The air you are breathing just now, reading this book on the surface, is comprised of 79 per cent nitrogen and about 21 per cent oxygen. Although largely inert on the surface, at high pressure levels nitrogen has a narcotic effect – the nasty diving problem called nitrogen narcosis. So both of the elements that make up ordinary air, nitrogen and oxygen, can become problematic when you are diving deep.
Nitrogen narcosis is a creeping (and at first largely unnoticeable) debilitating effect, which starts for me (when I’m diving on air) at a depth of about 30–35 metres. You need to know a little about the mechanics of diving to understand how it becomes a problem.
As a diver descends, the increasing weight of water surrounding them tries to compress internal air spaces such as their lungs, which are, simplistically, just bags of air. Imagine taking an air-filled crisp bag down underwater – it would very quickly be compressed to a fraction of its size by the surrounding water pressure. To avoid this eventually fatal effect happening to a diver’s lungs, an aqualung (or breathing regulator) delivers increasing amounts of compressed air with each breath as they descend. The aqualung delicately and rather cleverly keeps the air pressure in the diver’s lungs exactly equal to the increasing water pressure around the diver. The lungs stay the same size as topside, and no catastrophic collapse happens.
Once a diver has descended to a depth of 10 metres, the weight of the surrounding water in which they are immersed is conveniently exactly equal to the weight of the whole atmosphere that presses down upon us whilst we are standing on land at sea level. On the surface, the weight of the atmosphere (atmospheric pressure) is called one atmosphere or one bar. So, adding the 1 atmosphere weight of the atmosphere itself to the 1 atmosphere weight of water at 10 metres produces a pressure (water pressure) of 2 bar (or 2 atmospheres): at 10 metres, the water pressure is exactly double the air pressure we experience on the surface. The doubled weight of water and atmosphere above the diver will compress the volume of any air spaces such as lungs to half its normal size if an aqualung is not used.
To combat this ‘squeeze’ as the old hard-hat divers called it, at a depth of 10 metres a diver’s aqualung feeds them air at twice atmospheric pressure, that is at 2 bar. The delicate equilibrium between the air pressure in the lungs and the surrounding water pressure is maintained.
At a depth of 40 metres the water pressure is five times atmospheric pressure – that is, 5 bar – and comprises the 1 bar (atmosphere) on the surface plus 1 bar (atmosphere) for each of the four 10 metres. Any air spaces such as lungs would be compressed to a fifth of the volume they would be on the surface – not good. So, the aqualung again cleverly feeds a diver air that is compressed to five times atmospheric pressure – 5 bar. Again, the air pressure in the diver’s lungs is kept exactly the same as the surrounding water pressure – and the diver’s lungs remain exactly the same size as on the surface.
Boyle’s Law – the law of inverse proportions – governs this effect. When scientists were trying to work out what happened to air underwater, some brave, hardy men would sit in an upturned barrel cut in half, which was lowered into the water. As the barrel was taken below to predetermined depths, the air inside was compressed and the water level rose. Marks would be made on the side of the barrel at different depths. The depths and compression marks were correlated, and the law became clear.
If each breath the diver takes holds five times as much air as normal (compressed into the same volume), the diver is absorbing five times as much of the individual constituents. Therefore, in every breath the diver breathes in five times as much nitrogen, and five times as much oxygen.
Nitrogen is largely inert on the surface; the 79 per cent of nitrogen you are breathing right now as you read this is passing in and out of your body harmlessly. But the deeper you go, the higher are the volumes of compressed air breathed in each breath – and the more the increasing amounts of nitrogen in your body start to cause the debilitating effect known as nitrogen narcosis. Cousteau with typical flair called this effect the ‘Raptures of the Depths’.
For me, breathing air at 50 metres is roughly the same as drinking four pints of beer. The narcosis strips away your ability to understand and rationalise situations – and robs you of the ability to deal with things when they go wrong. The ‘narcs’, as they are affectionately known, affect people in different ways. Some get euphoric – some get paranoid. Some people get tunnel vision; others lose control and panic when the simplest thing goes wrong – something that could easily be dealt with normally by the same person on the surface.
Rather than breathing compressed air at any depth greater than 40 metres, nowadays I always dive on a trimix diluent, which replaces a large element of the dangerous nitrogen in the breathing mix with helium, which has no discernable narcotic effect. Although nitrogen narcosis is no longer an issue for me, if you want to get an idea of what nitrogen narcosis can do, I recounted a hit I got in the chapter entitled ‘Bail out on the Cushendall’ in The Darkness Below. This was a 58-metre air dive in 3-metre visibility, in a current, on the wreck of the World War II casualty SS Cushendall which lies off north-east Scotland. Oh, the things we do when we are young …
Whereas oxygen is very therapeutic and beneficial in normal use, as the aqualung feeds a diver increased volumes of breathing gas on descending, this means that in addition to getting higher partial pressures (or concentrations) of nitrogen, the diver also gets increased volumes of oxygen.
Oxygen, the very stuff that keeps us alive on the surface (and of course underwater as well) becomes increasingly toxic in the larger volumes breathed by divers as they venture deeper. The risk of an oxygen toxicity hit becomes a very real danger. This starts off with twitching and spasms but rapidly develops to uncontrollable convulsions where a diver will amongst other things, rip off their mask and spit out the breathing regulator. In water, a hit nearly always results in drowning, unless the diver is wearing a full-face mask. A number of leading technical divers have sadly ‘ox-toxed’ over the years and died of the uncontrollable convulsions – so deadly underwater. Some had mistakenly breathed their shallow water oxygen-rich decompression gases at too great a depth, quickly bringing on a fatal oxygen toxicity hit.
The trick is to use a nitrox mix which has the right amount of oxygen to safely accelerate decompression – and to use it at the right depth where the nitrox does not become toxic. The consequences of getting it wrong can be fatal.
A commonly used enriched air nitrox (EAN) mix that is suitable to breathe and shorten decompression (compared to breathing standard compressed air all the way to the surface) from a depth of 20 metres upwards is EAN50, which comprises 50 per cent oxygen and 50 per cent nitrogen. The increased amount of oxygen and reduced amount of dangerous nitrogen shortens (or accelerates) the time needed for decompression before surfacing.
At a depth of 20 metres, the water pressure on your body is three times what the atmospheric pressure on your body is as you read this right now. So the aqualung feeds the diver three times as much compressed breathing gas to keep the pressure in the lungs exactly the same as the surrounding water pressure – and avoid a lung collapse. This means that in every breath the diver is breathing in three times as much oxygen as on the surface. If each breath is 50 per cent oxygen, or half of the total mix, we could say that at the surface that the partial pressure of oxygen (abbreviated to PO2) is 0.5. At 20 metres, breathing three times as much oxygen the partial pressure is 3 x 0.50 = PO2 of 1.5 bar.
Trials have shown that a PO2 of 1.4 is relatively safe, but above a PO2 of 1.6, you are entering an area where the oxygen concentration in your body is starting to become toxic – and if the levels increase or if that same level is breathed for more than a certain time, you risk an oxygen toxicity hit, convulsions and death. That’s why we put a maximum depth limit on breathing EAN50 of 20 metres, where the PO2 is 1.5 bar.
But EAN50 has a fixed percentage of oxygen in it at all times – 50 per cent. Thus, at 10 metres, the PO2 is twice 0.5 = 1.0 bar. There’s less therapeutic oxygen in the breathing mix compared to breathing, say, EAN80 with 80 per cent oxygen, where the PO2 is 1.6 bar. So, although EAN50 is good because you can start breathing it deeper, at 20 metres, and start reducing the level of nitrogen in your body early, in the shallows it is not giving you as much oxygen as you could safely breathe. You