What Is Water? - Chemical Composition, Earth Mover, Hydrologic Cycle, Freshwater, Sometimes It Rains; Sometimes It Doesn't

Most people living in the United States assume they will have plenty of clean, safe water for drinking, that crops and gardens can be regularly irrigated, and that sewage will be taken care of by their local treatment plant. In many parts of the world, however, the availability of water for personal and public use cannot be taken for granted. In fact, according to the World Health Organization (WHO), 42% of the population around the world—2.6 billion people—did not have access to safe water in 2002 (http://www.who.int/water_sanitation_health/publications/facts2004/en/index.html, November 2004). WHO estimates that as many as 300 million people annually become ill with or die from waterborne diseases.

Water is vital to human survival. Although people can survive for about a month without food, without water they would die in about a week. Water is the most common substance on earth. It covers three-fourths of the earth's surface and makes up 65% of the adult human body, including 90% of its blood and 75% of its brain. Water is the main ingredient in most of the fruits, vegetables, and meats that people eat. According to the U.S. Environmental Protection Agency (EPA), water makes up about 75% of a chicken, 80% of a pineapple, 80% of an ear of corn, and 95% of a tomato. Growing enough food to produce an adequate diet for a human being for one year requires about 300 tons of water—nearly a ton (2,000 pounds) a day.

While essential to human existence, water can cause severe damage and destruction. It is terror to the swimmer caught up in a current. It rusts cars and rots the foundations of houses. Water in the form of hailstones destroys crops, and in winter ice coats roads and causes auto accidents. Too much water becomes a flood that destroys homes and people. Too little water is a drought that causes living things to wither and, eventually, die. Water can carry disease; in developing countries waterborne diseases account for over three-fourths of all illnesses and many deaths.