Decompression Theory

During scuba diving, the compressed air you breathe contains nitrogen and oxygen. The inhaled nitrogen is dissolved in your tissues under high pressure. If the pressure is too great, bubbles can form in blood and tissue, leading to decompression sickness. Decompression theory tries to determine how long and how deep you can dive without suffering decompression sickness.

Fast and Slow Tissue

• According to Canada's Defense and Civil Institute of Environmental Medicine (DCIEM), not all parts of your body absorb nitrogen at the same rate. Organs that are well supplied by blood, like the lungs and abdomen, absorb and excrete nitrogen faster than other tissues. Tissues that contain fat, marrow and cartilage absorb and excrete nitrogen at a slower rate. Fatty tissues hold more nitrogen than watery tissues, so it takes longer for nitrogen to fill and leave.

Figuring Safety

• Over the years, physiologists have created models and charts for the optimal depth and timing of dives to avoid decompression sickness. These models have been changed over time, as new information about tissues and nitrogen is learned.

M-Values

• Nitrogen pressure in your body is referred to as nitrogen tension. There is a maximum nitrogen tension for tissues, known as an M-value, which indicates the maximum tension before bubbles are thought to form. Faster tissues will have higher M-values. Scientists marry what they know about water pressure at different depths with the M-values of tissues, to come up with rules for ascent in order to avoid decompression sickness.