What are the dangers of diving at different depths? Risks for each depth

Last updated on 24 Sep 2024

Diving, although not risk-free, is generally a safe hobby if the standards and guidelines are respected. That being said, submerging yourself in a completely new environment does come with challenges: and those challenges increase the deeper you go.

First few meters

The first few meters aren't usually that dangerous for experienced divers, as they will be used to the surface-to-underwater transition. At that depth, there are no major changes in the physiological effects of the gas that we normally breathe. That doesn't mean that it is risk-free either (because if it was I wouldn't have included it on this list). Especially new/novice divers will have to be on the lookout for a couple of things:

Firstly, when staying on the surface, the primary objective is to establish positive buoyancy. New divers don't have the automatic reflex to inflate their jackets, and if on top of that, they don't keep their regulator in their mouth it may result in a stressful situation.
Secondly, the first few meters may be tricky because of equalizing: although a second nature for most seasoned divers, a lot of new divers surprisingly have some problems with that! For some, it poses no problems; others may have a harder time but eventually be able to equalize; and some, just cannot do it, no matter how slow you go! 1-5m is the make-or-break depth for most people regarding equalizing. In my experience, if a new diver can get to 5m without ear issues, going deeper will usually not pose too much of a problem.

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My recommendations for dive professionals taking out new divers: make it very clear that feeling pressure in the ear is normal, but feeling pain isn't. Right before descending, remind them again to equalize, and descend as slowly as you can. It is always a good idea to ask them to equalize on the surface (during your briefing, for example) and ask them if they feel anything in their ears, to make sure they equalize properly. Remember that while that feeling is normal for you it isn't to the new diver: to make them "mentally prepared" for feeling pressure in the ear, I like to compare it to what the ear experiences during a flight.

10m (33 feet)

All right, so you've been able to descend to 10m without too many issues. Now that you're there, you may ask yourself: "What is the most dangerous thing for me here?". Well as it turns out, it's going back up!

A depth of 10m wouldn't be considered too deep yet and there still aren't any major modifications to the physiological effects of the breathing gases you would be using at that depth (except if you're breathing pure oxygen).

What does matter at that depth is the relative change in pressure: you are gaining one additional atmosphere of pressure from the initial atmosphere you already had on the surface. You've essentially doubled the pressure you're subjected to. To double it again, you would now have to descend 20m, to 30m of total depth (where the pressure is 4 atmospheres).

Why does that fast change in pressure matter? As we know, air gets compressed as pressure increases and expands as it decreases. By doubling the pressure, you would be dividing the volume of an air pocket by two, and vice-versa: doubling its volume when halving the pressure (which is what happens when going from 10msw to the surface). This compression-expansion play affects the lungs (as it is an air pocket) and can cause serious injuries if a diver holds their breath while ascending: this happens faster at shallow depths, as the relative change in pressure is faster as well.

Another thing that gets affected when the volume of an air pocket varies is your buoyancy. Shallowing up 1 meter when you are shallower than 10m will increase your buoyancy far more than shallowing up 1m at 40m of depth. That is the reason why new divers might have a hard time maintaining proper buoyancy while shallow, but have more control over it whenever they go deeper.

30m (98 feet)

Below 30m, you will start to feel the first effects of nitrogen narcosis. Although not dangerous in itself, narcosis can have negative repercussions on your choices and lead to dangerous situations. People sometimes describe it as being drunk, or a feeling akin to euphoria. The truth is that the effect may change between person and you might experience it differently from how your buddy would.

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The effects of nitrogen narcosis can start shallower than 30m: I once had a diver that almost blacked out at 25m. However, this kind of case is very rare, and we usually don't notice the narcosis until passing the 30m mark.

Another issue arises at that depth: you are in-gassing nitrogen at a fast rate and have reduced bottom time (if you want to respect the no-deco limit, that is). At 30m on air, you would have 20 minutes before hitting the no-deco limit. This time would be reduced further in case of consecutive dives.

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Doing decompression isn't inherently a bad or dangerous thing, but it should be planned.

40m (131 feet)

40m is generally acknowledged as being the boundary between recreational and technical diving. Most agencies's "deep specialty" will allow you to dive up to 40m and will require another separate certification beyond that.

At 40m, you are subjected to 5 atmospheres of pressure and therefore breathe 5 times as much as on the surface. On top of that, the no-deco time comes way faster: at 40 meters on air, you only have 5 minutes before entering decompression. A recreational dive at 40m is basically a race against the clock!

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To get the full experience of a deep dive and stay as long as possible, try to remove any stress factors that may increase your air consumption (such as an improper amount of weights) and dive on Nitrox to increase your bottom time.

Below 40m, the no-deco time will come too fast to enjoy the dive and you need to plan for decompression. Even if you aren't planning for decompression it is always a good idea to have an extra stage with you or a pony tank for some redundancy at that depth.

57/66m (187/216 feet)

Of course, as you delve deeper and deeper, the narcosis will worsen if you're not diving on Trimix and the amount of decompression time required will increase at a much faster rate. Still, I put these depths in their own category, not because of nitrogen but because of oxygen: Oxygen, at a higher partial pressure, is toxic and will lead to physical impairments, going up to convulsions in extreme cases (and being underwater is not the best place to convulse, for obvious reasons).

Now, you may ask: "Why 57 and 66 meters specifically?". In recreational diving, we are careful to never breathe a gas with a higher PPO2 than 1.4 bar. This means that the partial pressure of oxygen should always be below that limit. For instance, if we are diving at 40m/131 feet (for an absolute pressure of 5 bars) on air (which is made up of 21% oxygen), then we would be breathing gas with a partial pressure of oxygen of 1.05 bars (0.21 x 5).

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Different scenario: Imagine that we are at the same depth (40m), but we are using nitrox, with a 28% part of oxygen. We would then be subjected to 1.4 bars of partial pressure induced by oxygen (0.28 x 5), which is on the edge of the limit! Here, 40 meters would be the MOD (or Maximum Operating Depth) of the 28% Nitrox gas mix, and ideally, we would stay 2-3m above the MOD to avoid accidentally exposing ourselves to a higher PPO2.

Now, if we try to calculate the maximum operating depth for air (meaning, how deep would you have to go to be subjected to 1.4 bars PPO2 using a gas that has 21% oxygen), you would find that that depth comes out to 57m! (0.21 x 6.7 bar ≈ 1.4 bar)

As to the 66m, that corresponds to a PPO2 of 1.6 when using air. This higher partial pressure would generally only be used during decompression stops, to accelerate decompression.

To dive deeper than 57m without being exposed to higher oxygen partial pressure you would need to reduce the oxygen concentration of your mix (i.e. breathe a gas mix with less than 21% oxygen). That is known as hypoxic diving, it requires special training as breathing a hypoxic mix above a certain depth can lead to hypoxia (lack of oxygen).

150m+ (500+ feet)

From 40m onwards, you would dive with helium in your gas mix. At a depth of 150m, even helium starts to harm your physiology: HPNS (standing for High-pressure nervous syndrome) is a condition that occurs when breathing gas containing helium at that depth and whose severity depends on the amount of helium in the mix and on the rate of descent.

Symptoms of HPNS range from mild dizziness and nausea to tremors and vertigo, going up to loss of consciousness below 300m. Of course, the deeper you go, the more severe the effects are.

It is not currently known to be preventable, but some methods, such as reducing the helium concentration, and adding a narcotic gas in the mix, have been observed to delay the effects.

HPNS is currently one of the main barriers to "ultra-deep diving", and there are alternatives to helium being looked into, such as gas mixes containing hydrogen (presenting their own challenges, such as a high reactivity).

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An infographic about the dangers of diving at each depth