Diving at any elevation above 300 metres (1,000 feet) requires adjusted procedures because the lower atmospheric pressure at the surface changes how your body absorbs and releases nitrogen. Standard dive tables and no-decompression limits are built for sea level. Use them unchanged at altitude, and you face a meaningfully higher risk of decompression sickness, even on dives that would be perfectly safe at the coast.
Why Altitude Changes the Risk
At sea level, atmospheric pressure pushes down on the water’s surface at roughly 1 atmosphere. As you ascend in elevation, that surface pressure drops. A lake at 2,000 metres sits under noticeably less atmospheric pressure than the ocean. That matters because decompression sickness is driven by the ratio between the pressure of dissolved nitrogen in your tissues and the pressure of the environment you surface into. A smaller number on top of the fraction means a bigger pressure difference when you come up, and that difference is what causes nitrogen to form bubbles in your blood and tissues.
Think of it this way: if you open a carbonated drink at sea level, it fizzes gently. Open the same drink on a mountaintop, where ambient pressure is lower, and it fizzes more aggressively. Your body works on the same principle. During a dive, you absorb extra nitrogen under pressure. When you return to a surface that already has reduced atmospheric pressure, the dissolved nitrogen in your tissues is proportionally more supersaturated than it would be after the same dive at sea level. The result is a greater tendency for bubbles to form.
Acclimatization Before Your First Dive
Your body arrives at altitude already fully saturated with nitrogen at sea-level pressure. That means you’re carrying more dissolved nitrogen than the new, lower surface pressure can comfortably hold. You need time for that excess to off-gas naturally through breathing before you add more nitrogen by going underwater.
The standard recommendation is a minimum of 12 hours at the new elevation before your first dive. At extreme elevations above 3,000 metres (10,000 feet), a full three-day acclimatization period is recommended, with daily increases in sleeping elevation limited to no more than 500 metres and a rest day built in every three to four days. Skipping this waiting period means you start your dive with a nitrogen load that your dive plan doesn’t account for.
Converting Depth for Sea-Level Tables
If you’re using standard sea-level dive tables rather than an altitude-compensating computer, you need to convert your actual depth to an equivalent sea-level depth. The concept is straightforward: because each metre of water at altitude represents a larger fraction of the reduced surface pressure, a dive to 20 metres in a mountain lake stresses your body more than a 20-metre ocean dive. You must plan as though you’re diving deeper than you actually are.
The conversion uses what’s sometimes called the “theoretical ocean depth” or standardized equivalent sea depth. It accounts for two factors: the ratio of surface pressure at altitude versus sea level, and the density difference between fresh lake water and seawater (fresh water is slightly less dense, at roughly 1,000 kg/m³ versus 1,033 kg/m³ for seawater). The practical effect is that a 20-metre lake dive at 2,000 metres of elevation might need to be planned as a 25- or 26-metre sea-level dive, depending on the exact formula used. Many training agencies publish altitude correction tables so you don’t have to do the math yourself.
How Dive Computers Handle Altitude
Modern dive computers measure absolute pressure and subtract the surface atmospheric pressure to calculate your depth. They can’t measure surface pressure while you’re underwater, so they sample it before you enter the water and save that value. If the stored surface pressure is wrong, every depth reading during your dive will be wrong too.
A review of dive computers on the market found that all current models automatically compensate depth readings for altitude. That’s reassuring, but the method each manufacturer uses to determine surface pressure varies. Some sample pressure the moment you turn the unit on. Others activate automatically. The risk comes when a computer defaults to a standard sea-level pressure value, or when it samples pressure while slightly submerged (say, sitting in shallow water at the lake’s edge). Using a surface pressure that’s too high makes the computer think you’re shallower than you are, which can lead you to exceed your planned depth and stay longer than your no-decompression limit actually allows.
To avoid this, turn your computer on at the surface, well away from the water, before you kit up. Confirm it’s reading correctly and shows adequate battery life. This gives it a clean atmospheric reading at your actual elevation.
Capillary Gauges: A Built-In Advantage
One piece of older technology actually works better at altitude than more modern alternatives. Capillary depth gauges, which measure depth by compressing a trapped air column, automatically compensate for altitude. Because the air column inside starts at ambient atmospheric pressure rather than a calibrated sea-level value, the gauge’s readings naturally reflect the correct pressure ratios for that elevation. If you use the gauge’s depth readings directly in a standard decompression table, your ascent rate and safety stop positions will produce the correct tissue overpressure values without any manual correction.
Dial-type bourdon tube gauges and digital gauges that default to sea-level calibration need to be corrected for both altitude and water density before you can use their readings with standard tables. This double correction is easy to get wrong, which is one reason altitude-aware dive computers have largely replaced manual calculations for recreational divers.
Ascent and Post-Dive Precautions
The lower surface pressure at altitude means the final phase of your ascent, where the pressure differential is greatest, carries more risk than the same phase at sea level. Conservative ascent rates and safety stops become more important, not less. Many divers and training agencies treat the standard 9-metre-per-minute ascent rate as a maximum and aim slower, particularly in the last 10 metres.
What catches many altitude divers off guard is what happens after the dive. Driving over a mountain pass to get home can take you to a significantly higher elevation than where you dived. That further drop in atmospheric pressure acts like an additional, unplanned ascent. You’re essentially reducing the ambient pressure around your body while you still have residual nitrogen in your tissues. The same 12-hour minimum recommended before diving at a new altitude applies in reverse: you should allow adequate surface interval time before traveling to any elevation substantially higher than your dive site. The exact wait depends on your dive profile and the elevation gain, but the principle is simple. Any post-dive increase in altitude is an additional decompression stress.
Decompression Sickness at Altitude
Decompression sickness from altitude exposure has a few characteristics worth knowing. Symptoms typically appear some time after arriving at (or returning to) a lower-pressure environment, not instantly. Bubble formation requires time: even though bubbles are frequently detected in the body at altitudes as low as 18,000 feet, symptoms rarely appear below that threshold because the bubbles haven’t grown large enough. And symptoms rarely appear after four hours at altitude, suggesting that if your body is going to react, it will do so within that window.
For divers, this means the first few hours after surfacing are the critical period, especially if you’re gaining elevation. Joint pain, unusual fatigue, skin tingling or mottling, dizziness, and neurological symptoms like confusion or visual changes are all signs of decompression sickness that warrant immediate attention. Altitude diving locations are often remote, making evacuation slower. Carrying emergency oxygen and knowing the location of the nearest recompression chamber before you dive is more than a formality in these settings.