Henry's Law and Gas Absorption
Henry's Law explains why gases dissolve into your tissues under pressure — and why they come back out when you ascend. It is the foundation of decompression theory.
If Boyle’s Law explains squeezes and buoyancy, Henry’s Law explains decompression. It is the reason dive tables exist, the reason you do safety stops, and the reason you cannot fly immediately after diving.
What it is
Henry’s Law states that the amount of gas that dissolves in a liquid is proportional to the partial pressure of that gas above the liquid. Higher pressure means more gas dissolves. Lower pressure means gas comes back out of solution.
Your blood and tissues are the liquid. The breathing gas in your lungs provides the pressure. As you descend and pressure increases, nitrogen (and helium, if you are diving trimix) dissolves into your tissues. As you ascend and pressure drops, that dissolved gas comes back out.
The formula
| Variable | Meaning |
|---|---|
C | Concentration of dissolved gas in the liquid |
k | Henry's Law constant (depends on the specific gas and liquid, and temperature) |
Pgas | Partial pressure of the gas above the liquid |
In diving, we do not usually calculate Henry’s Law directly. Instead, it provides the theoretical basis for compartment models used in dive tables and dive computers.
How it works in your body
On descent and at depth
As you descend, the partial pressure of nitrogen in your breathing gas increases. This creates a pressure gradient — the nitrogen partial pressure in your lungs is now higher than in your blood and tissues. Nitrogen begins dissolving into your blood, which carries it to tissues throughout your body.
Different tissues absorb gas at different rates:
- Fast tissues (blood, brain, spinal cord): Saturate quickly — within minutes
- Medium tissues (muscle, skin, organs): Take 30-120 minutes to approach saturation
- Slow tissues (fat, bone, cartilage, ligaments): Can take hours to saturate
The longer you stay at depth and the deeper you go, the more nitrogen dissolves into your tissues. This is your “nitrogen loading” and it determines your decompression obligation.
On ascent
When you ascend, the ambient pressure drops and the gradient reverses. Now the dissolved nitrogen in your tissues has a higher partial pressure than the nitrogen in your lungs. The gas begins to come out of solution and is carried by your blood to your lungs, where you exhale it. This process is called off-gassing.
If you ascend slowly enough, the dissolved gas comes out of solution gradually and is eliminated through normal breathing. If you ascend too fast, the gas can come out of solution faster than your body can eliminate it — forming bubbles in your blood and tissues. This is decompression sickness.
The soda bottle analogy
Henry’s Law is easy to visualize with a sealed bottle of carbonated water:
- Sealed bottle (at depth): The CO2 is under pressure and stays dissolved. The liquid looks clear
- Slowly opening the cap (slow ascent): A small hiss of gas, maybe a few small bubbles. The CO2 comes out of solution gradually
- Ripping off the cap (rapid ascent): Explosive fizzing. Gas comes out of solution all at once, forming countless bubbles
Your body works the same way. A controlled, slow ascent lets nitrogen off-gas gradually. A rapid, uncontrolled ascent can cause bubbles to form in your tissues — decompression sickness.
Practical implications
No-decompression limits
Dive tables and computers calculate how long you can stay at a given depth before the nitrogen loading in your fastest tissues exceeds a safe level. Stay within these limits and you can ascend directly to the surface (with a safety stop) without mandatory decompression stops.
Decompression stops
If you exceed no-decompression limits, you must pause at specific depths during ascent to allow excess nitrogen to off-gas before continuing. These stops give your body time to eliminate dissolved gas gradually.
Safety stops
Even within no-deco limits, a 3-5 minute stop at 5 meters is recommended. This provides extra time for off-gassing during the phase of ascent where pressure changes are most dramatic (remember Boyle’s Law).
Repetitive diving
After a dive, you still have excess dissolved nitrogen in your tissues — you are not fully off-gassed. Your surface interval allows continued off-gassing. If you dive again before fully off-gassing, you start the next dive with residual nitrogen, which reduces your available no-decompression time.
Flying after diving
At altitude, the ambient pressure is lower than at sea level. If you fly too soon after diving, the reduced cabin pressure (equivalent to about 1,800-2,400 meters altitude) can cause remaining dissolved nitrogen to form bubbles. DAN and most agencies recommend waiting at least 12-18 hours after diving before flying.
Temperature effects
Henry’s Law includes a temperature component: gases are more soluble in colder liquids. In practical diving terms:
- Cold water may increase nitrogen absorption slightly
- A warm shower or hot tub after diving can reduce gas solubility in your tissues, potentially promoting bubble formation
- This is why some agencies recommend avoiding hot tubs immediately after diving
Safety considerations
- Ascend slowly: 9-10 meters per minute is the maximum recommended ascent rate for most agencies
- Do your safety stop: Three to five minutes at 5 meters costs almost nothing and adds a meaningful safety margin
- Plan surface intervals: Allow adequate time between dives for off-gassing. Longer intervals mean more nitrogen elimination
- Stay hydrated: Dehydration thickens blood and may impair gas elimination
- Avoid strenuous exercise after diving: Heavy exercise increases perfusion and may promote bubble formation in tissues that are still off-gassing
Sources
- Henry’s law — Wikipedia
- NOAA Diving Manual, 6th Edition
- Doolette, D.J. & Mitchell, S.J. (2005). “Biophysical basis for inner ear decompression sickness.” Journal of Applied Physiology
- DAN (Divers Alert Network) Flying After Diving Guidelines