Tools · Heat & off-gassing

Heat, blood flow, and the gas you carry

When it comes to the surface and the gas still dissolved in you, common sense tends to follow the science. The exact numbers here are still being researched and argued, and the place to get the current specifics is DAN — but the broad picture is settled enough to be worth carrying in your head. So treat what follows as something to think about, not a rule to dive by: temperature, and what it does; blood flow, and what it does; and the what-ifs.

The soda tool showed one lever on dissolved gas: drop the pressure and it fizzes out. Temperature is the other lever. Warm a liquid and it holds less gas — warm soda goes flat — and after a dive your tissues are still a loaded soda. Warm them and their capacity falls; whatever they can no longer hold has to go somewhere.

Three outcomes, and they're the whole story. Cold tightens the vessels — vasoconstriction — and blood flow falls; the tissue can still hold its gas easily, but with the blood barely moving, the gas leaves only slowly. That's the cold diver on ascent, and the heater that quit. Gentle warmth opens the vessels — vasodilation — and because it comes on slowly, the blood carries the surplus away about as fast as the capacity falls; done this way, warming actually helps you off-gas. That's the gentle rewarm on the way up. Sudden warmth is the trap: the capacity plunges below what the tissue is carrying before the blood can catch up, and the surplus comes out as bubbles in the tissue itself. That's the hot shower, the hot tub — and skin bends begin right there.

The same logic runs through the dive itself, which is where it gets counterintuitive. Being warm at depth opens your vessels and lets you take on more nitrogen; being cold on the way up closes them and lets you give off less. So the worst possible thermal profile is warm-on-the-bottom, cold-on-the-ascent — and a Navy study found exactly that combination produced dramatically more decompression sickness than its reverse, even on far shorter dives. The takeaway the researchers drew is the one worth keeping: stay cool while you're loading, warm gently while you're unloading.

Which is what makes an electric heater in a drysuit a quiet trap, and you've probably thought this through if you dive dry. Run it warm on the bottom and it does two things at once — loads you up faster, and sets you up to fail into the worst case, because a heater's failure is always toward cold and tends to come late, when the battery fades, which is to say on the ascent. Bank on warmth at depth and a failure drops you straight into warm-bottom, cold-ascent. Run cool on the bottom and add heat only coming up, and even a failure just leaves you cool throughout — uncomfortable, but roughly what the tables were built around. Same part, same failure, completely different consequence, decided entirely by when you chose to depend on it. Warm coming up.

And the what-ifs, the ones that stack up when you're travelling. The hot shower on the dive boat and the resort hot tub are both sudden warmth on a loaded body — delay them; stow your gear, eat, log the dive, then warm up slowly. Alcohol is the compound case: it dilates the vessels, it dehydrates you (and good hydration is one of the few things shown to help off-gassing), and it blurs the line between a hangover and the first signs of a hit. Put the evening together — hot shower, hot tub, a few drinks, an early flight the next morning into a low-pressure cabin — and you've layered warming, dehydration, vasodilation, and altitude onto a body that may have been doing repetitive dives all week. None of these is a verdict; they're nudges, all in the same direction, and worth being awake to. It's the picture the safety people draw as slices of Swiss cheese: every defense has holes, and a hit comes not when one slice fails but when the holes in enough of them line up and the hazard passes clean through. No single hole is the problem. Lining them all up is.