Thermal Stress in Diving

How temperature changes a dive

Water carries heat away from you about 25 times faster than air. Below your core temperature, every minute underwater is bleeding off heat, and your body fights back in ways that directly change how you take up and shed inert gas (on-gassing and off-gassing: the nitrogen going into your tissues at depth and coming back out on the way up). Thermal management is a decompression tool, not a comfort preference. This matters most on longer, deeper, and decompression dives.

The vasoconstriction mechanism

When your skin and the tissue underneath get cold, the body clamps down the blood vessels feeding the surface (cutaneous vasoconstriction) to hold heat in. The decompression catch is direct: less blood flow (perfusion) to those tissues means less inert gas moving through them. Nitrogen that dissolved into your arms, legs, and skin during the dive cannot be carried back to the lungs efficiently if those tissues are cold and starved of blood. In model terms, their half-times (how fast a tissue loads and unloads gas) effectively get longer, not because the physics changed but because the blood flow that drives gas exchange slowed.

Your computer doesn't know this. It calculates your decompression assuming normal blood flow everywhere, so cold, clamped-down tissues are off-gassing more slowly than it thinks, and you surface with more residual nitrogen than the algorithm accounts for.

The risk doesn't end at the surface. When you warm up afterwards (on deck, in the sun, in a hot shower), that vasoconstriction reverses fast, restoring blood flow to tissues still loaded with nitrogen. That sudden flush can drive gas out of solution as bubbles. It is the mechanism behind post-dive "hot shower DCS."

When timing matters most: the warm-cold trap

It is not just whether you are cold, but when. The US Navy Experimental Diving Unit tested this directly (Gerth et al. 2007), and the result is the single most important thing to take from this article.

The worst combination is being warm on the bottom and cold during decompression. Warm, well-perfused tissue on the bottom soaks up more gas than the model assumes. Then, cold on the ascent, those same tissues clamp down and off-gas more slowly than the computer expects, exactly when elimination needs to be working hardest. You finish your stops still carrying more gas than the algorithm thinks, especially in the joints and skin, the classic sites of Type I DCS (the milder, painful kind).

The protective combination is the reverse: cool on the bottom, warm on the ascent. Staying cool while you load limits how much gas goes in; warming during the stops restores blood flow exactly when you want gas coming out.

Your dive computer doesn't track thermal state

Most computers show water temperature as an information field. None measure your skin or tissue temperature, so none can tell that your peripheral blood flow has shut down, or that your wetsuit has compressed to thin, inadequate insulation at depth. The computer keeps calculating as if you were warm and normally perfused the whole dive. Closing that gap is on you, through planning and gear.

Equipment and thermal protection

Wetsuits compress under pressure. A 5 mm wetsuit loses much of its thickness, and with it much of its insulation, by 20 m (msw, metres of sea water), and that loss does not come back during the dive. So you begin the ascent, the phase when off-gassing must run well, colder than you started. Fit matters: cold water flushing through a loose suit accelerates heat loss, and "still comfortable" is not the same as "warm enough to decompress well."

Drysuits hold their insulating gas layer because you add gas as you descend, so they don't lose insulation to compression the way a wetsuit does. A drysuit with a proper undergarment keeps core and limb temperature far better than a wetsuit of similar rating in cold water. The trade-offs: flooding (a sudden loss of insulation and buoyancy), overheating in warm water, and the need for training to manage suit buoyancy on ascent.

Heated systems (vest heaters, heated undergarments, commercial hot-water suits) are all about timing. Heating on the bottom is generally a bad idea: it raises blood flow during the loading phase and drives more gas into your tissues, the opposite of what you want. Heating during the decompression stops is the right move: it restores blood flow during off-gassing, right when the algorithm assumes elimination is happening. Studies of peripheral temperature and bubble formation (Mekjavic & Kakitsuba 1989), alongside the NEDU thermal work, support warming the body during the decompression phase to lower venous gas bubble counts (measured by Doppler ultrasound).

Heat illness in diving

Thermal stress is not only about cold. In the tropics, the mix of hauling gear in neoprene that traps sweat, plus sun on deck, can raise your core temperature fast. Unlike cold, heat doesn't announce itself with shivering; it builds quietly and dulls the mind at a temperature rise that feels like mild effort.

Moderate heat stress overlaps with nitrogen narcosis: poorer short-term memory, slower reactions, worse risk judgement, less drive to act on warnings. A diver who enters the water already overheated is impaired before nitrogen does anything, and the two effects add up. In busy tropical operations this is a real, practical problem, not a theoretical one. Manage it: get in the shade between gearing up and entry, keep your suit open until the last minute, and hydrate. Feeling warm before you splash is a red flag.

Cognitive and physical effects of cold

Cold cuts manual dexterity (regulator, inflator, valves, navigation), slows reactions, and shrinks attention and working memory. Even mild hypothermia (core 35 to 36°C) can impair the judgement behind gas management, turn-around timing, and ascent pace, without the diver realising their judgement is off. That alone is reason to treat thermal protection as safety kit, not a comfort upgrade.

What to actually do

• Plan as if you will be colder than you expect, and dive a conservative profile when you know you will be cold.
• If you use active heating, keep it low or off on the bottom and turn it up for the ascent and stops.
• Carry a plan that survives a dead heater battery: a vest that quits mid-deco recreates the dangerous warm-bottom, cold-deco pattern.
• Rewarm gradually after a dive, and skip the hot shower straight out of the water.
• In the tropics, treat overheating as real: shade up, open your suit until entry, hydrate.
• Call the dive if you are shivering, losing dexterity, or your judgement feels off. Cold degrades decisions before you notice.

Related reading

• Decompression Sickness: What It Is and Why It Happens

• Predisposing Factors for DCS

• Immune Suppression and Diving

References

• Gerth WA, Ruterbusch VL, Long ET. The Influence of Thermal Exposure on Diver Susceptibility to Decompression Sickness. NEDU Report TR 06-07. Navy Experimental Diving Unit, 2007.
• Gaustad SE, Kondratiev TV, Eftedal I, et al. Effects of cold decompression on hemodynamic function and decompression sickness risk in a dry diving rat model. Frontiers in Physiology 2021;12:763975. doi:10.3389/fphys.2021.763975
• Pollock NW. Re: Don't dive cold when you don't have to. Diving and Hyperbaric Medicine 2015;45(3):209.
• Mekjavic IB, Kakitsuba N. Effect of peripheral temperature on the formation of venous gas bubbles. Undersea and Hyperbaric Medicine 1989;16(5):391–401.
• Madden D, Lozo M, Dujic Z, Ljubkovic M. Exercise after SCUBA diving increases the incidence of arterial gas embolism. Journal of Applied Physiology 2013;115(5):716–722.
• Buzzacott P, Pollock NW, Rosenberg M. Exercise intensity of recreational SCUBA diving. Diving and Hyperbaric Medicine 2014;44(2):80–86.
• Pollock NW. Thermal protection and decompression. Alert Diver, DAN, 2008.

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