The underwater environment is vastly different from the above water environment. The body in the underwater environment must be able to adapt to much greater pressure changes and more rapid pressure changes than encountered in performing military aviation, high altitude parachuting and high altitude mountaineering activities.
The underwater environment is not an atmospheric environment and is a hostile physiological environment. Extended underwater activities beyond breath holding require use of some sort of supplied breathing provision.
Diving techniques include free (breath-hold) diving, snorkeling, SCUBA (Self Contained Underwater Breathing Apparatus) diving, surface supplied air and mixed-gas bounce diving, and saturation diving. Free diving is the only underwater activity lacking use of some sort of supplied breathing provision (life support).
Under certain circumstances, any type of diving may result in decompression sickness, barotraumas (tissue damage due to increased pressure), and pulmonary overinflation syndromes (trapping of gases in lungs, with potential for rupture of alveoli leading to arterial gas emboli).
Human performance capacity to physiologically adapt (cope) to the at all times hostile underwater environment is much more than the simple imminent danger of drowning should any mishap of occur with equipment or as a result of inclement diving conditions.
The perplexing physiological adaptation (acclimatization) includes individual tolerance to increased underwater pressure varies significantly as depths increase beyond 33 feet of sea water (2 atmospheres-the combination of atmosphere and water weight at that depth). At the greatest depths of the ocean (approximately 36.000 feet), the pressure is more than seven tons per square inch (1,100 atm).
Physiologically there is a limit for compressed air diving as breathing compressed air at depth approaching 130 feet of sea water (fsw) or 4.5 atm has been implicated in diver performance degradation. Generally the risks of compressed 80% nitrogen and 20% oxygen breathing air mix causing serious life threatening maladies increase as the diver descends deeper.
A malady known as Nitrogen Narcosis can occur at 33 fsw or 2 atm, with increasing effects on the brain as the descent. Oxygen Toxicity becomes a concern in environments in which oxygen is inspired at pressures greater than 2.5 atm and at depths of 300 fsw the nitrogen narcosis at that depth renders all but the most experienced divers incapable of any useful work.
Inspiration of oxygen at increasing depths is a paradox as it develops a nature of rapidly becoming pulmonary and/or central nervous system (CNS) toxic. CNS toxicity is more dangerous than in the pulmonary type because it can cause a sudden loss of consciousness which can quickly progress to coma and death. There is huge variation in the amount of oxygen individuals can tolerate before they show signs of CNS oxygen toxicity and of even more concern, a huge variation in the same person on different days.
The risks of Nitrogen Narcosis and Oxygen toxicity contribute as much to dive depth and duration limits as does other compression and decompression (bends, caisson disease, chokes) sickness concerns. The maximum operational depth limit for compressed air diving is generally considered to be 190 feet because of the risks of nitrogen narcosis and oxygen toxicity beyond this depth. The normal working limit for open circuit SCUBA is 130 fsw (40m) is based on a practical consideration of working time versus decompression time and oxygen-tolerance limits.
Most Air Force diving operations are what is called bounce diving. A bounce dive commonly refers to any non-saturation dive. Often multiple bounce dives of different depths and duration are conducted in a single day, and repeated decompressions are necessary. Both bounce diving and saturation diving are considered to be stressful types of diving.
In recreational diving, a bounce dive is a descent of short duration (usually less than ten minutes) with an almost immediate ascent to keep the time needed for decompression to a minimum.
As a diver descends, increasing pressure causes increasing amounts of gases in the lungs to diffuse into the body. Increased oxygen can be partially metabolized by the body, but the nitrogen (or other inert gas) that is absorbed remains in the body until removed by pulmonary gas exchange during ascent.
When a diver ascends and the pressure decreases (decompression), the gas that was absorbed on descent diffuses back out of the tissues and is exhaled. However, if the ascent is too rapid, gas bubbles form in tissues and blood vessels, and may result in mechanical blockage of blood vessels to vital organs, compression of nerves, pressure pain within joints, and disruption of other tissues, resulting in symptoms of decompression sickness (DCS). DCS is treated by recompression in a hyperbaric (high pressure) chamber, which reduces the size of bubbles and creates a favorable diffusion gradient that allows a slow, controlled release of the gas from body tissues without allowing the reformation of bubbles.
Deeper dives have a greater risk of DCS. What is considered to be a deep dive varies with the type of equipment used, the gas used, and other factors. While 100 feet would be considered a deep dive in recreational diving, a deep dive in commercial and military diving is often 300 feet or more. A diver may need to stop and wait for a period of time several times during the ascent to allow gas diffusion out of the tissues to take place slowly, in order to decrease the risk of DCS. Oxygen may be used during decompression to facilitate the elimination of inert gas.
Flying shortly after diving increases the risk of DCS because the ascent to altitude (with decreased pressure) allows residual nitrogen in body tissues to diffuse out as gas bubbles, as on ascent from the dive.
In summary there are short- and long-term medical effects of diving causing the need for all diver candidates to must undergo a giving physical exam and be able to demonstrate ability to adapt to adapt to pressure of 112 feet (50 psig or about 3.39 atmospheres absolute) and tolerate a decent rate of 75 fpm, an accent of 60 fpm, and tolerate breathing 100% oxygen at 60 feet for 30 minutes.