Thermal stress and decompression for conservation divers

 

Every diver learns the same rule early on: your dive plan comes down to depth and time. Whether you’re running tables or trusting a computer, those two numbers are what decide how much inert gas you’re carrying, and how long you need to off-gas before it’s safe to surface or to dive again.

Except that’s not the whole story. There’s multiple other variables that neither tables nor computers account for, and some of them can move your decompression risk by an order of magnitude: One of these is how warm or cold you are, and, this is the part that trips people up, when in the dive you’re warm or cold.

How gas loading actually works

Nitrogen (or, if you’re mixing gases, helium too) moves into and out of your tissues on the back of your blood flow. When you’re warm, your peripheral blood vessels dilate and circulation to your tissues increases. When you’re cold, they constrict, shunting blood toward your core. More blood flow means faster gas exchange — in both directions.

That has an uncomfortable implication. Being warm during the deep, gas-loading part of a dive means you absorb inert gas faster than you would if you were cool. Being warm during the ascent and stops, when you’re trying to off-gas, helps you eliminate it faster. Flip either one around and you get the opposite effect. So the worst possible combination isn’t being cold, or being warm – it’s being warm while you’re loading and cold while you’re trying to unload. Which, if you think about a typical dive (cranking a heated vest on during the coldest, deepest part, then getting chilly and just wanting to be done by the safety stop), is exactly the pattern a lot of divers fall into by accident.

What the research actually shows

This isn’t a theoretical concern. A Navy Experimental Diving Unit (NEDU) chamber study, widely cited in technical diving circles and summarized by DAN Research Director Dr. Neal Pollock — put divers through identical dive profiles (37m/120ft) and varied only when they were warm versus cold:

• Warm during descent and bottom time, cold during ascent and stops: 22% incidence of DCS at a 30-minute bottom time.
• Cold during descent and bottom time, warm during ascent and stops: 0.1% incidence of DCS — even with bottom time extended to 70 minutes.

That’s not a rounding error. Reversing the order of warm and cold, with the more conservative profile actually made longer, cut the bends rate by roughly two orders of magnitude.

More recent data backs this up. A 2026 DAN analysis of roughly 128,000 logged dives found that divers who reported feeling comfortably warm during a dive had a higher rate of decompression sickness than those who reported feeling cold, the exact opposite of what most divers would guess. Comfort, it turns out, is a poor proxy for safety.

Why your computer can’t help you here

Dive computers measure water temperature. They don’t measure your thermal status, how warm your tissues actually are, which depends on your exposure protection, your heating system if you have one, your body composition, and how hard you’re working. Two divers on the same computer, same profile, same water, can have meaningfully different decompression stress simply because one of them is wearing a heated vest and the other isn’t.

This is worth knowing if you dive with active heating: a heated undergarment that keeps you toasty on the bottom and then dies, or gets switched off, on the ascent is close to the worst-case scenario the NEDU study identified. If you’re going to use one, the safer pattern is to stay slightly on the cool side of comfortable early in the dive and let yourself warm up on the way up.

The hot shower mistake

One more piece of DAN’s guidance worth knowing: don’t rush straight into a hot shower or a hot tub the moment you’re out of the water. If you’re still carrying a meaningful gas load, rapid rewarming can bring inert gas out of solution faster than your circulation can clear it, and the most common result is cutaneous (“skin”) bends. Give yourself time — rinse your gear, log the dive, eat something — before you turn up the heat.

Practical takeaways

• If you use active heating, it’s suggested to not run it at depth, but rather run it hard during your stops.
• Don’t treat “I felt comfortable” as a proxy for “that was a low-stress dive” — the data says the opposite may be true.
• Hold off on the hot shower or hot tub for a while after diving, especially after a demanding profile.
• Your computer is tracking depth and time. It isn’t tracking you.

Why this matters for scientific diving

Most of what’s written about thermal stress and decompression is aimed at cold-water technical divers, which can make it feel irrelevant if you’re diving the Bohol Sea in a 3mm wetsuit. It isn’t. Divers on our scientific survey program are often doing multiple dives a day, repeatedly, for weeks — which means small, repeated thermal effects add up in a way that a single recreational dive never would. Zamboanguita’s reefs also aren’t thermally uniform at depth; thermoclines are common enough that “warm tropical water” isn’t always the full picture below 40 metres, as our technical divers learn.

If this kind of decompression physiology interests you, it’s exactly the territory our technical diving program is built around, and our PADI Enriched Air / Nitrox specialty touches on the same gas-loading principles from a different angle. It’s also just one piece of the broader safety culture we try to build into every course we teach.

If you’re curious about other angles on decompression physiology, we’ve also written about how marine mammals avoid the bends entirely despite diving repeatedly to serious depths.

Further listening

Dr. Neal Pollock — Research Director at Divers Alert Network and Senior Research Associate at Duke University Medical Center’s Center for Hyperbaric Medicine — goes deeper into all of this on Pod Diver Radio, in a conversation that’s technical in places but worth the 22 minutes if any of this piqued your interest. Listen to the full episode here.