Typically, water supply networks deliver potable water, whether it is to be used for drinking, washing or landscape irrigation. One counterexample is urban China, where drinking water can optionally be delivered by a separate tap. In the United States, public drinking water is governed by the Safe Drinking Water Act (SDWA).

Metabolism

Main: Fluid balance Water is necessary for all life on Earth. Humans can survive for several weeks without food, but for only a few days without water. A constant supply is needed to replenish the fluids lost through normal physiological activities, such as respiration, perspiration and urination. In terms of mineral nutrients intake, it is unclear what the drinking water contribution is. However, inorganic minerals generally enter surface water and ground water via storm water runoff or through the Earth's crust. Treatment processes also lead to the presence of some mineral nutrients. Examples include fluoride, calcium, zinc, manganese, phosphate, and sodium compounds. Water generated from the biochemical metabolism of nutrients provides a significant proportion of the daily water requirements for some arthropods and desert animals, but provides only a small fraction of a human's necessary intake. There are a variety of trace elements present in virtually all potable water, some of which play a role in metabolism. For example sodium, potassium and chloride are common chemicals found in small quantities in most waters, and these elements play a role (not necessarily major) in body metabolism. Other elements such as fluoride, while beneficial in low concentrations, can cause dental problems and other issues when present at high levels. Water is essential for the growth and maintenance of our bodies, as it is involved in a number of biological processes.

Requirement

The exact amount of water a human needs is highly individual, as it depends on the condition of the subject, the amount of physical exercise, and on the environmental temperature and humidity. In the US, the reference daily intake (RDI) for water is 3.7 liters per day for human males older than 18, and 2.7 liters for human females older than 18 including water contained in food, beverages, and drinking water. Food contributes 0.5 to 1 liter, and the metabolism of protein, fat, and carbohydrates produces another 0.25 to 0.4 liters, which means that 2 to 3 liters of water for men and 1 to 2 liters of water for women should be taken in as fluid in order to meet the RDI. Because in general, RDI values incorporate a safety margin to account for individual variations, it does not mean that this amount is necessary for every person. The folk wisdom that everyone should drink two liters (68 ounces, or about eight 8-oz glasses) of water per day is not supported by scientific research. Various reviews of all the scientific literature on the topic performed in 2002 and 2008 could not find any solid scientific evidence that recommended drinking eight glasses of water per day.