Never Hold Your Breath. The most important rule in scuba diving. Certified divers learn this during their PADI, SSI or RAID courses. They also learn the reasons why we should not hold our breath while diving. But what happens when you hold your breath while scuba diving?
What Happens When You Hold Your Breath While Scuba Diving?
When scuba divers descend, they expose themselves to additional pressure exerted on them by the water weight. This pressure affects how flexible air containers, such as your lungs, ears, and sinuses behave.
While a human lung can function under pressure and withstand shrinking by large percentages, it cannot expand much past 100%. When scuba divers take a breath at depth, their lung volume is 100%. This is due to the density of air at depth. The deeper, the denser air gets. Divers fit more physical air into their lungs at depth than what could fit on the surface.
Why Must Divers Exhale While Ascending?
A diver holding their breath during an ascent risks air not escaping naturally. Air volume in their lung expands due to less pressure at shallower depths. Air has to escape and the diver’s lung is forced to break.
This is a Lung Over Expansion Injury. The easiest way to prevent this injury is to continuously breathe in order to keep the lung volume at 100% regardless of the diver’s depth. This is different to decompression sickness or the bends.
Can your lungs explode scuba diving? Not exactly. They can rip and tear, causing severe damage, but they generally won’t explode.
But Why Does This Happen?
The primary physics principle behind this is Boyle’s Law. Boyle’s Law dictates the linear relationship between a diver’s depth, therefore the pressure they are under and the correlating volume and density changes. The deeper a diver goes, the more pressure they are under. This means that flexible air containers shrink more and more. In order to keep lung volume at 100% volume, denser air fills the lung.
For example, at 20 meters depth, a diver is under 3 bar of pressure. That means the volume of air is at 1/3 and the density is 3 times what the same amount of air would be on the surface. In order to fill the lungs, this diver breathes 3 times the amount of air.
This directly relates to a diver’s air consumption. This is to say that air does not last as long deeper.
See how long a scuba tank lasts for more information on air consumption.
What About Freedivers?
Freedivers hold their breath all the time when diving. In fact, that is how they conduct their dives. Will pressure affect them?
Actually, yes. Pressure changes affect free divers too. The biggest difference, however, is that they do not take a breath of compressed air at depth in order to equalize their lungs.
A freediver’s lung physically shrinks in size and rapidly expands on the ascent. Because these divers did not add additional physical amounts of air at depth, the remaining air volume in their lungs simply expands back to 100% by the time they reach the surface. The risk of lung overexpansion injury is therefore minimal.
This is why freedivers learn not to take a breath from a passing scuba diver and ascend holding their breath.
Can You Hold Your Breath Underwater?
Some of my students like to go down the line of asking why? and I really appreciate the curiosity. A junior diver I once taught said during equipment setup that this is only a problem if they were to hold their breath and ascend at the same time.
Very smart kid. That is correct. Because beginner divers might not have as much control over buoyancy and even ascent speed, it is safer to train them to never hold their breath. Furthermore, it is a mammal instinct that you cannot breathe underwater and the first thought is to hold one’s breath and go up.
This could potentially be fatal.
The first thing I learned when I trained to become an Underwater Photography Instructor was that it is in fact a requirement in many circumstances to hold your breath in order to take a clear photo. Holding your breath limits body movement and keeps the camera nice and still. By that stage, however, I had very good buoyancy and breath control and knew not to even move a centimeter up or down while taking a photo.
Let’s Take a Breath
Now that you know what happens when you hold your breath while scuba diving you can explain it to your curious friends. Maybe even inspire them to try out diving for themselves.