Drinking seawater is dangerous for human consumption because it contains a high concentration of dissolved salts, primarily sodium chloride. Ocean water salinity averages around 3.5%, meaning every liter contains approximately 35 grams of salt. This far exceeds the body’s capacity to process, as human blood plasma maintains a tightly regulated salt concentration of about 0.9%. Ingesting this hyper-saline fluid introduces a massive influx of sodium into the bloodstream, immediately disrupting the delicate balance of fluids and electrolytes. The body’s attempt to correct this severe imbalance leads to rapid, life-threatening dehydration and cellular stress.
The Immediate Physiological Danger
The danger of drinking seawater begins at the cellular level due to osmosis. Osmosis is the movement of water across a semipermeable membrane, like a cell wall, from an area of low solute concentration to an area of high solute concentration. Seawater is a hypertonic solution because its salt concentration is significantly higher than the body’s internal fluids.
When hypertonic seawater salts are absorbed into the bloodstream, the surrounding fluid outside the body’s cells becomes highly concentrated with sodium. To achieve equilibrium, water is drawn out of the cells and tissues into the saltier bloodstream to dilute the excess sodium. Instead of hydrating the body, drinking seawater causes water to be pulled out of the cells, leading to cellular dehydration.
This loss of water causes cells to shrink, which can fatally damage them and disrupt normal physiological processes. This systemic fluid shift quickly leads to symptoms such as dry mouth, muscle cramps, nausea, and delirium.
Kidney Function Under Salt Stress
The kidneys filter waste products and excess electrolytes, including sodium, from the blood to produce urine. The human kidney has a maximum concentrating power, known as the renal concentrating capacity, of approximately 1,200 to 1,400 milliosmoles per liter (mOsm/L). Seawater has an osmolarity, or total solute concentration, between 1,000 and 1,200 mOsm/L, which is already near this maximum limit.
The kidney must also excrete other waste products, particularly urea, which contributes to the urine’s total solute concentration. When working at maximum capacity, the kidney can only excrete sodium at a concentration much lower than that of seawater, typically around 600 mOsm/L. This means that for every liter of seawater ingested, the body requires a volume of fresh water greater than one liter to flush out the salt load.
To source this extra water, the kidneys must draw it from the body’s own reserves, including fluid pulled from the cells. This creates a worsening cycle where drinking salt water results in a net loss of water, intensifying dehydration. The strain on the kidneys can eventually lead to acute kidney injury and a buildup of nitrogenous waste in the blood, a condition called uremia.
Survival Myths and Desalination
The scientific reality is that drinking seawater in a survival situation accelerates dehydration and hastens systemic failure. Attempting to ingest seawater, even in small amounts, is counterproductive because it increases the body’s water requirement beyond the amount consumed. Boiling seawater is also ineffective as a purification method, as the water evaporates and leaves the salt behind, creating a more concentrated and toxic brine.
The only way to make ocean water safe for drinking is through desalination, a process that physically removes the dissolved salts. Desalination techniques separate pure water molecules from the salt, often through distillation or reverse osmosis. Purified seawater is safe to drink only because the hypertonic salt load has been eliminated.