When you dive underwater, the increased pressure forces extra nitrogen from your breathing gas into your blood and tissues. Decompression is the process of ascending slowly enough that this dissolved nitrogen can safely exit your body through your lungs. Rise too fast, and that nitrogen forms bubbles inside you, much like opening a shaken soda bottle. Those bubbles can block blood flow, damage tissue, and cause a painful and sometimes dangerous condition called decompression sickness.
How Pressure Changes Your Body Underwater
Air is roughly 78% nitrogen. On land, your body maintains a steady balance of dissolved nitrogen in your blood and tissues. But water is heavy, and for every 10 meters (33 feet) you descend, the pressure on your body doubles. At 30 meters, you’re under four times the surface pressure.
That rising pressure does something important: it forces more gas molecules into solution in your blood. This follows a basic principle of physics. The higher the pressure surrounding a gas, the more of that gas dissolves into a liquid. Your lungs act as the exchange point. With each breath at depth, the elevated pressure in your lungs pushes extra nitrogen across into your bloodstream, which then carries it throughout your body. Your muscles, fat, joints, and nervous system all absorb this excess nitrogen the longer you stay down and the deeper you go.
This absorption is completely painless and happens without you noticing. The problem only starts when you head back to the surface.
What Happens If You Ascend Too Quickly
As you rise, the water pressure around you drops. The nitrogen dissolved in your tissues is now at a higher concentration than the surrounding pressure can hold in solution, so it starts coming out of your blood and tissues. If you ascend gradually, this nitrogen travels back to your lungs and you simply breathe it out. But if you ascend too fast, the pressure drops so steeply that the nitrogen can’t reach your lungs in time. Instead, it forms gas bubbles right where it is: inside your blood vessels, your joints, your spinal cord, or your brain.
These bubbles physically obstruct blood flow and trigger inflammation. Small bubbles in your joints cause deep, aching pain, which is why decompression sickness is commonly called “the bends.” Bubbles in your spinal cord can cause numbness, weakness, or even paralysis. Bubbles that reach the lungs in large numbers can cause chest pain and difficulty breathing. In severe cases, bubbles in the brain can cause confusion, vision changes, or stroke-like symptoms.
No-Decompression Limits: How Long You Can Stay
Recreational divers follow time limits that allow them to ascend directly to the surface (at a controlled rate) without needing to make extended decompression stops. These are called no-decompression limits, and they get shorter as you go deeper. NOAA’s dive tables illustrate this clearly:
- 12 meters (40 feet): 180 minutes
- 18 meters (60 feet): 121 minutes
- 24 meters (80 feet): 95 minutes
- 30 meters (100 feet): 73 minutes
- 40 meters (130 feet): 44 minutes
Stay within these limits and ascend at a safe rate (typically 9 meters or 30 feet per minute), and most of the excess nitrogen leaves your body during the ascent and shortly after surfacing. Exceed them, and you must make decompression stops at specific depths on the way up, pausing long enough for nitrogen to off-gas safely. Technical divers who go deeper or stay longer plan these stops in advance, sometimes spending more time decompressing than they spent at depth.
Even within no-decompression limits, most divers make a safety stop at about 5 meters (15 feet) for three to five minutes. This gives your body extra time to release nitrogen and adds a margin of safety.
How Dive Computers Track Nitrogen
Your body doesn’t absorb nitrogen uniformly. Fat tissue soaks it up slowly but holds onto it for a long time. Blood-rich organs absorb it quickly but release it quickly too. Modern dive computers account for this by modeling your body as a series of theoretical tissue compartments, each with a different absorption and release rate. Common algorithms like ZHL-16C model 16 separate tissue types, while others use 9 or 20.
The computer continuously reads your depth and time, calculates how much nitrogen each tissue compartment has absorbed, and tells you in real time how many minutes you have left before decompression stops become mandatory. When you ascend, it recalculates as the pressure drops. This is far more precise than printed dive tables because it tracks your actual depth profile rather than assuming you spent the entire dive at your maximum depth.
Factors That Raise Your Risk
Two divers can make the exact same dive and have very different outcomes. Several physiological factors influence how efficiently your body handles dissolved nitrogen.
Dehydration is one of the most significant and controllable risk factors. Research using controlled animal models found that dehydration significantly increased both the incidence and severity of decompression sickness, and dehydrated subjects developed symptoms sooner. When you’re dehydrated, your blood is thicker and circulates less efficiently, which slows nitrogen removal from your tissues. This is why divers are encouraged to drink plenty of water before and between dives.
Fatigue, poor physical fitness, excess body fat, cold water exposure, and older age also appear to increase susceptibility. Fat tissue absorbs more nitrogen and releases it more slowly, so higher body fat percentages mean a larger nitrogen load after a dive. Cold water constricts blood vessels in your extremities, reducing circulation and trapping nitrogen in those tissues longer. Doing multiple dives in a single day also compounds your risk because residual nitrogen from earlier dives hasn’t fully cleared before you go back down.
How Decompression Sickness Is Treated
The primary treatment for decompression sickness is hyperbaric oxygen therapy, which involves entering a pressurized chamber. The increased pressure inside the chamber physically shrinks the gas bubbles in your body, following the same physics that caused the problem in reverse. Breathing pure oxygen at this elevated pressure also floods your tissues with oxygen while accelerating the removal of nitrogen. Most patients start to feel relief during the first treatment session, though severe cases may require multiple sessions over several days.
The sooner treatment begins, the better the outcome. Mild joint pain that is treated promptly tends to resolve completely. Neurological symptoms like numbness or weakness have a much better prognosis when treated within hours rather than days.
Why You Should Wait Before Flying
The decompression process doesn’t end when you step out of the water. Even after a safe ascent, your body still carries excess nitrogen that continues to off-gas over hours. Boarding a plane too soon is essentially doing another ascent, because aircraft cabins are pressurized to the equivalent of roughly 1,800 to 2,400 meters (6,000 to 8,000 feet) of altitude. That pressure drop can push residual nitrogen out of solution and form bubbles.
Consensus guidelines from the Divers Alert Network and the Undersea and Hyperbaric Medical Society recommend waiting at least 12 hours after a single no-decompression dive before flying, and at least 18 hours after multiple dives or multiple days of diving. The Divers Alert Network and the U.S. Air Force recommend a full 24-hour wait as an extra precaution. Research has shown that some divers still develop detectable bubbles during flights even after a 24-hour surface interval, so erring on the longer side is the safer choice.