A no decompression limit (NDL) is the maximum amount of time you can spend at a given depth and still ascend directly to the surface without needing to make mandatory decompression stops along the way. Go deeper, and that time shrinks. At 60 feet, the U.S. Navy tables give you 60 minutes. At 100 feet, you get just 25 minutes.
How Nitrogen Creates the Limit
The air you breathe underwater is roughly 79% nitrogen. On land, that nitrogen is harmless. Underwater, increasing water pressure forces more of it to dissolve into your blood and tissues. This relationship is governed by a basic physics principle: the amount of gas that dissolves into a liquid is directly proportional to the pressure of that gas above the liquid. Double the pressure, double the dissolved nitrogen.
Every 33 feet of seawater adds one full atmosphere of pressure. At 66 feet, the pressure on your body is three times what it is at the surface, and your tissues are absorbing nitrogen three times faster than normal. Your body can handle a certain amount of this extra dissolved nitrogen. The NDL represents the point at which you’ve absorbed as much as your body can safely release during a normal, controlled ascent to the surface.
When you ascend, the pressure drops and that dissolved nitrogen needs to come back out of your tissues. If you’ve stayed within your NDL, the nitrogen releases slowly and harmlessly, mostly through your lungs as you exhale. If you’ve absorbed too much, the nitrogen can come out of solution too quickly and form bubbles in your blood and tissues, similar to the way carbonation fizzes out of a soda bottle when you crack the cap. Those bubbles are what cause decompression sickness.
Why Deeper Means Shorter
The relationship between depth and NDL isn’t linear. It drops off steeply. At shallow recreational depths like 40 feet, your NDL can stretch well past an hour. At 60 feet, you get about 60 minutes on the U.S. Navy tables. At 100 feet, that window closes to just 25 minutes. By 130 feet, the common recreational depth limit, you may have only 10 minutes or so depending on the table or algorithm you’re using.
This steep curve exists because deeper water means higher pressure, which means faster nitrogen absorption. Your tissues don’t load nitrogen at a steady rate, though. Different types of tissue in your body absorb and release nitrogen at different speeds. Blood and brain tissue saturate quickly, while cartilage, fat, and joints absorb nitrogen much more slowly. Decompression models account for this by tracking multiple hypothetical “tissue compartments,” each with its own absorption rate. Your NDL at any given depth is determined by whichever compartment reaches its tolerance threshold first.
How Dive Tables and Computers Calculate NDL
The original NDL values most recreational divers learn come from the U.S. Navy air decompression tables, developed through experimental testing where the endpoint was the onset of decompression sickness. These tables treat every dive as a square profile: you descend to your maximum depth, stay there for the entire bottom time, then ascend. That’s a conservative assumption, since most recreational dives involve moving between different depths.
Modern dive computers calculate your NDL in real time using algorithms that track your actual depth every few seconds. Because you’re rarely sitting at your maximum depth for the entire dive, a computer typically gives you more bottom time than a printed table would. The computer continuously updates how much nitrogen each tissue compartment has absorbed and displays a countdown of how many minutes you have left before you’d need a decompression stop.
Most dive computers also let you adjust conservatism settings. These settings work by reducing the maximum tolerable nitrogen pressure the algorithm allows in each tissue compartment. Divers who are older, less fit, dehydrated, or who have certain medical conditions (like a small hole between the heart’s chambers called a patent foramen ovale) often set their computers to be more conservative, which shortens the displayed NDL but adds a safety margin. In practice, changing these settings on some computers produces only modest differences in allowed bottom time, so the specific brand and algorithm matter.
Safety Stops vs. Decompression Stops
If you stay within your NDL, you can technically ascend straight to the surface. In practice, nearly every diver training agency recommends a safety stop: pausing at about 15 feet (5 meters) for 3 minutes near the end of every dive. This stop is voluntary, not mandatory. It gives your body extra time to off-gas nitrogen in a low-pressure environment before you hit the surface, adding a buffer against the imprecision of decompression models.
A decompression stop is fundamentally different. If you exceed your NDL, you have taken on more nitrogen than your body can safely release during a direct ascent. You now must stop at specific depths for specific durations on the way up to let nitrogen leave your tissues gradually. Skipping or shortening a required decompression stop carries a significantly higher risk of decompression sickness than skipping a safety stop. This is why recreational dive training emphasizes staying well within NDLs: it keeps you in the “no-stop” zone where a direct ascent remains an option if something goes wrong, like a low-air situation.
What Happens If You Exceed Your NDL
Accidentally going a minute or two past your NDL doesn’t guarantee decompression sickness, but it does mean you’ve entered territory where a direct ascent is no longer the recommended protocol. Most training agencies teach an emergency procedure: ascend slowly, make an extended stop at 15 feet (typically 8 to 15 minutes depending on the table), then avoid diving for at least 24 hours afterward. Your dive computer will typically switch to a decompression mode and display the required stop depth and time.
If nitrogen does form bubbles in your tissues, the symptoms of decompression sickness vary depending on where those bubbles lodge. Milder cases (sometimes called Type I) involve joint pain, skin rashes, or localized swelling. More serious cases (Type II) affect the nervous system and can cause numbness, tingling, difficulty walking, dizziness, or vision changes. In rare but severe cases, bubbles can accumulate in the lungs, causing chest pain, coughing, and difficulty breathing. Symptoms typically appear within hours of surfacing, though they can occasionally be delayed.
Treatment for decompression sickness involves breathing pure oxygen and, in significant cases, spending time in a hyperbaric chamber. The chamber raises the surrounding pressure, which forces the nitrogen bubbles back into solution in the blood, then allows controlled, gradual pressure reduction so the gas can be exhaled normally.
Factors That Shorten Your Real-World NDL
The numbers on a dive table assume a healthy, well-hydrated diver at a standard temperature. Real conditions often reduce your effective margin. Dehydration thickens blood and slows gas exchange, making nitrogen harder to eliminate. Cold water causes blood vessels near your skin to constrict, trapping nitrogen in peripheral tissues. Fatigue, poor cardiovascular fitness, excess body fat (which absorbs and holds nitrogen readily), and older age all reduce your body’s efficiency at off-gassing.
Repetitive diving also chips away at your NDL. Residual nitrogen from your first dive stays in your tissues during the surface interval and carries over into your second dive. Your dive computer or table accounts for this by giving you a shorter NDL on subsequent dives. A depth that gave you 60 minutes on your first dive of the day might give you only 35 minutes on your second dive a few hours later. This is why many divers plan their deepest dive first and go progressively shallower throughout the day, maximizing bottom time while managing nitrogen load.