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Why Can't Humans Drink Seawater?

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K S Lane is a student of science and is deeply passionate about educating others on her favorite topics.

Why can't humans drink seawater? Keep reading to find out!

Why can't humans drink seawater? Keep reading to find out!

The fact that humans shouldn’t drink seawater is a lesson we all learn in childhood. Whether through a teacher or parent or by personal (and unpleasant) experience, by the age of ten virtually every child knows that if you’re stranded in the middle of the ocean, drinking salty seawater just isn’t a viable option. However, precious few actually learn why drinking seawater only increases your thirst. It’s only water with a little bit of extra salt added, after all. Why can’t we drink it? In this article, I’ve outlined the science behind seawater and why drinking it really isn’t a good idea.

How Much Salt Is in Seawater?

Seawater, as we all know, has a high salt content. On average, a litre of water will contain about 35g of salt. This may not sound so bad, but considering that the recommended daily intake of salt for an adult is just six grams it’s a lot of salt. By doing a little arithmetic, we can deduce that one glass (240ml) of seawater contains on average 8.4g of salt, which is almost one and a half times the recommended daily intake. Yikes.

What Role Do the Kidneys Play?

To understand why humans can’t drink seawater, you need to have a little bit of background physiological knowledge. The kidneys are an essential human organ located on your sides, halfway tucked under your ribs. Their key role is to filter our blood, making sure that the body’s salt and water content is just right. They do this primarily by regulating how much urine we pass. This is important because too much salt in the blood causes water stored in the body to be released, which in turn causes an increase in blood volume and an increase in blood pressure (or hypertension). Hypertension puts you at a greater risk for strokes, heart disease and kidney failure, and therefore it's critical that the kidneys are able to maintain the correct salt-water balance in the blood.

The kidneys, which are located half under the ribs, help to maintain the body's salt balance.

The kidneys, which are located half under the ribs, help to maintain the body's salt balance.

What Happens When We Drink Seawater?

When you drink seawater, you’re flooding your body with salt. To maintain the correct balance, the kidneys kick into high gear. To remove the excess salt they start producing more urine than usual, attempting to flush the sodium out of your body. However, there’s a limit to the salt concentration of urine, and that limit is much lower than the salt concentration in seawater. This means that more water is needed than the amount you took in from the seawater to get rid of the extra salt. You get thirsty, which is your body’s way of telling you to increase your water intake. If the only liquid available to you is seawater and you drink it then you’re only going to increase your body’s salt content and get even thirstier as your kidneys cry out that they need more water in order to flush out all of the extra salt. It’s a vicious cycle, and one that can get deadly incredibly quickly. As your body continues to urinate out all of the water you’ve drunk dehydration kicks in, causing fatigue, dizziness and confusion. Death can occur in as little as three days; more if you keep drinking seawater to try and alleviate your thirst. Dying of thirst after drinking water; it's cruelly ironic.

Are There Other Bad Things in Seawater?

To put it simply, yes. For one, there are an estimated 3.5 trillion fish in the sea, which amounts to a heck of a lot of fish excrement. Whales can produce over 900 litres of urine every day. It’s diluted, of course, given the massive scale of the ocean, but still. Not nice. The amount of human waste in the ocean also isn’t something to be ignored. If you live in a coastal town, chances are your sewage is dumped right into the ocean. It’s treated first, so it doesn’t harm the marine ecosystem, but that doesn’t mean that it’s not still, well… poop. Keep that in mind the next time you feel an urge to open your mouth while swimming at the beach.

This whale is swimming in its own urine.

This whale is swimming in its own urine.

I've Drunk Seawater Before; Am I Going to Die?

If you’ve gulped down a few mouthfuls of seawater on a day at the beach, it’s nothing to be concerned about. Drinking normal water can protect you from dehydration and give your kidneys the power they need to flush the extra salt from your body. As for the other nasty things in seawater, a little excrement never killed anybody. Well, it probably has, but hopefully you catch my meaning. Drinking seawater isn’t usually deadly. If you’re guzzling litres of the stuff daily, though, you might want to consider what all of that extra salt is doing to your body.


In conclusion, don’t drink seawater. A single glass contains almost one and a half times the recommended daily intake of salt and a not inconsiderable amount of fish and human excrement. Saltwater has the nasty effect of increasing your thirst and causing dehydration, as the kidneys are forced to mass-produce urine to flush the salt from your body. In the long term, an excess of salt can lead to stroke, heart failure and kidney disease. A few accidental mouthfuls of salty seawater are unlikely to be harmful, but if you’re drinking from the ocean on purpose then you definitely need to reevaluate your life choices.


© 2018 K S Lane


Christopher Wanamaker from Arizona on September 08, 2018:

Very thorough explanation. I've accidentally swallowed seawater and fortunately did not experience any ill effects. Great article.

K S Lane (author) from Melbourne, Australia on September 08, 2018:

Thanks Pamela- I've swallowed a few mouthfuls myself swimming at the beach. Not very pleasant!

Pamela Oglesby from Sunny Florida on September 08, 2018:

I certainly wouldn't want to drink seawater, but I did get a swallow while swimming in a rather rough ocean one time. I have been on islands where I knew they dumped a lot of junk in the ocean. As you stated, thi is another reason not to drink this water. Interesting article.