressure, and the effect this has on the body.
The first one is that gasses are compressible – they can be put under pressure, squeezing a large volume of gas into a smaller space, and they can also expand, making the same volume of gas fill a much larger space. In the metric system we measure gas pressure in bar. One bar is (not quite, actually, but for all intents and purposes) the pressure of gas at sea level. You’re probably familiar with your scuba tanks being filled to 200 bar (if you use the metric system) – this essentially means that they have been packed with 200 times the normal volume of your tank. So, if it is a 12 litre tank, it effectively has 2400 litres of air stuffed into it. Which is why you can breathe it for so long.
Ok, the next key point is that water exerts pressure on gases, and will therefore also squeeze them into smaller volumes. Conveniently, if you use the metric system, the pressure exerted by 10 metres of water is equivalent to the pressure exerted by the entire atmosphere’s worth of air if you are standing at sea level. This is known as 1 ata – an atmosphere equivalent. So, if you are at 10 metres depth, the pressure being exerted on any gas is double what it would be at the surface. 1 ata from the atmosphere, and 1 ata from the 10 metres of water. At 20 metres it is three times as much – again 1 ata from the atmosphere and 2 atas from the 20 metres of water. And so on. If you were to take a balloon that you had blown up at the surface down to 10 metres, it would shrink to half the size. It has the same amount of air in it, but compressed into a smaller space. If you took it up to the surface, it would expand again. By the same token, if you could somehow blow up a balloon at 10 metres depth and then take it to the surface, it would expand to twice the size, and maybe even burst. Hold that thought because it’s going to be important.
The effect of all this is critical in diving, because it has a huge impact on how you dive. The most obvious point is that, as you’re no doubt aware, you will use up your air much faster the deeper you go. Why? Because it is compressed, so although you have 2400 litres of air in your tank (if it is a 12l tank at 200 bar) it takes much more air to fill your lungs. Average human lung volume is about 6 litres, so at the surface you need 6 litres of air to fully fill your lungs (you probably don’t completely fill them with a single breath, but that doesn’t matter for the purposes of this explanation). At 10 metres, though, your 6 litres of air has shrunk to half the size, but your lungs haven’t, so to fill them you now need effectively 12 litres of air (albeit it only takes up 6 litres of space now). At 20 metres you need 18 litres of air, and so on. So clearly the deeper you go, the faster you will breathe the 2400 litres of air in your tank.
This has another vital effect though, which is the last principle you need to understand before I finally get on to the injuries. When a gas is compressed it still contains the same number of molecules, they’re just all a lot closer together. So, at 20 metres when you are filling your lungs with three times as much air squashed down to a third of the size by the pressure, you are still breathing in three times as much oxygen as you are used to and three times as much nitrogen. This is the partial pressure, which is expressed as a number where 1 is the equivalent of breathing 100% of the gas at sea level. So, air has 21% oxygen and 79% nitrogen and therefore at sea level we are breathing oxygen at a partial pressure of 0.21, and nitrogen at a partial pressure of 0.79. This is fine. At 10 metres, you are getting twice as much of both, so the oxygen has a partial pressure of 0.42 – the equivalent of breathing 42% oxygen at sea level. Meanwhile the nitrogen has a partial pressure of 1.58. And that’s just at 10 metres. At 40 metres, the maximum for recreational divers, you are breathing oxygen at a partial pressure of 1.05 and nitrogen at a partial pressure of 3.95. That is the equivalent, if such a thing was not logically impossible, of breathing 395% nitrogen at sea level. In other words, you’re taking in a lot more nitrogen than you are used to.
And if you understand all that, you will be able to understand the following possible diving injuries.
- What is it? This is probably the simplest one to understand, and the one a lot of non-divers writing about diving focus on because it sounds impressive as ‘your lungs will literally explode!!’. Well, they won’t, but they may rupture, which is bad too. Remember that hypothetical balloon that we filled at 10 metres and then took up to the surface, where it would have expanded to twice the size? This is what could happen to your lungs if you aren’t careful – at depth you breathe in air under pressure, so it takes up a smaller space. Then you swim up, and the air expands (the ’18 litres’ of air which actually only takes up 6 litres and therefore just fills your lungs at 20 metres re-expands to 18 litres at the surface), which clearly puts enormous pressure on the walls of your lungs, and is likely to tear them and force air into the bloodstream, which is not where it’s meant to be.
- How to avoid it? It’s simple – never hold your breath underwater. As long as you are regularly breathing in and out, your lungs are not sealed like the balloon and therefore the expanding air will take the path of least resistance and simply force it’s way out through your windpipe out out of your mouth. This is why even when you have your regulator out, you always keep steadily breathing out, as lung overexpansion injuries can occur with even quite small changes of depth such as a few metres. It’s also important to keep to a safe ascent rate, although this is primarily to avoid the next problem.
- What is it? You know how at depths we’re breathing a lot more nitrogen in – well where does it all go? The answer is that it is stored in our body tissues. That’s perfectly normal, and is the same thing that happens at surface pressure; nitrogen is not used by our bodies so is simply stored, dissolved in tissues. Now, one of the properties of a gas being under pressure is that considerably more of it can be dissolved in a given liquid than at surface pressure.It’s the same principle as how more gas can fit into our lungs at pressure as at the surface. For a visual example of this, look at a sealed bottle of a fizzy drink. No bubbles, obviously. Now open it, and all the bubbles appear. All that has happened is that you have taken the drink from a higher pressure suddenly down to normal atmospheric pressure. The amount of gas, in this case normally co2, that can be dissolved in the coke is less at atmospheric pressure than it was at the higher pressure of the bottle, and so a lot of the co2 has to un-dissolve and bubble out. The exact same thing would happen to you if you suddenly went from 30 metres to the surface, and you can probably imagine that having all that nitrogen bubbling out of your tissues would not end well. And indeed it wouldn’t – with such a sudden change in pressure, the bubbles that would form would be large and could easily block major arteries causing a heart attack, a stroke, nerve damage and paralysis, and severe pain especially in the joints where the bubbles gather. This is ‘the bends’. The longer you have been underwater the more nitrogen you will have stored in your tissues, and the faster you ascend the bigger the bubbles it will form as it is released.*
- How to avoid it? The way to prevent the bends, then, is firstly to limit your time underwater and secondly to alter the pressure so slowly that the bubbles that form are smaller and easily removed from the body without causing harm. This is why divers ascend very slowly and take a safety stop to give the nitrogen an extra chance to be safely released from the body, and why they dive to well-established limits as to how long they can stay down at a given depth. If they do exceed these limits, the nitrogen in their bodies will have built up to potentially dangerous levels and they must then conduct various ‘decompression’ stops, pausing at various depths so that it can be slowly released from the body in small, harmless, bubbles.Incidentally, if nitrogen build up limits the time you can spend underwater, one of the other ways around this is to breathe a gas with less nitrogen and (generally) therefore more oxygen. This is commonly known as Nitrox and comes in various varieties depending on how much oxygen is in it. Nitrox 28, for example, which is one I commonly use for reasons I will come on to, is 28% oxygen and 72% nitrogen. Much higher mixes are available though, usually for recreational divers up to 40% but higher mixes are used by technical divers when decompressing as it has been discovered that breathing a high mix of oxygen speeds up the rate at which nitrogen is safely removed from tissues, and therefore decreases decompression stop times.
Central Nervous System Toxicity
- What is it? This is a nasty one, and to understand it you have to go back to the fact that, at depth, you can effectively breathe in what would, at surface pressure be more than 100% oxygen. This is significant because even ‘pure oxygen’, meaning 100% oxygen at 1 atmosphere of pressure, is not harmful**. But by breathing oxygen under pressure we are taking on even more than ‘pure oxygen’ and, at sufficiently high quantities, this can result in central nervous system toxicity which causes seizures. These seizures are not especially dangerous on land but, underwater, they result in regulator loss and almost invariably drowning. Before this was properly understood, there is little doubt that many divers diving deep who simply drowned inexplicably were in fact victims of this.Personally I suspect one of the deaths related in Shadow Divers was caused by CNS toxicity since the diver was well outside the now accepted limits (though he would not have known that at the time). If you breathe enriched Nitrox to reduce your exposure to nitrogen, you increase your exposure to oxygen so increase the risk of a CNS hit.
- How to avoid it? Subsequent research has shown that a generally accepted limit to avoid CNS toxicity is to never breathe oxygen at a partial pressure of more than 1.6, and PADI and other training agencies recommend for added safety that a limit of 1.4 is used in practice. That, if you recall my primer above, is the (theoretical) equivalent of breathing 140% oxygen at the surface. In more realistic terms, it is breathing 100% oxygen at 4 metres, or 50% oxygen at 18 metres. (In case you’re struggle with the maths here, 18 metres is 2.8 atas, 1 from the atmosphere, 1 from the first ten metres and 0.8 from the 8 metres. Ok? So 50% oxygen at 1 ata has a partial pressure of 0.5, so at 2.8 ata its partial pressure is 0.5 x 2.8, which is 1.4, or our safety limit. H’ok?). Why do I like Nitrox 28? Because Nitrox 28 has an o2 partial pressure (po2) of 1.4 at exactly 40 metres, which is my training limit anyway, so I can breathe it safely on any dive I might do currently.
- So, you may already have got ahead of me here and thought, hang on, does that mean there is a limit to what depth you can safely breathe even normal air? Correct – the po2 of air reaches 1.4, the safety margin limit, at 56 metres. At 66 metres it reaches 1.6, the absolute outside safety limit. U-869, the u-boat dived in Shadow Divers lies at 73 metres, and many of the early dives were done on ordinary air, so you can start to see why I suspect that a CNS hit may have been the reason one of the divers suddenly and inexplicably lost consciousness and died. In later dives, and in almost all modern deep diving, helium is added to the mix to create helitrox or trimix. This allows the reduction in nitrogen which reduces decompression requirements, but keeps the o2 to within safe partial pressures as well.
Ok, I lied, this wasn’t a very brief intro, but the subject is fascinating and every detail leads to a dozen more details so I found it hard to limit what I included! I hope it was interesting and, if you are a diver, has taught you something you didn’t already know.
* I had already gone off-topic enough in this post so I thought I’d put this little aside down here instead, but have you ever heard about why astronauts wear pressurised suits and what happens to humans in a vacuum? Sometimes people express it melodramatically as ‘your blood would literally boil!!’. Well, my assumption is that this is basically just another expression of decompression sickness. After all, a vacuum has 0 pressure, so going from normal pressure (1 ata) into a vacuum (0 ata) suddenly is similar to going from 10m (2 ata) to the surface (1 ata) very quickly – it’s going to cause a lot of the gasses dissolved in blood and tissues to un-dissolve. Of course, I assume the effect is far worse because you have spent a lifetime at 1 ata so your tissues are fully saturated with all the gas they can absorb at that pressure – the equivalent of spending hours if not days underwater (I’m not sure how long it takes the body to become saturated with nitrogen at a given depth and I can’t be bothered to google it right now). But yes, your blood would ‘boil’ in that all the gas would bubble out of it with all the disastrous consequences of a very quick ascent after a very long dive. This raises the question of whether a human could safely be very slowly exposed to a vacuum, assuming they had breathing apparatus. The answer? Not sure. But it would seem logical. If you know, tell me in the comments.
**Update here, as a reader pointed out that breathing 100% oxygen at the surface is not entirely harmless as I implied. This is a good point and worth clarifying: while there are no short term consequences to breathing enriched air (and if 100% o2 is being given to treat DCI, for example, the benefits far outweigh any potential risks), breathing oxygen at partial pressures of over 0.5 for an extended period can have side-effects. At a po2 of 1 (100% oxygen on the surface) this usually takes several hours of continuous exposure, which is why it is never really a concern when using o2 therapeutically. However when diving and being exposed to po2s of up to 1.4 or occasionally 1.6 when decompressing (technical divers will sometimes decompress at a po2 of 1.6 due to the fact that the reduced exertion during a deco stop lowers the risk of a CNS hit) this can be a concern. I didn’t discuss it here and it is very rarely an issue in recreational diving because you will almost always reach your no-stop limit or run out of gas long before oxygen toxicity would be an issue, unless you were doing extremely long, very shallow dives with very rich nitrox mixes. In technical diving where you could be doing length decompression stops with up to 100% oxygen it can be more of a concern, and technical divers learn to track their oxygen as well as their nitrogen exposure, and plan dives appropriately.