Salt poisoning

Salt poisoning typically results in a feeling of confusion and jitteriness; more severe degrees of intoxication can cause seizures and coma. Death can result if medical intervention is not forthcoming. These symptoms are generally a consequence of hypernatremia—an abnormally high sodium level in the blood. (There are myriad causes of hypernatremia, which is frequently encountered in medical practice; salt poisoning is not a common cause.)

Early on, the intoxicant will cause a strong feeling of thirst, followed by weakness, nausea, and loss of appetite. More severe symptoms ensue, including confusion, muscle twitching, and bleeding in or around the brain. Death results by the swelling of the brain against the skull. (Normal serum sodium levels are 135 – 145 mEq/liter (135 – 145 mmol/L). Severe symptoms typically only occur when levels are above 160 mEq/L.) The human renal system actively regulates sodium chloride in the blood within a very narrow range around 9 g/L (0.9% by weight).

Accidentally consuming small quantities of clean seawater is not harmful, especially if the seawater is taken along with a larger quantity of fresh water. However, drinking seawater to maintain hydration is counterproductive; more water must be excreted to eliminate the salt (via urine) than the amount of water obtained from the seawater itself.[6]

In most open waters concentrations vary somewhat around typical values of about 3.5%; drinking seawater temporarily increases blood's NaCl concentration, which signals the kidney to excrete sodium. However, seawater's sodium concentration is above the kidney's maximum concentrating ability. Eventually the blood's sodium concentration rises to toxic levels, removing water from cells and interfering with nerve conduction, ultimately producing fatal seizure and cardiac arrhythmia.

Survival manuals consistently advise against drinking seawater.[7] A summary of 163 life raft voyages estimated the risk of death at 39% for those who drank seawater, compared to 3% for those who did not. The effect of seawater intake on rats confirmed the negative effects of drinking seawater when dehydrated.[8] (In contrast to humans, pelagic birds and other sea animals can (and must) drink sea water without ill effects.)

Historical experiences

Some historians have suggested that the mysterious sicknesses afflicting the early English colonists at Jamestown, Virginia (1607–1610)—which nearly extinguished the settlement—reflect sea water poisoning. The settlers arrived in the spring, when the James River water was relatively fresh, but by summer a drought of historical magnitude had rendered it much more brackish. The historical geographer Carville Earle, among others, holds to this view.[9]

The temptation to drink seawater was greatest for sailors who had expended their supply of fresh water, and were unable to capture enough rainwater for drinking. This frustration was described famously by a line from Samuel Taylor Coleridge's epic poem The Rime of the Ancient Mariner (1798):

"Water, water, everywhere,
And all the boards did shrink;
Water, water, everywhere,
Nor any drop to drink."

Although humans cannot survive on seawater alone—and, indeed, will sicken quickly if they try—some people have claimed that up to two cups a day, mixed with fresh water in a 2:3 ratio, produces no ill effect. During the 18th century, British physician Richard Russell (1687–1759) advocated the practice as part of medical therapy in his country. In the 20th century, René Quinton (1866–1925), in France, would also endorse the practice. Currently, the practice is widely used in Nicaragua and other countries, supposedly taking advantage of the latest medical discoveries.) In his 1948 book, Kon-Tiki, Thor Heyerdahl reported drinking seawater mixed with fresh in a 2:3 ratio during the 1947 expedition.[10] The French physician Alain Bombard (1924–2005) survived an ocean crossing (1952–53) in a small Zodiac rubber boat using mainly raw fish meat, which contains about 40 percent water (like most living tissues), as well as small amounts of seawater and other provisions harvested from the ocean. His findings were challenged, but an alternative explanation was not given. A few years later, an American sailor and adventurer, William Willis (1893–1968), claimed to have drunk two cups of seawater and one cup of fresh per day for 70 days without ill effect when he lost part of his water supply.[11]