Does a hot shower, a heater, exercise or a beer change my off-gassing?

Short version: your dive computer's half-times are fixed numbers. Your blood flow isn't. Warmth, cold, exercise and — less clearly — alcohol all change how fast blood carries nitrogen into your tissues and back out again, and the computer can't see any of it. The pattern that falls out of the research is simple: cool and easy on the way down, warm and gently moving on the way up, and take it easy afterwards. The hard part is being honest about how strong the evidence is for each piece, so every section below says so.

A half-time is a blood-flow number

In What are fast and slow tissues? the half-time came down to a sponge and a hose:

half-time ≈ 0.693 × (how much the tissue holds) ÷ (how much blood flows through it)

Read that equation the other way round. Double the blood flow and you halve the half-time. Halve the flow and you double it. And it works in both directions: a tissue with more blood flow loads faster at depth and unloads faster on the surface. Less flow, slower both ways.

Your body changes that flow all the time:

• Vasodilation — the vessels widen and more blood flows. Warmth does this to skin; exercise does it to working muscle; alcohol does it to skin.
• Vasoconstriction — the vessels narrow and less blood flows. Cold does this: your body shuts down flow to the skin, arms and legs to protect your core.

Your computer's sixteen half-times (see What does my dive computer actually measure?) are set at the factory. They assume one blood flow, all dive long, for everybody.

Warm going down, cold coming up: the heater that fails

Here is the scenario I always walk through in class. You're diving with a heated vest. It's on during the descent and the bottom — you're warm, your skin and limbs are vasodilated, and you're loading nitrogen fast. Near the end of the dive the battery dies. Now you're ascending and doing your safety stop cold and vasoconstricted, so you're unloading slowly. That's the worst combination there is: fast in, slow out.

This isn't just theory. The US Navy Experimental Diving Unit tested it directly, keeping divers warm or cold in controlled water temperatures during the bottom phase and the decompression phase.^1,2 Divers who were warm on the bottom and cold during decompression had a decompression sickness rate of about 22%. Divers who were cold on the bottom and warm during decompression had about 1.3% — even though their bottom time had been more than doubled. Same divers, same kind of dive; the difference was when they were warm.

To see what that means inside the model, take the dive from the table-versus-computer FAQ — 30 minutes at 80 ft (24 m) on EAN32 — and follow compartment 4, the one that set the limit there. Suppose warmth doubles its blood flow (half-time 18.5 → 9.25 min) and cold halves it (18.5 → 37 min). Those are round numbers for illustration; real changes differ from tissue to tissue.

Compartment 4 of Bühlmann ZH-L16C. The ascent is drawn as instant and the safety stop is left out, to keep the picture clean.

Look at the middle row. The computer, assuming normal flow, thinks this compartment is at 1.81 bar — comfortably inside its 2.05 bar limit. The warm diver's real compartment is at 2.15, already past it. Then the heater fails and that compartment drains at half speed: half an hour after surfacing it's still where the neutral diver was ten minutes out of the water. Being warm at depth turned a 47-minute limit into 23.

Don't read the bottom row as a way to buy bottom time. Nobody controls their blood flow that precisely; the point is the direction, not the numbers.

DAN's advice on heated garments follows from this: don't plan a dive around the heater working, keep it low on the way down if you use it, and expect that a failure before the ascent leaves you worse off than if you'd never switched it on.³ The same logic applies without any gadget: a warm tropical bottom followed by a long, chilly safety stop in a thin wetsuit pushes in the same direction, just less.

Evidence: strong for the direction — controlled Navy trials. The heater-failure case itself hasn't been studied directly; it's the Navy's worst condition happening by accident.

The hot shower

After the dive you're cold, and a hot shower is the best thing in the world. Here two effects pull against each other:

1. Gas comes out of solution as tissue warms. Warm tissue holds less dissolved gas. A hot shower heats your skin and the fat under it from the outside, and gas that tissue was holding can come out as bubbles right there.
2. Warming opens the vessels, and more blood flow carries gas away faster.

The trouble is timing. Heat reaches the skin in seconds; the blood flow to clear the gas takes longer to catch up. Rapid warming soon after a dive, while your tissues are still carrying a lot of gas, lets the first effect win.^2,4 DAN has a case report that fits it exactly: an experienced diver, nine dives over four days, takes a hot shower about three hours after the last dive and gets intense itching, a rash, joint pain and blurred vision — skin bends, in the fatty areas.⁵

DAN's advice: delay the hot shower or hot tub as long as you reasonably can — the more gas you're carrying, the longer — and if you can't wait, make it lukewarm.⁴

An honest wrinkle: the Navy found that being warm during decompression was protective. So warmth on the way up isn't the enemy; sudden, hot warming of skin and fat that's still full of gas is.

Evidence: moderate. The physics is solid and there are case reports, but no controlled trial of hot showers after diving.

The cold shower

Some divers reason the other way: if heat is bad, cold must be good. It isn't. Cold constricts the vessels and slows the very off-gassing you want — that's the cold-on-the-way-up half of the Navy's worst condition. No one has studied cold showers after diving specifically, but the direction is clear. Warm up gradually.

Exercise

Muscle is the tissue whose blood flow changes the most: working hard, it can take many times its resting flow. So exercise follows the same rule as heat, with a few twists. DAN has written a lot about this.^6,7

Right before the dive. You'd expect a hard workout just before a dive to be risky, and most guidance says to avoid one: DAN Southern Africa reports a common consensus of four to six hours,⁸ and DAN US says 24.^6,7 But the studies DAN reports mostly point the other way. Divers who ran for 45 minutes one hour before a 30-m (100-ft) sea dive had fewer bubbles afterwards, not more;^9,8 Navy divers who exercised two hours before did too.⁸ Not every study agrees: one found that hard exercise 24 hours before reduced bubbles but the same exercise two hours before didn't.^10,6 DAN's caution is about impact: pounding exercise like running may create tiny gas nuclei that bubbles later grow from.⁷ None of this has been tested on the average recreational diver, so the guidance stays cautious.

At depth. Working hard — finning into a current, chasing a turtle — increases blood flow to the muscles and loads them faster. To your tissues it's a longer or deeper dive than the one your computer is counting. Keep the bottom as easy as possible.⁶

On the way up and at the safety stop. Here gentle movement helps. In one controlled study, light exercise during a three-minute decompression stop reduced bubbles after the dive.¹¹ Light is the word: easy finning, moving your arms and legs, not a workout.⁶

Right after the dive. This is the window that matters most. Your tissues are still carrying extra gas, and hard work can shake it out of solution. In a NASA study DAN cites, subjects decompressed to spacewalk pressure who stayed still had no decompression sickness, while 20% of those who did a few short bouts of relaxed walking did.⁷ That isn't diving, and one study of well-trained Navy divers found that exercise ten minutes after surfacing actually reduced bubbles — yet DAN Southern Africa, reporting it, still advises waiting at least four hours,⁸ and DAN US says to take it easy after diving.⁶ Remember what counts as exercise here: hauling your gear up a boat ladder, swimming back to shore against a current, carrying tanks up the beach.

Evidence: moderate for exercise at depth and at the stop. Right before and right after the dive, the studies and the guidance disagree, and the guidance errs on the cautious side.

Alcohol

Read this section for what it is. It is only about what alcohol does to blood flow and off-gassing. It is not about alertness, judgment, reaction time, narcosis, or any of the other reasons alcohol and diving don't mix — those are real, well established, and a separate discussion (for narcosis, see Will I get narced?). And where this section says the evidence is weak, that means we don't know. It does not mean alcohol is safe before or after a dive.

“Alcohol dilates your vessels, so it changes your off-gassing.” Alcohol mainly opens vessels in the skin. What that reliably does is lose heat, so a diver who drank can end up colder on the way up — and cold is the wrong direction.¹² No study I know of has measured alcohol speeding or slowing off-gassing in divers.

“Alcohol dehydrates you, and dehydration causes the bends.” Plausible, and widely repeated, but the evidence is mostly indirect. And one controlled study points the other way: divers who sat in a sauna an hour before a chamber dive to 30 m were measurably dehydrated, yet had fewer bubbles afterwards, not more.¹³ Dehydration may matter; it just isn't proven to matter the way it's usually told.

The gap is not for lack of drinking divers. In a survey of more than 4,300 French divers, 30% said they had drunk alcohol within six hours before a dive, and the authors note how little research there is on alcohol and diving accidents.¹⁴

Evidence: weak — the effect of alcohol on off-gassing simply hasn't been studied well. Not knowing is not the same as safe.

All of it on one page

What your computer doesn't know

None of this reaches the computer on your wrist. It doesn't know you were warm at depth, cold on the stop, finning into a current, or out late the night before. It runs the same half-times either way. That's one more reason to dive with margin — a conservative setting covers some of the gap between the body the model assumes and the one you actually brought.

For education only. This page explains the physiology behind decompression and the reasoning behind it. It is not for dive or gas planning. Plan every dive with your training, your dive computer and your gauges, and follow your instructor or dive professional.

References

1. Gerth WA, Ruterbusch VL, Long ET. The influence of thermal exposure on diver susceptibility to decompression sickness. NEDU Technical Report TR 06-07. Panama City, FL: Navy Experimental Diving Unit; November 2007. ↩
2. Divers Alert Network. Factors contributing to decompression stress. Chapter 5 in Decompression Sickness, DAN Dive Medical Reference Books. 2020. ↩ab
3. Tillmans F. Heated garments. Alert Diver. May 2022. Divers Alert Network. ↩
4. Pollock NW. Hot tubs after diving. Divers Alert Network health resources. ↩ab
5. McCafferty M. Cutaneous DCS with transient neurological symptoms after a hot shower. DAN case summary. 2020. ↩
6. Pollock NW. Juggling physical exercise and diving. Divers Alert Network. 2020. ↩abcdef
7. Pollock NW. Exercise and decompression risk. Alert Diver. August 2025. Divers Alert Network. ↩abcd
8. Divers Alert Network Southern Africa. Physical exercise before, during & after a dive. DAN SA blog. August 2017. ↩abcd
9. Castagna O, Brisswalter J, Vallee N, Blatteau JE. Endurance exercise immediately before sea diving reduces bubble formation in scuba divers. European Journal of Applied Physiology. 2011;111:1047–1054. ↩
10. Dujić Ž, et al. Aerobic exercise before diving reduces venous gas bubble formation in humans. Journal of Physiology. 2004;555(3):637–642. ↩
11. Dujić Ž, et al. Exercise during a 3-min decompression stop reduces postdive venous gas bubbles. Medicine & Science in Sports & Exercise. 2005;37(8):1319–1323. ↩
12. Marshall M. Diving under the influence. Alert Diver (DAN Europe). September 2015. ↩
13. Blatteau JE, Gempp E, Balestra C, Mets T, Germonpré P. Predive sauna and venous gas bubbles upon decompression from 400 kPa. Aviation, Space, and Environmental Medicine. 2008;79(12):1100–1105. ↩
14. Cuminetti L, D’Andrea C, Bouscaren N. Alcohol and diving: a cross-sectional study of attitudes towards alcohol consumption among 4322 French divers. La Presse Médicale Open. 2022. ↩