Seawater, which makes up approximately 97% of the Earth’s water, is defined by its high concentration of dissolved salts. On average, ocean water contains about 35 grams of salt for every liter of water, a level toxic to human consumption. Consuming salty ocean water initiates a dangerous biological process that accelerates the body’s water loss. This leads to severe dehydration and eventual organ failure as the body attempts to maintain a delicate balance of salts and fluids.
The Core Problem: Why Seawater is Dangerous
The high salinity of seawater is incompatible with human biology. Seawater typically contains approximately 35 parts per thousand (ppt) of dissolved salts, primarily sodium chloride. In contrast, the salt concentration of human blood is tightly maintained at a much lower level, around 9 ppt, or 0.9%.
The ocean is roughly four times saltier than the internal environment of the human body. When seawater is ingested, it introduces a massive salt load into the bloodstream. The body cannot excrete this excess salt without assistance from fresh water.
The kidneys regulate blood salt concentration by producing urine. However, the human kidney can only produce urine with a maximum salt concentration of about 20 ppt, which is less salty than the seawater consumed. Consequently, the body must sacrifice a greater volume of its existing fresh water reserves to dilute and excrete the excess salt load. This biological deficit ensures that drinking seawater results in a net loss of water.
Physiological Effects of Excessive Salt Intake
Accelerated dehydration is caused by osmosis, which describes the movement of water across a semipermeable membrane from a lower to a higher solute concentration. When concentrated salts from seawater enter the bloodstream, the fluid surrounding the body’s cells becomes hypertonic, or saltier, than the fluid inside the cells.
To equalize the concentration gradient, water is involuntarily pulled out of the body’s cells and into the bloodstream. This osmotic stress causes cells throughout the body, including those in the brain and organs, to shrink. This cellular shrinkage is the root cause of the intense thirst and physiological damage experienced after drinking seawater.
The kidneys attempt to filter the highly concentrated blood but are overwhelmed by the salt, demanding more water to create dilute urine. This obligatory water loss depletes the body’s fluid reserves, leading to severe dehydration. As dehydration progresses, the high salt concentration can lead to hypernatremia, which disrupts nerve function and can culminate in delirium, convulsions, and eventual organ failure.
Methods for Obtaining Safe Drinking Water from the Sea
Seawater can be rendered safe for consumption through desalination, a process that removes the dissolved salt.
Reverse Osmosis (RO)
On a large scale, the most common and efficient method is Reverse Osmosis (RO). RO systems use high-pressure pumps to force seawater through a fine, semipermeable membrane. This membrane is engineered to allow water molecules to pass through while physically blocking the larger dissolved salt ions.
Distillation
For emergency or survival situations, distillation is the preferred method, relying on evaporation and condensation. The basic principle is that when water turns into vapor, it leaves all the non-volatile salts and impurities behind. The steam is then collected as pure, fresh water.
A simple survival device known as a solar still uses this principle by capturing the sun’s energy to heat seawater placed in a basin. The resulting water vapor condenses on a cooler surface, such as a plastic sheet, and drips into a collection container. Another method involves boiling seawater in a pot and capturing the steam that condenses on an inverted lid or a separate collection vessel.