The most significant effect of breathing air under pressure is the increased effect of nitrogen absorbtion by the body. During every dive, the increased pressure causes nitrogen from your breathing air to dissolve into your body tissues. The amount your body absorbs on a given dive depends on how deep and how long you your dive is, but the deeper you dive and the longer you stay, the more excess nitrogen your body absorbs.

When you ascend, decreasing the surrounding pressure, the nitrogen you absorbed begins to leave your body. This excess nitrogen must be slowly eliminated through respiration, because, unlike oxygen, your body doesn't use nitrogen. What goes in must come out.

As long as the amount of excess nitrogen is kept within reasonable limits, your body can eliminate it without complication. Special dives have been developed that establish the limits necessary to keep nitrogen within aceptable bounds.

If, however, you should remain under water too long, the excess nitrogen will begin to form bubbles in your blood vessels and tissues when you ascend. These bubbles cause a very serious medical condition called decompression sickness (DCS), sometimes referred to as the bends.

Besides the time and depth of your dives, there are other factors that influence how your body absorbss and eliminates excess nitrogen. These include fatigue, dehydration, vigorous exercise (before, during, or after the dive), cold, older age, illness, injuries, alcohol consumption before or after a dive and being overweight. Also an increase in altitude after diving (by flying or driving through the mountains) can also contribute to getting decompression sickness. Recreational diver should dive well within established limits and use extra caution if any other influencing factors are involved.

Because bubbles can form inmany different places in the body, the symptoms of decompression sickness can vary. Symptoms include: paralysis, shock, weakness, dizziness, numbness, tingling, difficulty breathing, and varying degrees of joint and limb pain. In the most severe cases. unconsciousness and death can result.

In recreational diving, decompression sickness may have symptoms that are subtle. These can include a mild to moderate dull ache, usually but not necessarily in the joints. DCS can produce a mild to moderate tingling or numbness, usually but not necessarily in the limbs. Other common symptoms of DCS are weakness and prolonged fatigue. Decompression sickness symptoms can occur together or individually, and can occur anywhere in the body, and may be accompanied by a feeling of lightheadedness.

Symptoms of decompression sickness usually occur anywhere from 15 minutes to 12 hours after a dive, though they can occur later. Most symptoms come on gradually and persist, though they can be intermittent. Regardless of the severity of the symptoms, all cases of decompression sickness are considered to be serious. If a diver suspects he has symptoms of decompression sickness, or isn't sure, he should 1)discontinue diving, and 2)immediately seek medical attention and consult a diving physician. Some areas have special services available especially for diver emergencies.

This information was obtained from the Padi Open Water Diver Manual