Henry’s law in diving: nitrogen in, nitrogen out

You surface feeling fine, yet hours later your computer still won’t clear you to fly. It’s tracking nitrogen you took on at depth and haven’t finished breathing off. Henry’s law explains how that nitrogen gets into your tissues, and why the speed of your ascent matters.

What is Henry’s law?

Henry’s law states that, at a given temperature, the amount of a gas that dissolves in a liquid is proportional to the partial pressure of that gas in contact with it. HyperPhysics gives a simple picture: double the partial pressure and twice as many molecules hit the liquid’s surface in the same time, so twice as many dissolve. Each gas has its own solubility.

For divers, the liquid is your blood and tissues, and the gas that matters most is nitrogen. As DAN puts it, the greater the gas pressure in your lungs, the more gas dissolves into your blood and tissues.

Try a question from the app

Henry’s law

Which statement is Henry’s law?

1. AThe total pressure of a mix is the sum of its partial pressures
2. BGas volume varies inversely with absolute pressure
3. CThe amount of gas dissolved follows its partial pressure
4. DA gas dissolves faster as its partial pressure falls

Show the answer

Answer C. Henry’s law links dissolved gas to partial pressure. A is Dalton’s law and B is Boyle’s law; a falling partial pressure makes gas leave the liquid, not dissolve.

What does Henry’s law mean for a diver?

Henry’s law means you load nitrogen at depth and release it as you ascend. The ppN2 you breathe comes from Dalton’s law: 0.79 × absolute pressure on air. A tissue can be in three states:

• Undersaturated: its nitrogen tension is below the ppN2 you breathe, so nitrogen enters. That’s the descent and bottom time.
• Saturated: tension and ppN2 are equal, with no net exchange.
• Supersaturated: its tension is higher, so nitrogen leaves. That’s the ascent and the hours after the dive.

Some supersaturation is normal: it’s how you offgas. Too much, too fast, and nitrogen comes out of solution as bubbles, which can cause decompression sickness.

Why does nitrogen take time to dissolve?

Nitrogen takes time because the transfer follows a gradient: the difference between the ppN2 in your lungs and the nitrogen tension in each tissue. The bigger the gradient, the faster the flow, so a tissue loads quickly at first, then more and more slowly. Well-perfused tissues load and unload fast. Fat, poorly supplied with blood, is slow, yet nitrogen is about five times more soluble in fat than in water.

Decompression models describe this with theoretical compartments, each with a half-time: the time to cover half the remaining difference. After one half-time, 50%; after two, 75%; after three, 87.5%; after six, about 98%, which models treat as saturated. Offgassing follows the same curve.

Why does a slow ascent let nitrogen out safely?

A slow ascent keeps supersaturation moderate, so nitrogen leaves dissolved instead of forming bubbles. A bottle of soda behaves the same way: open it fast and it foams, open it slowly and the gas escapes quietly. On the way up, your blood carries dissolved nitrogen to your lungs, and you breathe it out. That’s why divers ascend slowly and add a 3-minute safety stop at 5 m (15 ft).

Try a question from the app

slow ascent and bubbles

Why does a slow ascent with a safety stop reduce bubble formation?

1. AIt stops nitrogen from entering the tissues
2. BNitrogen has time to leave dissolved, via the lungs
3. CIt keeps the ppO2 high enough to dissolve any bubbles that form
4. DIt lets the slow tissues saturate before surfacing

Show the answer

Answer B. Like a bottle opened slowly, tissues release gas gradually when the pressure drops gradually. The blood carries dissolved nitrogen to the lungs, where you breathe it out.

What are the common Henry’s law mistakes?

• Using gauge pressure. At 20 m (66 ft), a saturated tissue holds three times the surface nitrogen, not twice.
• Thinking tissues fill instantly. Loading slows as the gradient shrinks; slow compartments take hours.
• Calling supersaturation DCS. Supersaturation is how you offgas on every ascent. Bubbles and DCS come when it’s too high.
• Trusting the limit as a guarantee. DCS can happen well within computer limits. Keep a margin: it’s the first of the dive theory myths.

Which dive course covers Henry’s law?

In Octo, the Open Water Diver level teaches the idea as “nitrogen in, nitrogen out”, with the soda bottle and the safety stop. The Divemaster level covers Henry’s law itself, then decompression theory: compartments, half-times and why dive computers give different limits. The half-time arithmetic is in the dive theory formulas cheat sheet, and you can check yourself with the Divemaster practice questions.

Frequently asked questions

Is Henry’s law the cause of decompression sickness?

Henry’s law explains the nitrogen load behind it. DAN describes decompression sickness as linked to inert gas absorbed under pressure, then an ascent during which the gas may form bubbles. Bubbles don’t always cause symptoms: ultrasound has detected “silent” bubbles in divers who stayed within the limits and felt fine.

How long does it take my body to saturate with nitrogen?

It depends on the tissue. A model compartment counts as saturated after about six half-times. Haldane’s early model used half-times of 5 to 75 minutes; many modern models use 16 compartments from 4 to 635 minutes. Fast compartments saturate within a dive, while the slowest would take days.

Why do I still have nitrogen in my body after the dive?

Because slow tissues release nitrogen as slowly as they took it up, so your body holds extra nitrogen for hours. Your computer keeps tracking it between dives and gives you shorter limits on the next one. Wear the same computer for every dive of the day, and leave a reasonable surface interval.

What does Henry’s law have to do with flying after diving?

Airliner cabins are kept at a lower pressure than sea level. For a body still holding extra nitrogen, that means more supersaturation, which can let leftover nitrogen form bubbles. That’s why there are waiting times before flying: see the decompression sickness page, and follow your computer’s no-fly time.

Sources

1. Georgia State University, HyperPhysics, Henry’s Law (Laws of Gas Transport)
2. Nicholas Bird, Back to Basics, DAN Alert Diver (2014)
3. Schellart et al., Body fat does not affect postdive bubble grade, Journal of Applied Physiology (2013), PDF hosted by Hyperbaar Centrum