The most important rule to follow over anything in scuba diving is to never hold your breath

What is the most important rule in scuba diving - Never hold your breath

When I taught beginner scuba divers to dive, the most important rule I explained was for them to not hold their breath. I always encourage safe diving practices.


The most important rule in scuba diving is to never hold your breath. If you were to hold your breath, and in particular if you ascend whilst holding your breath, you will rupture your lungs. The worst case scenario for scuba divers to ignore this rule is death! Never hold your breath and you'll be okay.


What is the most important rule in scuba diving?

To answer the question that many beginner scuba divers ask, which is "What is the most important rule in scuba diving?" I need to explain a bit more about the science behind diving and depth.

I always feel it's important to understand the reasons behind why something is important, not just to know that it's important. If I'm able to convey the theory behind the most important rule in scuba diving, you're more likely to remember to follow the rule.

The deeper you dive underwater, the higher the pressure

If you didn't know this already, water is nearly 800 times more dense than air. For example, at sea level you experience what is known as atmospheric pressure. Atmospheric pressure is also known as barometric pressure and is one bar of pressure.

The 'bar' is a unit of pressure defined as 100 kilopascals and is about equal to the atmospheric pressure on Earth at sea level

But now you're diving at 10 metres (33 feet) underwater. The pressure you're now experiencing has just doubled. At 10 metres, you are experiencing two bar. Or twice that of atmospheric pressure.

For each additional 10 metres ( 33 feet) you dive underwater, the water pressure you experience increases by one bar. So at 20 metres (66 feet), the pressure is three bar; at 30 metres (100 feet) the pressure is four bar, and so on.

Why is pressure key to the most important rule in scuba diving?

Why these pressure changes are important to scuba divers is because in the same way that pressure increases as you go deeper, on the contrary, as you ascend and get shallower, the pressure reduces once more.

But why is this important you may ask? Plus why are scuba divers told not to hold their breath when ascending as a result of these pressure changes?

The answer to this question can be demonstrated using a balloon filled with air.

The balloon experiment with water pressure underwater

To demonstrate the importance of why should you never hold your breath while scuba diving can be explained using the simple balloon experiment.

You can try this ballon experiment, but please never experiment holding your breath when scuba diving!

If you were to fill a balloon with air at the surface and then take it down to 10 metres (33 feet), the volume of this balloon would halve in size.

This phenomenon is due to the doubling in pressure of water at this depth.

If you then brought the same balloon back to the surface, it would return to the same size it was before it was submerged.

The phenomenon that is demonstrated with a balloon is explained by Boyle’s Law.

What is Boyle’s Law in scuba diving?

Boyle's Law in scuba diving explains how the volume of gas varies with the surrounding water pressure. For the purpose of scuba diving, the gas that varies is air.

Boyle's Law is explained as follows; as the water pressure increases, 'air spaces' decrease in size in direct proportion to the pressure increase, but only when the temperature is held constant.

Noting that as already explained, the deeper you dive underwater, the higher the water pressure. Similarly as water pressure decreases the, 'air spaces' increases in size in direct proportion to the pressure decrease, but only when the temperature is held constant

Understanding this simple 'gas law' is key to staying safe when scuba diving. Take a moment to grasp this basic concept.

What happens if you fill the same balloon with air at 10 metres (33 feet) underwater

If you were to take the same balloon, but instead of filling it with air at the surface, you took it to 10 metres empty and fully inflate it underwater, you would get a completely different outcome.

Well not completely different....

The balloon filled with air at 10 metres, which is then brought to the surface, would attempt to double in size, which is the same as before.

However, as the balloon is already fully inflated, and unless the balloon is able to expand to double its size, you guessed it...the balloon would burst.

Think of your lungs as balloons

Perhaps you've never thought of your lungs this way before, but essentially your lungs are like balloons. They inflate in the same way as balloons and are made from flexible material like a balloon.

However, the difference with your lungs to balloons is that your lings are surrounded by your rib-cage.

This means that even without the limitations of the expandability of the material your lungs are made from, which is similar to the balloon above, your lungs will not be able to expand any further than how much they are constrained by your rib-cage.

This means that if you did a similar experiment as the balloon one above, but with you lungs instead, what do you think might happen?

But I can't stress this enough, please do not try this experiment...this is only a hypothetical question.

Why should scuba divers not hold their breath?

To answer this question, let's continue with where I left off in the last paragraph in the section above.

If you were to fill your lungs with air at 10 metres (33 feet). And if you were to hold your breath and come to the surface, your lungs would attempt to expand to twice the size they were at the depth of 10 metres by the time you reach the surface.

You may have already worked this out, but if you haven't yet, let me explain this further.

If your lungs are fully inflated already, there's no way they could double in size without injury. Even if your lungs were able to double in size, your rib-cage would prevent this amount of expansion.

This would result in a lung rupture!

A lung rupture of this nature is extremely serious and dangerous and should be avoided at all costs.

Which is why this is the most important rule in scuba diving.

Holding your breath underwater whilst scuba diving is extremely dangerous and will cause lung-over-expansion

Holding your breath while scuba diving is extremely dangerous.

As you ascend from a dive, the air in your lungs will expand. If you don't breathe-out as you ascend, your lungs will keep expanding until they over-expand.

Lung over expansion can result in injuries such as a pneumothorax, also referred to as a collapsed lung. It can also lead to an arterial gas embolism or AGE.

An AGE is where air escapes from the lungs into the blood vessels (arterial gas embolism). Air or gas embolisms can cause serious and potentially fatal conditions, such as a stroke or heart attack.

This is why it's bad to hold your breath underwater when scuba diving. It's also why this is the most important rule in scuba diving.

I hope you enjoyed this article about what is the most important rule in scuba diving

I’d love to hear from you. Tell us about your adventures of diving and snorkeling, in the comments below. Please also share your photos. Either from your underwater cameras or videos from your waterproof Gopro’s!

If this article hasn’t answered all of your questions. If you have more questions either about snorkeling or types of scuba diving (or specifically about what is the most important rule in scuba diving), please comment below with your questions.

There will also be many more articles about scuba diving (and snorkeling) for you to read and learn about these fabulous sports.

Have fun and be safe!


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