Decompression sickness (DCS) is classed as a Decompression Illness - also known as Caisson Disease or 'the bends' - is a series of ailments which can strike when the human body is compressed and breathing compressed air. This write-up focusses on decompression sickness in divers, as this is where DCS currently most commonly occurs
The first recorded incidence of DCS was in the 1840s, when workers in pressurised mines in France were exhibiting symptoms of DCS when they were emerging. Whilst the phenomenon was described in the 1850s, the physicians weren't able to explain the illness displayed by the miners.
What causes DCS?
To understand DCS, we have to turn to physics; specifically, to Henry's law: "At a constant temperature, the amount of a given gas dissolve in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid".
At the surface at sea level, we have about 100km of air above us, in the atmosphere. This is defined as 1 atmosphere of pressure, and it's the pressure we're used to in our everyday lives. (If you live on a mountain top, this is slightly less, but that's not really relevant in this discussion)
To get another atmosphere of pressure in sea water, you only have to go down 10 meters. Go down to 20 meters, and you have 3 atmospheres. 30 meters, 4 atmospheres, and 40 meters will take you to 5 atmospheres. So, within the limits of recreational diving, you are exerting 5 times more pressure than usual on your body.
Liquids don't compress much, so your body is perfectly fine under the water. The problem, then, is that we breathe a lot of nitrogen. At the surface, we breathe around 79% nitrogen. No problem: our bodies don't do anything with this nitrogen, so we just breathe it in and out. A typical lungful is about 5 litres - so you'd breathe 3.95 liters of nitrogen.
At depth, however, we are under a lot of pressure. Gas under pressure becomes soluble in liquids, as mentioned above. As we said, your body contains a lot of liquid, so when we go down to depth, our body takes on gases. For Oxygen, that's not a problem - your body will use it and burn it off. For Nitrogen, however, it starts to saturate your bloodstream and body tissues.
At 30 meters, you are under 4x the pressure as you were at the surface, so that same 5 litre breath you took now actually takes 20 litres of air from your scuba cylinder, and 15.8 litres of that is Nitrogen. Since your body is now 4x undersaturated of gas compared to at the surface, this nitrogen starts to dissolve into your body tissues. Again, no problem.
The challenge comes when you start ascending. As the pressure on your body decreases, the gas that has been dissolved into your blood and body tissues, comes back out of solution. When this happens slowly, the nitrogen comes out of solution the same way it came in: You would simply breathe it out.
It's when the pressure is released quickly that the nitrogen could become a problem. Why? Well, have you ever shaken a Coke bottle, and then opened it really fast? The same thing happens in your body. The gas comes out of solution, and forms bubbles. If you come up too fast (or have too much gas dissolved in your body), these bubbles can cause serious problems.
Tissue compartments
To understand absorption of nitrogen into tissues better, dive physiology (and decompression theory) operates with theoretical 'tissue compartments'. The idea is that some tissues of the body absorbs nitrogen very quickly (with, say, a 5-minute half-life), whilst others absorb nitrogen very slowly.
For a tissue compartment with a 5-minute half-life at 10 meters depth, for example, the tissue would absorb half the partial pressure of nitrogen after five minutes. That means that the partial pressure of nitrogen for that particular theoretical tissue compartment would be the same as if it were saturated at 5 meters depth. After another five minutes, it would be the same saturation as at 7.5 meters. After another five minutes, it would be the same as 8.75 meters. After 6 cycles (30 minutes), the tissue compartment would be 98.4% saturated - which is considered full saturation.
When you come back up from pressure, you would have to do so slowly, to ensure the body can offgas the pressure again bit by bit - but even if the 5-minute compartment was fully saturated, it would only take 30 minutes before the compartment would be completely offgassed at the surface.
Different decompression models take different tissue compartments into account. A dive computer, for example, might take as many as 30 different compartments into account, and will constantly be computing how much nitrogen you have in your body. Other decompression models use other numbers (and focuses on different aspects of) of tissue compartments, and can be drastically different.
Dive tables are calculated to ensure that a recreational scuba diver can dive safely, by calculating how much Nitrogen is absorbed into their body tissues. There are two main dive tables currently in use - the US Navy dive tables for decompression diving (usually just known as 'the Navy tables') and the PADI RDP - or Recreational Dive Planner. The Navy tables were tested on Navy divers - generally men aged 17-20 years, and in good health. Obviously, recreational divers nowadays come in all sizes (obese people tend to have a drastically different nitrogen absorption than skinny people, as fat readily accepts nitrogen), ages (any cardiovascular issues strongly increase the risk of DCS, and older people tend to have more heart- and blood-vessel issues than younger people) and genders.
The main difference between the Recreational Dive Planner (RDP) tables and the navy tables, is that the Navy tables use the 120-minute theoretical tissue compartment as their 'leading' compartment for calculations, whereas the RDP uses the 60-minute tissue compartment. As a result, the RDP (and the PADI dive courses) advises a maximum ascent rate of 18 meters per minute, and the Navy table-based dive organisations (most other diving certification agencies) advise a max ascent rate of 9 meters per minute.
The other big difference between the RDP and the Navy tables is surface time: At the surface, the half-time of the 60-minute compartment is half of that of the 120-minute compartment, so from the dive-table's perspective, you get twice the 'credit' for surface intervals between repetitive dives, as divers who use the Navy tables.
Symptoms of DCS
Decompression sickness, then, is the problem of the little gas bubbles that float around your body. Symptoms of DCS can include general pain, skin troubles (caused by blood not reaching the skin properly), unusual extreme weakness and/or fatigue, numbness, tingling in the extremities (usually on one side first, spreading to the other side), breathing troubles, problems with vision, hearing, or speech, vertigo, nausea (and possible vomiting), paralysis, decrease- or loss of consciousness, coma, and (worst case scenario), death.
All the above symptoms are related to air bubbles. Bubbles getting trapped in the joints can be extremely painful - and people suffering from DCS would often curl up into a ball to try to reduce the pain in their joints. The bent-over, kneeling down, bent joints posture is what gave DCS the nickname 'the bends'.
Air bubbles travelling around your body can give problems with your heart, brain, joints, and cause all sorts of general pain - and symptoms that are generally associated with heart- and circulation problems.
First aid for DCS
First aid for decompression sickness is to place the victim in the recovery position, and administer 100% oxygen. The oxygen helps the body off-gas the remaining nitrogen. In addition, if there is a reduction in bloodflow anyway, the higher concentration of oxygen can help the body deliver adequate oxygen, if at least a little of the blood gets through to the tissues in question.
In serious cases of DCS (i.e. in cases beyond a little fatigue and some tingling in the fingers), evacuating the victom to a hyperbaric chamber (or 'pressure chamber' / 'recompression chamber' as it is sometimes known), is required. In a hyperbaric chamber, the victim can be re-compressed under controlled circumstances, which would bring the Nitrogen bubbles back into solution in the tissues and blood of your body, before slowly decompressing allows you to breathe the nitrogen out again.
How can you avoid DCS?
In recreational diving, we use dive tables or dive computers to keep track of how much nitrogen we have in our bodies at a given time - by staying within those limits, we decrease the risk of DCS. By using the tables or computer conservatively, we stay within the no-decompression limits, and are always able to ascend (slowly - at a rate of 9 or 18 meters per minute maximum, depending on which tables you are using) directly to the surface.
If you overstay your no-decompression limits, you have a problem: There is too much gas dissolved in your body's tissues to be able to surface, and you have to make a decompression stop to give your body time to offgas. Dive computers have made these calculations a lot easier, but it's still not good practice for recreational divers.