Shallow Water Blackout: What You Need To Know

shallow blackout

Whether you are a fisherman, diver, spear fisherman, cruiser, floater, or even a sailor (sometimes they troll) this is a topic worth understanding.

First off why do people blackout?

Hypoxia-Not enough Oxygen to the Brain

Before we can really get into the nuts and bolts of this we need to review some basic normal physiology.

lung function

Lets first think about the lungs. This organ is responsible for gas exchange, bringing oxygen into the blood and getting rid of carbon dioxide.

The air we breathe is 21% oxygen, 78% nitrogen, and the rest trace elements.

free diving

When you breathe, air comes into your mouth, down the trachea (wind pipe) and branches off to each lung via another pipe (left or right bronchus), from here it branches into even smaller pipes eventually ending in millions of tiny air sacs called alveolus. This is where the business happens and oxygen and carbon dioxide get exchanged.


These gases move the direction they do because of gradients, based on pressure and concentration (on land pressure isn’t important since it doesn’t change but it becomes important when diving). Oxygen in the blood (pumped from right ventricle of heart) coming by the alveoli is low, thus oxygen from lungs goes into the blood. The opposite is true for carbon dioxide, which is higher in the blood and goes into the lungs to be exhaled.

O2 pump

The heart is simply a pump that pumps the blood into your lungs for the exchange, get oxygen and get rid of carbon dioxide, then pump the blood out of the lungs and throughout the body to feed hungry specialized cells in the body making up different organs. Oxygen is used in all these cells to help burn fuel like sugars, fats, and proteins. The byproduct of all this is CO2 that then goes back to the lungs and the pathway starts again.

Ok so that is our basic pump and oxygenator that you need to understand. Now lets discuss a regulatory mechanism.

Breathing and heart rate is involuntary. That is when you need more oxygen or need to get rid of more carbon dioxide you will breathe faster automatically. Your heart rate will even increase.

How does the body know?

Through special peripheral and central chemoreceptors

chemoreceptorsIn your carotid arteries, aorta, and brain there are specialized cells that are in communication with your heart and lungs via our signaling network (nerves).  These cells are very sensitive to pH changes in the body.

Lets look at this equation. (don’t freak out)

blood levels

What this shows us is that the more CO2 in our blood we produce more H+ which lowers the pH. This triggers these cells to tell the body to breathe more because we have too much CO2 and need to get rid of it.

Why is this important?

Carbon Dioxide CO2 is the greatest driving force for the urge to breathe!

Having low oxygen levels also will kick in the breathing response but it’s not as robust as the CO2, thus it’s a late reflex.

So holding your breathe, your CO2 rises, while your O2 decreases. At some point the CO2 rises to a point where you get the urge to breathe. (makes sense right?)

Trivia: This is so important that in the hospital one of the criteria to pronounce someone brain dead that’s on a ventilator is to stop their ventilator and let their CO2 rise to see if they get a trigger to breathe.

Ok so that is the understanding of basic physiology you need to move forward and really understand this topic. We also need to introduce a little bit of diving physiology but will get to that as we need.

So now on to shallow water blackout.  First a few facts:

  • It can happen to anyone who participates in breath holding activities. Due to the sports involved you will notice this has killed young, elite athletes.
  • Proper education and safety protocols in place can prevent bad outcome.
  • It can happen at any depth if oxygen stores get too low, however as you will learn the greatest change in pressure happens in less than 33 ft of water so there is a mechanism of why the blackout can happen in shallow water.

Remember it’s all about hypoxia (low oxygen levels). Anything that lowers your oxygen level puts you at risk, ex: not enough recovery time on surface to build O2 levels back up, etc, however I would like to address two key points:

  1. Hyperventilation prior to diving
  2. Hypoxia of ascent

Hyperventilation prior to Diving = BAD, BAD, BAD

Remember what we discussed above? The importance of CO2 as a key player that allows our chemoreceptors to send off signals to breathe!

So this is what is happening. You are breathing deep and fast to blow off more CO2 before the dive. This lowers your CO2 initially. So it will take longer before you gain enough CO2 while holding your breath, to get the urge to breathe. You will indeed be able to hold your breath longer, but now you are getting closer to risking hypoxia!!

You are trying to reset your body’s warning system. This can be very dangerous.

Lets look at this in graphic form.

Here is normal breathing

shallow-water blackout

As you see here. When you hold your breath normally before a dive, your O2 will drop and your CO2 will rise. However, your CO2 will hit the threshold to give you the urge to breathe before O2 ever reaches a low level causing blackout.

Ok so now lets look at hyperventilation.

shallow water

As you can see here. With hyperventilation you are getting rid of CO2. So when you start the dive the CO2 levels are much lower than normal. Before the CO2 levels can rise enough to give you the urge to breathe, you have reached low enough levels of oxygen that causes you to blackout.

Take home message: Yes you will hold your breath longer, but you are starting to play a dangerous game, that could lead to death!

Now that you understand this mechanism you can understand more complex ideas such as type of fuel burned during your free diving. This may be too much for some readers but I just want to introduce it to those that want to read more. When you dive initially your body is burning fuel via glycogen stores (sugars) through the famous glycolysis and kreb cycles. This uses O2 and give off CO2 as a byproduct. When you run out of glycogen stores you burn fat. It turns out that fat doesn’t give off as much CO2 when it is burned. So what does this mean? Your are reducing the amount of CO2 formed thus reducing how quickly your CO2 will rise in the blood when holding your breathe thus getting into the same dangerous scheme!

Ok now lets move on the to the second point.

Hypoxia of ascent

To really understand this we have to introduce a little bit of diving physiology. It’s a little bit complicated but lets see if I can give you the right idea.

First off we need to talk about atmospheric pressure. We have air molecules surrounding us also being pulled down by gravity. This air, obviously doesn’t weigh much but it still is pushing on us. So we say on earth we have 1 atmosphere of pressure on us or 760 mm Hg or 14.7 pounds per square inch. These are all units of pressure that can be used. However, going forward we will use the units atmospheres.

Ok so if you go underwater now you have the pressure of water pushing on you + the air above. Water weighs more than air so the deeper you go the pressure becomes significant pretty quick. It turns out the pressure down to 33 feet of water is the same pressure as the entire atmosphere pushing on you! Think of it this way, if you go down 5 feet all that water above is pushing on you. If you go 10 feet even more water above is pushing on you, etc… That’s the idea.

Ok so lets introduce the different atmospheres and Boyles law.

diving rules

So what does this image show us? Well a few things first off look at the pressure in atmospheres. It turns out every 10 meters (33 feet) of water is an atmosphere of pressure. So if you go down 33 feet you gained an atmosphere from the water + the 1 atmosphere of air above so you your body now sees 2 atmospheres of pressure.

Ok now quickly to understand Boyles law. The formula is Pressure (surface) x Volume (surface)= Pressure (at new depth) x Volume (at new depth).

So look at the balloon in the picture. On the surface it’s blown up to its greatest size called 100%. When you go down 1 atmosphere to 33 feet, there is a total of 2 atmospheres now pushing on the balloon. That’s twice as much pressure, thus we have to decrease the volume by half. That formula should now make sense but visually it should also make sense. If you keep going down and increasing the pressure the volume gets smaller. This happens to your lungs.

Trivia: For the non-divers this is why if you are scuba diving and take a deep breathe at depth and hold it and come up, your lungs will expand and eventually rupture!

The greatest pressure change is from 1 atmosphere (surface) to 2 atmospheres (33 feet deep). The change in pressure is 50% nowhere else does it change that much from one level to the next!

This is important and will get to it in a second.

Ok one more principle everyone needs to understand. Henry’s law.

gas properties

So here’s the idea. In a closed environment, chamber 1 on the left and chamber 2 on the right. If you have more pressure (chamber 2) then these molecules will be more soluble in the liquid.

To be thorough there’s a law called Dalton’s law. Which states “the total pressure exerted by a mixture of gases is equal to the sum of the pressures exerted by each gas.” All this means is when you breathe air which is essentially 21% O2 and 78% Nitrogen and take a dive, increasing the total pressure, the pressure exerted from each gas has to increase. (If you don’t understand this, don’t worry you can still understand the topic, keep reading)

gas exchange

This principle is also happening in your lungs. Remember the picture above of the alveoli with capillaries coming by exchanging oxygen and carbon dioxide?

Remember it’s a gradient that allows oxygen to go into the blood and carbon dioxide to leave. So now imagine you add more pressure to these alveoli (diving down), you are allowing the oxygen to still continue to flow into the blood (normally at 1 atmosphere it would stop sooner) to get to your brain. In fact if you go a little deeper you increase the pressure even more and it keeps forcing oxygen into the blood, to an extent when things equilibrate.

Ok so the driving force to keep oxygen going into your blood stream from your lungs is pressure driven (allowing you to keep O2 up enough to prevent blackout). Remember this is all from 1 breath at the surface. So not only have you been depleting your lungs of oxygen now we are about to come up to the surface.

When you do this, the pressure that was really helping the oxygen get into your blood, is no longer there. Now you have two big issues: no pressure to help this driving force and minimal amount of oxygen left in the lungs- all the dangerous factors to lead to hypoxia and blackout!

With all kinds of fancy physiology equations and math you can show that less than 10 meters (33 feet) the pressure reaches a point that if you have a low amount of oxygen you can reach blackout levels before quite making it to the surface.

This principle can be seen in Deep Dives.  However, both of these principles are not exclusive and both are contributing factors to blackouts.

If you understand this you understand a lot about the physiology leading to blackouts. To learn even more pick up a dive physiology book.

Ok thanks for getting through that with me. I hope you all learned something.

shallow water blackout

For those of you thinking about getting into free diving or spearfishing I would recommend you taking a free diving course to learn more, especially on safety. If you talk to free divers with a lot of experience they only like to dive with buddies they know understand this physiology and have been trained to deal with emergencies in the water. If that’s not the case, then Darwin may get them in the end!


  • Blackouts happen when your brain does not have enough oxygen (hypoxia)
  • Never Hyperventilate before a breath hold, since this will lower your body’s ability to trigger itself to breathe prior to reaching hypoxia and blackouts
  • When diving deep always have a buddy come meet you on the ascent to assure you don’t blackout
  • Always Dive with an experienced partner and take a freediving class to understand more!
  • Really never hold your breath without supervision. But definitely don’t dive without a buddy!!!!!

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Dr. Brad Genovese
Brad Genovese. I was born and raised in Laguna Niguel, California. He has been greatly blessed to have incredible parents that introduced him to sport fishing, hunting, and travel at an early age. Growing up he spent many summers fishing and diving the Sea of Cortez. Brad says, “Although my entire...