Shallow Water Blackout: The Secret Killer

Shallow Water Blackout kills more swimmers than anything else.  Year after year.
See, it doesn’t matter how well you swim.  If you faint underwater, there’s nothing you can do.  Someone has to rescue you within three minutes or you’ll drown.
So what causes Shallow Water Blackout?  And what can you do about it?  The more you know, the safer you are.

How it Works

Here is a great diagram that explains the process leading up to SWB (Shallow Water Blackout).

shallow water blackout

Your body is constantly measuring the levels of oxygen and carbon dioxide in your blood.  When there’s too much carbon dioxide, it tells you to breathe.  This is how it regulates the ratio, making sure that all your cells have enough oxygen.

When you hyperventilate, you’re blowing out way more carbon dioxide than normal, increasing the percentage of oxygen in your blood.  But remember, you have lower carbon dioxide levels, not more oxygen.

You dive.  Then there’s no way to get more oxygen…because you’re underwater.  So your oxygen levels go down, down, down.  But you don’t feel out of breath.  Why?

You have lowered your carbon dioxide levels, which is the trigger to take a breath, so you never feel like you need to although your oxygen levels are dangerously low.


In a normal dive, your CO2 levels increase proportionally as O2 decreases.  You feel like you need to breathe long before you lose consciousness.

After hyperventilating, CO2 and O2 are still correlated, but since CO2 starts so low, it doesn’t hit the trigger level until it’s way too late.

There are no warning signs.  You feel perfectly normal.  You pass out underwater and don’t even feel out of breath.

Why is it so Dangerous?

Normally, drowning can take 5-10 minutes depending on a variety of factors.
Shallow Water Blackout is faster.  Within 2 minutes, you could experience brain death.  Why?
Once you hit the trigger level of CO2, you are already unconscious.  So your body tries to help out.   It instinctively draws a huge breath for you.  But you’re underwater, so your lungs immediately flood.
If there’s no rescue within a minute or two, you’ll drown quietly, with no struggle, at the bottom of the pool.  No warning signs, no splash, no sounds.

What Can I Do?

1. DON’T HYPERVENTILATE!  Yes, you can go longer in the pool without feeling like you need to breathe.  But that doesn’t mean you don’t need oxygen!  Hyperventilating on purpose to hold your breath longer is the #1 cause of SWB.  Don’t do it.  It’s not worth it.
2.  If you see someone trying to hold their breath as long as possible, WATCH THEM CLOSELY.
They may have low levels of CO2.  This is a dangerous game and/or drill, and should never be performed without close supervision.
3.  If you do see someone unconscious in the pool, get them out of the water as fast as possible.  Every second counts.
Start CPR right away.  Call 911.

Your Thoughts

Do you know CPR?  Would you be able to respond in an emergency situation?  It’s a good idea to get CPR and First Aid certified, just in case you’re ever needed.

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