Why Humans Can’t Drink Saltwater
Water is vital for life, but most of Earth’s liquid supply is off-limits to us. More than 96% of the planet’s water is stored in the oceans, and it contains so much salt that it’s undrinkable. In fact, drinking too much seawater can speed up dehydration and even lead to death. But why exactly is saltwater so harmful when it’s still, technically, water?
The Problem Lies in Salt
Seawater is about 3.5% salt by weight, according to the National Oceanic and Atmospheric Administration (NOAA). To visualize that, if all the salt from the oceans were spread across every land surface on Earth, it would form a layer more than 500 feet (166 meters) thick — about the height of a 40-story building. That level of saltiness, known as salinity, is far too high for human bodies to handle safely.
How the Kidneys Work
Our kidneys are responsible for filtering impurities and excess substances from the blood. They do this by producing urine, which carries waste products and dissolved salts out of the body. However, there is a limit: human kidneys can only produce urine that is less salty than our blood.
Normal human blood has a salt concentration much lower than seawater. Because seawater is more than three times saltier than blood, the kidneys cannot simply eliminate the extra salt without losing even more water in the process.
Why Saltwater Makes You Thirstier
When you drink seawater, the body suddenly takes in a huge load of salt. To remove that excess salt, the kidneys require extra water. For every cup of seawater consumed, you would need to drink at least an equal amount of fresh water just to flush the salt out. Without access to fresh water, the body ends up losing more water than it gains, causing dehydration to worsen instead of improving.
As Rob DeSalle, curator at the American Museum of Natural History, explained: “What happens when you drink saltwater is you ingest a lot of salt that the body now needs to wash out.” The cycle quickly becomes impossible to maintain — drinking more saltwater only adds to the salt load, intensifying dehydration and thirst.
Lessons from the Ocean
Marine animals have adapted to this challenge in remarkable ways. Many species of fish and seabirds possess specialized glands or efficient kidney systems that allow them to excrete excess salt and survive on seawater. Humans, however, lack these adaptations, making seawater a dangerous choice for hydration.
The Takeaway
Saltwater is abundant, but it is no substitute for fresh drinking water. The very process that keeps us alive — the filtration system in our kidneys — is precisely what makes seawater lethal if consumed. Far from quenching thirst, it accelerates dehydration, proving that not all water on our planet is drinkable. For humans, survival depends on access to fresh, pure water.
