Decompression is the controlled process of eliminating dissolved inert gas, primarily nitrogen, from body tissues following exposure to increased underwater pressure. The duration needed to safely off-gas this nitrogen depends entirely on the dive profile, meaning the depth, the duration of the dive, and whether it was a single or repetitive exposure. A safe decompression time is a calculated period that minimizes the risk of Decompression Sickness (DCS), commonly known as “the bends.”
The Mechanism of Nitrogen Saturation and Desaturation
The entire process of gas absorption and release in the body is governed by Henry’s Law. This law states that the amount of gas dissolved in a liquid is directly proportional to the partial pressure of that gas above the liquid. When a diver descends, the ambient pressure increases, which raises the partial pressure of the nitrogen in the breathing gas, causing more of it to dissolve into the bloodstream and then into the body’s tissues.
The body is modeled as a collection of theoretical “tissue compartments” that absorb and release nitrogen at different rates. These compartments are defined by their “half-time,” which is the time it takes for a tissue to become half-saturated with nitrogen at a new pressure, or to release half of its excess nitrogen upon pressure reduction.
Tissues with a high blood supply, such as the brain and spinal cord, are considered “fast tissues” and have shorter half-times, meaning they absorb and release nitrogen quickly. Conversely, tissues with lower blood flow, like fat and bone, are called “slow tissues” and have half-times that can extend for hours. The total time required for safe decompression is determined by the slowest tissues that have accumulated a significant amount of nitrogen during the dive. While fast tissues may release their excess gas in minutes, the slower tissues can take many hours to fully return to their pre-dive nitrogen level.
Calculating Surface Intervals for Repetitive Dives
The concept of a Surface Interval (SI) is the time spent on the surface between dives, during which the body begins to eliminate the nitrogen absorbed during the previous dive. This period must be long enough to reduce the amount of “residual nitrogen” remaining in the body before the next descent.
Divers rely on dive computers or specialized dive tables, such as the Recreational Dive Planner, to calculate the minimum safe SI. These tools track the nitrogen load from the previous dive and assign the diver a pressure group, which represents the current level of residual nitrogen. The duration of the surface interval is then used to determine the reduction in this pressure group.
The amount of residual nitrogen directly impacts the allowable bottom time for the subsequent dive, as the diver is effectively starting the second dive with a pre-existing nitrogen load. A longer SI allows the diver to off-gas more nitrogen, thereby increasing the safe no-decompression limit for the repetitive dive. For most recreational dive planning, the surface interval is measured in minutes or a few hours, as the purpose is only to reduce the nitrogen load enough to safely execute the next dive.
The Longest Wait: Guidelines for Flying After Diving
The longest waiting period for a diver occurs before ascending to altitude, such as when flying in a commercial airplane. This is because the cabin of a commercial aircraft is pressurized to an equivalent altitude of approximately 8,000 feet above sea level. This significant reduction in ambient pressure is similar to a rapid, uncontrolled ascent from a dive, which can cause any remaining dissolved nitrogen to form bubbles and lead to altitude-induced Decompression Sickness. To mitigate this risk, the Divers Alert Network (DAN) and other organizations have established minimum waiting periods based on extensive research and experimental data.
Single No-Decompression Dives
For a single, no-decompression dive, the recommended minimum pre-flight surface interval is 12 hours. This guideline applies to dives that remained within the limits requiring no mandatory decompression stops.
Repetitive and Multi-Day Dives
For divers who have completed multiple dives in a single day or have been diving on consecutive days (multi-day repetitive diving), the minimum recommended waiting time before flying is extended to 18 hours. This longer period accounts for the greater cumulative nitrogen load that builds up in the slower tissue compartments over multiple exposures. The most conservative advice suggests a 24-hour wait after multi-day diving.
Decompression Dives
In the case of dives that required mandatory decompression stops, the nitrogen load is considered substantially higher, and the waiting period must be significantly longer than 18 hours, with some recommendations extending to 48 hours. These guidelines are considered minimums for safety, and many modern dive computers will provide a personalized, conservative countdown to the time when it is safest to fly.