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Decompression sickness (DCS), also known as “the bends,” is a condition caused by bubbles of gas forming in a diver's bloodstream during ascent. Their size and number overwhelm the body's ability to eliminate them without complications.

As soon as you take a breath of air (or nitrox) at depth, gas will start to be dissolved in your body tissues. This is a function of the increased ambient pressure. Oxygen is metabolized (used by the body to create energy) as you descend; however, nitrogen is not used in the metabolic process and instead is absorbed by your body tissues as the surrounding pressure increases. When the tissues have absorbed an internal gas pressure equal to the surrounding water pressure, they are said to be “saturated.”

The actual process of gas uptake (on-gassing) in a diver’s body and gas elimination (off-gassing) is complicated by each individual’s body type, fitness level, and other considerations in conjunction with depth, duration, and the gas being used on the dive. Some experts suggest there are more than three dozen factors at play.

Luckily, you can follow a simple guideline and consider the way compressed gas interacts with your body in very simple terms.

While under pressure, any increased gas circulating in your blood stream is contained; but as you ascend, excess gas needs to be eliminated from your body. If the pressure is released slowly (if you ascend at the prescribed rate and under control), the nitrogen outgases slowly and will safely travel to the lungs where it is exhaled without any further action from you. However, if the pressure drops too quickly (typically from a rapid ascent), it is believed that nitrogen forms larger bubbles in the blood than your body can eliminate and that can be harmful. While tiny bubbles generated by slow and controlled ascent are harmless, larger bubbles can get stuck and create blockages in your circulatory system. To complicate the issue further, these bubbles may activate your body’s immune system, causing more complications. Quite obviously, this situation is one you should strive to avoid at all costs.

Signs and Symptoms

Signs and symptoms of DCS vary from skin irritation, a dull soreness in the joints, tingling, and minor swelling, to incapacitating pain, paralysis, and unconsciousness.

Divers suffering from DCS may say that they have the bends or have been bent or taken a “hit.” This happens when nitrogen bubbles block parts of the circulatory system (small veins of joints, elbows, knees, or shoulders), causing pain that divers may seek to relieve by bending those joints. Even very experienced divers can get bent, so take this very seriously.

If you suspect you, a buddy, or a dive professional has suffered DCS, remember the most common sign is denial, and denial will delay treatment. This can result in more serious complications. Another form of denial is to brush minor symptoms off as “only a mild hit” and thinking they will resolve on their own in time. Regardless of the severity of signs and symptoms, consider all cases of decompression sickness to be serious.


Treatment is simple: discontinue diving, administer 100% oxygen, and seek professional medical attention as soon as possible. It is important to have the correct contact information for the closest diving emergency facility or re-compression chamber because treatment by such a facility will most likely be necessary.


By following safe and proper diving procedures, you can reduce the risk of DCS. While it is very difficult to exactly predict how much nitrogen is absorbed by the wide range of human bodies, there are models and algorithms that give fairly accurate approximations. Also, modern personal dive computers (PDCs) provide a very acceptable guideline for your ascent behavior from your dives.

Current open circuit dive tables—the kind used by modern PDCs—inform divers of how much nitrogen they have absorbed (decompression status), how long they can safely remain at their current depth, and how quickly they may dive again. Although printed dive tables are available for divers who want to plan dives in the traditional way and calculate nitrogen loading manually, PDCs do the same thing only more quickly and more accurately, and they are recommended for this diving course. You will be trained to make a safety stop on every dive as a precaution to help reduce the risk of DCS.

The following are a few precautions you can take to stay as safe as possible.

  • Make sure you ascend slowly—9 meters/30 feet per minute or less.
  • Stay well hydrated before and after the dive.
  • Do not exceed your NDLs.
  • Be more conservative with cold or strenuous dives.
  • Do not dive if you are unwell.
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