Dysbarism refers to medical conditions resulting from changes in ambient pressure. Various activities are associated with pressure changes. Underwater diving is the most frequently cited example, but pressure changes also affect people who work in other pressurized environments (for example, caisson workers), and people who move between different altitudes. Ambient pressure
Ambient pressure is the pressure in the water around the diver (or the air, with caisson workers etc.). As a diver descends, the ambient pressure increases. At 10 meters (33 feet) in salt water, it is twice the normal pressure than that at the surface. At 40 meters (a common recommended limit for recreational diving) it is 5 times the pressure than at sea level. Pressure decreases as we rise above sea level, but less dramatically. At 3000 feet altitude (almost 1000 meters), the ambient pressure is almost 90% of sea level pressure. Ambient pressure does not drop to 50% of sea level pressure until 20,000 feet or 6,000 meters altitude. Effects of pressure on the body
Direct effects on tissues
This is not of practical importance, because the body is mostly composed of barely compressible materials such as water. People often wonder whether scuba divers feel their body being crushed by the pressure, but divers would have to reach depths of thousands of feet before their flesh began to suffer significant compression. Gas filled spaces
Gas is very compressible. Humans have many air spaces: sinuses, middle ears, gas in our bowels, cavities in our teeth, and largest of all, our lungs.
It is often impossible to distinguish AGE from DCS, but luckily it is rarely necessary for physicians to be able to distinguish between the two, as treatment is the same. Sometimes AGE and DCS are lumped into a single entity, Decompression Illness (DCI). Classification
In addition to the foregoing, dysbarism is sometimes classified according to the source of the excess gas, with "trapped gas" dysbarism referring to the expansion of pockets that were already in a gaseous state in the body, and "evolved gas" dysbarism referring to gasses (primarily nitrogen or helium) dissolved in the body coming out of solution to form gas bubbles. See also
List of diving hazards and precautions – Hazards associated with underwater diving
Dysbaric osteonecrosis – Ischemic bone disease caused by decompression bubbles

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