Salt is an essential component of human diets. All fish, reptiles, amphibians, birds, and mammals carry within their veins the elements of sodium, potassium, and calcium in almost the same proportions as the oceans. The "sea" within us has the same saltiness as the Precambrian seas of three billion years ago. Rachel Carson, in her book The Sea Around Us, gives us a clue to our origins: "When the animals went ashore to take up life on land, they carried part of the sea in their bodies, a heritage which they passed on to their children and which even today links each land animal with its origins in the ancient sea."

For humans, salt is as essential as water. We can perish from too little salt as we can of thirst. Salt regulates the exchange of water between our cells and their surrounding fluids. One component of salt, sodium (Na), is involved in muscle contraction including heartbeat, nerve impulses, and the digestion of body-building protein. Humans contain about eight ounces of salt. The amount of salt is regulated in our bodies by our kidneys and by perspiration.

What is salt? It is a compound and has a cubic crystalline form when seen under a scanning electron microscope. Its chemical formula is NaCl, or sodium (Na) chloride (Cl). The chlorine part (ion) accounts for 55% of the dissolved solids in sea water, while the sodium accounts for 30%.

The combined oceans contain about four and one-half million cubic miles of salt—enough to cover the entire depth of the United States to a depth of one mile.

Where did all this salt come from? Part of the salt content of earth came from the breakup of rocks by frost and erosion. The salt was locked in these rocks, and as the rain fell the dissolved salts were carried into the sea. The balance of the sea's salt was leached form the rocks beneath the sea's surface.

The sea is about 3.5% salt. Stated another way: for every 1,000 pounds of water, 35 pounds are salt; or we can say that the average salt content is 35 parts per thousand. In areas closer to the shoreline, this figure will vary because of evaporation and dilution. There is evidence that the salinity of the oceans has changed little since their formation. Through the process of evaporation and freshwater input, the salinity balance stays the same. In addition, much salt returns to land through salt spray or through the salt crystals that form nuclei for raindrops. It then finds its way back to the oceans.

Salinity is vital for animals that live in the ocean. Most marine organisms in the open ocean have body fluids whose salinity closely approximates that of the water around them. However, problems arise for marine organisms who live in the coastal environment. In the intertidal zones and estuaries, wide fluctuations demand mechanisms that will increase water uptake or remove salt. Some animals protect their internal salinity with shells or scales. Oysters thrive in low-salinity waters of 7 to 18%, but their predators (mainly starfish) cannot. Some species of fish can readily adapt to rapid changes in salinity, such as the common mummichog or killfish (Fundulus heteroclitus), which can tolerate a wide range of salinity. Perhaps this adaptation has permitted it to thrive in these stressful habitats. Particularly susceptible to salinity fluctuations are the eggs of many vertebrates and invertebrates.

Myths have arisen about salt. To spill salt at the table requires throwing a pinch over the left shoulder for good luck.

There are some examples of art that celebrates salt. In the Kunsthistorische Museum in Vienna resides a magnificent sixteenth-century Golden Salt Cellar, product of the craftsmanship of Benvenuto Cellini. Perhaps because of the myth about spilled salt, Leonardo da Vinci's famous "Last Supper" has a spilled saltcellar in front of Judas.

Salt could be used in Central Africa in the 1800s to purchase a bridge. The French, who detested the salt tax, the gabelle, began a revolution that helped to repeal the tax. In India, Mahatma Gandhi led a march to the ocean in protest against the British law that forbade Indians from making their own salt. "Why," reasoned Gandhi, "should they not march two hundred and forty miles to the shore where the salt from the sea was free?"

Poland's Wieliczka salt works is a museum made up of chambers with sculptures and chandeliers in salt 400 feet underground. Near Hutchinson, Kansas, a storage company operates the world's largest warehouse in a salt mine 650 feet beneath the surface—it covers 300 acres.

Some 14,000 uses have been found for salt. It is used in adhesives, batteries, explosives, meats, and metals. Before the days of refrigeration, people used to cure meats, thus preventing spoilage.

The next time you shake out some salt remember that it demands your attention. We all spent the first months of our lives in a sac of saline solution. We are dependent on this humble compound.

Return to Rhode Island Sea Grant Fact Sheets