Why Drinking Seawater Makes You Thirstier — and Can Kill You

A castaway surrounded by water can still die of thirst, and the reason is the salt. Drinking seawater does not quench thirst; it makes it worse, and it can be deadly.

A CTD rosette, one of the instruments scientists use to measure salinity — it records the Conductivity (salinity), Temperature and Depth of the water. Credit: NOAA.

Salt in, water out

The problem is your kidneys. Human kidneys can only make urine that is less salty than seawater. So to get rid of all the extra salt that comes with a drink of seawater, your body has to pass more water than you drank. Each swallow leaves you with less water than before. In the end you die of dehydration, even as you grow thirstier.

Why ordinary salt is fine

The body does need salt. Living cells depend on sodium chloride to keep the body's chemistry in balance and its reactions running, and people can safely take in small amounts of it. When salt is part of a normal diet, we also drink liquids that dilute it and keep it at a healthy level.

Seawater breaks that balance. Its salt content is far higher than the human body can process — and too much sodium can be deadly.

Sources

Based on "Can humans drink seawater?" from NOAA's National Ocean Service, a work of the United States government in the public domain.