Henry's Law

At the surface, with an atmospheric pressure of 1 bar (14.7 psi), your body contains a certain amount of dissolved nitrogen from the air you breathe. When you descend underwater, the ambient pressure increases significantly. For every 10 metres (33 feet) you go deeper, the pressure increases by approximately 1 bar (14.7 psi). This increased pressure directly affects the partial pressure of nitrogen in your breathing gas.

According to Henry's Law, as the partial pressure of nitrogen surrounding your body increases, more nitrogen will dissolve into your blood and, subsequently, into your body tissues until equilibrium is reached. The longer you stay at depth, the more nitrogen your tissues absorb. Different tissues, such as fat, muscle, and bone, absorb and release nitrogen at varying rates due to differences in blood supply and solubility characteristics.

The critical aspect of Henry's Law for divers comes into play during ascent. As you ascend, the ambient pressure decreases, which in turn lowers the partial pressure of nitrogen in the lungs. Now, the nitrogen dissolved in your tissues is at a higher partial pressure than the nitrogen in your lungs. To re-establish equilibrium, nitrogen must leave your tissues and be exhaled.

If the ascent is too rapid, or if you exceed no-decompression limits, the nitrogen cannot leave the tissues quickly enough and becomes 'supersaturated'. This supersaturation can cause the nitrogen to come out of solution and form bubbles, much like opening a fizzy drink bottle. These bubbles are the primary cause of decompression sickness (DCS), leading to symptoms ranging from joint pain and skin rashes to neurological damage and paralysis.

For divers on a liveaboard, Henry's Law underpins nearly every aspect of safe diving practices, especially given the multi-day, repetitive dive schedules. You are likely performing multiple dives a day, often to significant depths, which means your body continuously accumulates nitrogen over several days. This cumulative effect increases your overall nitrogen load, requiring more conservative dive planning.

Liveaboard dive guides and your dive computer will factor in this cumulative nitrogen. You'll typically find that your allowable bottom times become shorter on subsequent dives or later in the week, and surface intervals are carefully monitored. Understanding Henry's Law reinforces the importance of following dive computer recommendations, performing safety stops, and respecting no-fly times to ensure a safe and enjoyable liveaboard experience.

Myth: If I don't feel any symptoms, I'm not absorbing nitrogen. Fact: Your body is continuously absorbing and off-gassing inert gases like nitrogen whenever you breathe compressed air, regardless of whether you feel symptoms. Henry's Law dictates this process is happening even on shallow, short dives, just in smaller quantities. DCS symptoms only appear when the supersaturation exceeds a critical threshold and bubbles form.

Myth: My dive computer handles everything, so I don't need to understand Henry's Law. Fact: While dive computers are excellent tools, they are based on models derived from Henry's Law and other physiological principles. Understanding the underlying science empowers you to make smarter, more conservative decisions, especially in complex dive scenarios or when a computer might malfunction.

Myth: I can 'push' my no-decompression limits every time because I'm young/fit. Fact: Individual susceptibility to DCS varies greatly, but Henry's Law applies to everyone. Pushing limits increases the nitrogen load and the risk of bubble formation, regardless of fitness. Factors like dehydration, fatigue, and cold can exacerbate this risk, even for healthy individuals.