why can't we drink sea water?

Found it on Yahoo..very interesting..

A search on "drink salt water" in Yahoo!'s ever-ready search box linked us to Water Science for Schools, an educational site from the USGS (United States Geological Survey). There we learned the difference between salt water, which contains concentrations of dissolved salts, and fresh water. Salinity is usually measured by the weight or amount of salt contained in water expressed as "parts per million" or ppm.

Water is considered highly saline if it contains anywhere from 10,000-35,000 ppm of dissolved salts. Water is considered fresh if it contains less than 1,000 ppm of salt. In some regions of the United States, slightly saline water is used for tasks like crop irrigation. In regions where an abundant supply of clean, fresh water is difficult to come by, desalinization technologies convert seawater to drinkable fresh water. This is still a very expensive process, although costs are starting to drop, and methods, such as reverse osmosis, are improving. Today, the towns of Avalon and Santa Barbara in California, and Tampa Bay in Florida, are working on desalinization projects.

But that still doesn't answer your question. So, we typed "why can't humans drink salt water" directly into the search box. The U.S. government came through again with an answer from the Department of Energy's Ask a Scientist web site. Prof Bill's response is brief and to the point:

Humans can't drink salt water because the kidneys can only make urine that is less salty than salt water. Therefore, to get rid of all the excess salt taken in by drinking salt water, you have to urinate more water than you drank, so you die of dehydration.

As is so often the case, this answer raises some new questions. How much salt is too much? Salt, like water, is a key ingredient of life on earth. The right amount of sodium chloride (common table salt) is essential for human health. In fact, our blood is 0.9% salt and our body weight is about 1/400 salt. Living cells depend on sodium chloride to maintain the chemical balances required for complex processes and reactions that take place at the microscopic level.

Insufficient salt intake can lead to fatigue, illness, and death, although it is more common nowadays to hear about health problems associated with too much salt. According to the Salt Institute's encyclopedic site about salt, the National Academy of Sciences' recommended daily dose is 500 mg/day -- though most Americans consume closer to 3,500 mg/day. (A teaspoon of salt equals about 2,000mg.)

Optimal salt intake varies -- it depends on a person's genetic makeup, where they live, how active they are, and other lifestyle factors. However, nobody, except for a saltwater fish, is designed to drink saltwater.

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