Atmospheric Pressure in Diving

When a person dives underwater, they feel excitement, maybe a bit of fear, joy, and awe. But right now I want to talk not about that, but about what you must know to make your dive safe and enjoyable.

Atmospheric Pressure

Each of us is constantly under pressure. Right now, air pressure from the surrounding atmosphere is pressing on you. At sea level, this pressure is practically constant and is considered to be 1 atmosphere or 1 bar (Technically there’s a slight difference between an atmosphere and a bar, but it’s so minor that divers just treat them as equal).

So, 1 bar is the pressure from the entire column of the atmosphere.

Underwater, a person experiences even greater pressure. The BCD doesn't sit quite right, the mask fogs up, and on top of that, absolute pressure increases — because water has its own weight, which adds to the air's weight — the atmospheric pressure.

Water is much denser and heavier than air, so just 10 meters of saltwater exerts the same pressure as the entire atmosphere. What does that mean? That at 10 meters depth, the absolute pressure is already 2 atmospheres, at 20 meters it's 3, at 30 meters it’s 4, at 40… you get the idea. And logically, pressure drops by one atmosphere for every 10 meters of ascent. It's worth mentioning that freshwater is a bit lighter than saltwater, so you need about 10.3 meters of it to get the same pressure change. The difference isn't big, but it's good to know.

The Relationship Between Pressure, Volume, and Air Density

Water is nearly incompressible, so its volume and density don’t change with pressure. Since the human body is mostly water, pressure while diving doesn’t affect most of the body. But air is not water, and changes in pressure do affect the volume and density of air.

As you descend, pressure increases and the volume of gas decreases. Air becomes denser because the number of gas molecules stays the same, but now they occupy a smaller space.

This is important to know and remember because it affects what happens to the air-filled spaces in your body — ears, sinuses, lungs — and air spaces around you like your mask or drysuit. This relationship also impacts buoyancy control, air consumption, and other key safety aspects.

Let’s take the most classic and illustrative example, proving that air volume and density change proportionally with pressure.

Take a balloon holding 3 liters of air. As you descend from the surface to 10 meters where pressure increases to 2 atmospheres, the air volume inside the balloon will halve, and its density will double. So now the balloon contains 1.5 liters of air. At 20 meters, its volume will shrink to 1 liter — one third of what it was at the surface — and the air will be three times denser due to 3 atmospheres of pressure. And this pattern continues every 10 meters.

Naturally, it works the other way too. Suppose you half-inflate the same balloon (1.5 liters of air) at 30 meters, where the air density is 4 times higher than at the surface. To fully inflate, the pressure must halve — so at 10 meters (2 atmospheres), the balloon would be fully inflated. If we keep ascending, the expanding air would eventually burst the balloon.

How Pressure Affects Air-Filled Spaces in the Body

Balloons are fun, but how does this apply to divers?

Although most of the human body is water — and we know that water isn’t greatly affected by pressure — air spaces are affected. These include the ears, sinuses, lungs, the mask, and drysuit.

Pressure imbalance occurs when the pressure outside an air space is greater than inside. As the volume of air shrinks, water pushes harder against the tissues around that space, causing discomfort.

If you don’t equalize pressure, you’ll feel squeezing. And squeezing causes discomfort, and discomfort during diving is not good. If left uncorrected, it could even cause injury. Again, this happens because external pressure exceeds internal pressure in the air space.

Luckily, you can easily avoid this discomfort (squeeze). Just add air to these air spaces as you descend. That equalizes the internal pressure with the external water pressure, keeping the volume normal.

Let’s start with ears. Anyone who’s flown knows the feeling of ears popping during takeoff or landing. That’s also due to pressure changes. Many people pinch their nose and gently exhale through it to fix it. That’s called equalizing.

Ear discomfort during descent is caused by the eardrum and nearby tissues being squeezed inward by the water. To equalize, pinch your nose and gently blow into it. That pushes air from the throat into the ears and sinuses, relieving the pressure difference. You can also try moving your jaw side to side or swallowing, but the most effective method is that gentle nose-blow. It’s very important to equalize your ears as you descend to avoid barotrauma. But always do it gently — blowing too hard can also injure your ear.

Sounds a bit scary, but the rule is: equalize every meter (or even more often). Don’t wait for discomfort. If you equalize often during descent, you won’t feel pressure or pain at all.

One of the key rules of diving:

💡 Equalize early and often during descent

What about mask pressure?

As you go down, the air in the mask is also compressed. Tissues start to press into the mask, breaking capillaries. It can look nasty — bruises around the eyes, bloodshot eyes — but usually there are no serious consequences. To equalize the pressure in your mask, just exhale through your nose into it. Then you're selfie-ready after the dive :)

And of course, lungs.

Lungs react both to increases and decreases in pressure. While descending, the higher air pressure doesn’t hurt your lungs — you’re breathing air supplied at the same pressure as the water around you.

It’s absolutely critical not to hold your breath while diving, especially during ascent. Remember what happens to an air-filled balloon rising to the surface — it bursts from expanding air. Our lungs are like balloons too, and they can rupture if you ascend while holding your breath.

That leads us to another essential rule in diving:

💡 Never hold your breath

So as you can see — although pressure changes constantly while diving, it's nothing to worry about if you know how it works and what to do.