While Earth is mostly covered by water, the vast majority is seawater, unsuitable for human consumption. Scientific principles explain why attempting to quench thirst with ocean water leads to further dehydration and can be dangerous.
The Salty Truth About Seawater
Seawater is highly saline, with oceans averaging about 3.5% salinity (35 grams of dissolved salts per liter). Sodium and chloride ions are the two most abundant components, making up about 85% of the total. Other minerals like magnesium, sulfate, calcium, and potassium are also present. In contrast, human blood has a much lower salt concentration, around 0.9%. This significant difference is the primary reason seawater cannot be safely ingested.
Your Body’s Water Management System
The human body maintains a balance of water and salt through osmoregulation. This process ensures solute concentration, primarily mineral salts, remains within a narrow range in bodily fluids. Kidneys are central to this regulation, continuously filtering blood to control fluid and electrolyte balance. They produce urine to excrete waste products and excess salts.
Water movement in the body, including into and out of cells, is governed by osmosis. Osmosis is the diffusion of water across a semi-permeable membrane (like a cell membrane) from an area of higher water concentration (lower solute) to an area of lower water concentration (higher solute). Cells maintain an internal balance; if the external environment becomes too concentrated, water moves out of the cells to equalize the concentration.
The Dehydrating Impact of Seawater
When seawater is consumed, its high salt concentration creates an osmotic imbalance within the body. The bloodstream quickly becomes saltier than the body’s cells and tissues. To dilute this excess salt and restore balance, water is drawn out of the body’s cells, including those in the kidneys, through osmosis.
The kidneys, tasked with filtering the ingested salt, cannot produce urine saltier than seawater. To excrete the large amount of salt, the kidneys must use more water than was consumed, leading to a net fluid loss. This process intensifies dehydration, causing increased thirst, cellular shrinkage, and straining organs like the kidneys and heart. Drinking seawater exacerbates dehydration rather than alleviating it.
What to Do in a Survival Situation
Given the harmful effects of seawater, avoid it as a source of hydration in survival scenarios. Prioritize access to fresh, potable water. Natural sources like rainwater can be collected. Moisture from plants, through transpiration, can also provide a small amount of water.
For larger quantities, desalination is necessary to remove salts from ocean water. Basic methods include distillation, where seawater is boiled and the resulting steam is collected and condensed into fresh water. Solar stills, which use the sun’s energy to evaporate water and collect condensation, are another option. Portable desalination devices, often employing reverse osmosis technology, are available for emergency use, though they may require significant effort.