When air is breathed under pressure, nitrogen diffuses into various tissues of the
body. This nitrogen uptake by the body occurs at different rates for the various
tissues. It continues as long as the partial pressure of the inspired nitrogen in the
circulatory and respiratory systems is higher than the partial pressure of the gas
absorbed in the tissues. Nitrogen absorption increases as the partial pressure of the
inspired nitrogen increases, such as with increased depth. Nitrogen absorption also
increases as the duration of the exposure increases, until tissues become saturated.
As a diver ascends, the process is reversed. The partial pressure of nitrogen in the
tissues comes to exceed that in the circulatory and respiratory systems. During
ascent, the nitrogen diffuses from the tissues to the lungs. The rate of ascent must
be carefully controlled to prevent the nitrogen pressure from exceeding the
ambient pressure by too great of an amount. If the pressure gradient is uncontrolled,
bubbles of nitrogen gas can form in tissues and blood, causing
decompression sickness.
To reduce the possibility of decompression sickness, special decompression tables
and schedules were developed. These schedules take into consideration the
amount of nitrogen absorbed by the body at various depths and times. Other
considerations are the allowable pressure gradients that can exist without excessive
bubble formation and the different gas-elimination rates associated with
various body tissues. Because of its operational simplicity, staged decompression
is used for air decompression. Staged decompression requires decompression
stops in the water at various depths for specific periods of time.
Years of scientific study, calculations, animal and human experimentation, and
extensive field experience all contributed to the decompression tables. While the
tables contain the best information available, the tables tend to be less accurate as
dive depth and time increase. To ensure maximum diver safety, the tables must be
strictly followed. Deviations from established decompression procedures are not permitted except in an emergency and with the guidance and recommendations of
a Diving Medical Officer (DMO) with the Commanding Officer’s approval.
