Short answer: the air you breathe from a scuba tank is compressed, and compressed air expands as you rise and the pressure around you drops. If you keep breathing, the extra volume simply leaves your lungs with every exhale. If you close your airway and hold it in, that expanding air has nowhere to go and can damage delicate lung tissue. That is why "never hold your breath" is the first rule taught in every briefing, and the only one with no exceptions.
The Physics, Without the Diagram
Pressure underwater grows quickly. At the surface your body sits under one atmosphere of pressure. At 10 metres it is two atmospheres — double. The air delivered by your regulator arrives at whatever the surrounding pressure is, which means a lungful taken at 10 metres contains twice as much air as the same lungful at the surface, squeezed into the same space.
Now go back up. As the pressure falls, that squeezed air tries to return to its original volume. A breathing diver never notices, because the excess escapes naturally with every exhale. A diver with a closed throat is carrying a balloon that wants to grow inside a container that cannot.
The uncomfortable detail is that the biggest relative change happens in the shallowest water. Going from 10 metres to the surface doubles the volume; going from 40 to 30 metres changes it far less. This is why the rule matters just as much on a beginner dive at 5–8 metres as it does on a deep dive, and why we repeat it on every single trip regardless of depth.
Why Beginners Hold Their Breath Without Meaning To
The instinct is completely normal. Everything you have ever done in water — swimming, snorkelling, jumping in — rewards holding your breath. Under a scuba set the instruction inverts, and old habits take a few minutes to unlearn. The usual moments are:
- The first descent, when the water closes over your head and you brace instinctively.
- A surprise — a bit of water in the mask, a wobble in buoyancy, an unexpected fish.
- Concentration — many people hold their breath when focusing hard on a task, above water as well as below.
- Taking the regulator out at the surface, then putting it back and pausing before the first breath.
None of these is dangerous when you are aware of them, which is precisely what the briefing is for.
How the Briefing Drills It
Before anyone gets wet, your instructor states the rule, explains it in one plain sentence, and then builds every skill around it. You will hear a very simple version of it that works underwater when there is no time to think: always be blowing bubbles, or always be breathing in. If you can see bubbles leaving your regulator, you cannot be holding your breath.
The rule is then attached to the specific moments where it matters:
- During ascent: breathe normally and slowly. If your regulator is out for any reason, hum out a continuous stream of bubbles.
- While clearing the mask: you breathe in through the regulator and out through your nose — the breathing never stops.
- When adjusting buoyancy: never use your lungs as a lift by holding a big breath. Small, steady breaths, and let the instructor manage the rest.
- If something startles you: the first response is to breathe out, then signal. Read the hand signals guide so the second half is easy.
With a maximum of 3–4 divers per instructor on our trips, your instructor can watch each diver's bubbles — a steady stream is the clearest sign in diving that a beginner is doing exactly the right thing.
Holding Your Breath Also Makes the Dive Worse
Beyond safety, breath-holding is uncomfortable. It traps carbon dioxide, which is what actually creates the urge to breathe and, over a dive, a dull headache. It wrecks buoyancy, sending you gently up and down like a yo-yo. And it burns air faster than the slow, full breathing it replaces. Divers who breathe calmly get longer, prettier, more relaxed dives — that is not a coincidence.
What About Snorkelling and Freediving?
Holding your breath is perfectly safe when you have not been breathing compressed air. A snorkeller on the surface takes a normal lungful, dives down, and the air compresses and re-expands back to exactly where it started. The danger belongs specifically to scuba, where you fill your lungs at depth. If you are joining our snorkelling programme or coming as a boat visitor, the rule does not apply to you — but it is worth understanding why.
Dive Where the Rule Is Repeated Every Time
Our full-day boat trips include a full safety briefing before the first dive, two guided dives, all equipment, free hotel transfer both ways, lunch on board and insurance, from €43 to €50. Beginners stay at 5–8 metres in the morning and around 13 metres at most on the second dive, always with an instructor within arm's reach. Read more on our diving safety page or the guide to how safe diving in Side really is, browse the dive trips, and message us with any question. Reserve free online, pay on the day.
