The eight-glasses rule has no scientific basis, and most tap water flushes toilets
The familiar advice to drink eight glasses of water daily is not grounded in science; thirst tracks need better than any fixed number. Meanwhile an American home uses roughly 262 litres of piped water per person per day, and only about 1 percent of public supply goes on drinking and cooking.
Official figures vary widely. US Adequate Intake levels, derived from median consumption, are 4 litres of total water a day for adult men and 3 for women, assuming about a fifth arrives in food. Europe's food safety agency suggests 2.5 litres for men and 2 for women. Americans over 21 actually drink about 1,043 millilitres of plain water daily on average. Heavy work in hot climates can push requirements to as much as 16 litres, while drinking too much too fast can cause hyponatremia.
Bodies are mostly water, though the share shifts with age: about 60 percent of a man's weight, 55 percent of a woman's, 70 to 80 percent of a baby's and about 45 percent in old age. Farm animals need planning too. A dairy cow may need about 130 litres a day, a horse a third of that and a pig a tenth.
Safe water is far from universal. As many as two billion people lack it, and contaminated water is estimated to kill more than half a million people a year; in 2010 Ban Ki-moon said unsafe water killed more people than war. Cholera, dysentery, typhoid and hepatitis A top the list of linked diseases. Natural hazards matter as well: arsenic in groundwater affects about 140 million people in 70 countries, and fluoride leaching from granite is thought to have poisoned 60 million through wells.
Modern public health partly began with a water pump. In 1854 John Snow traced a cholera outbreak in London's Soho to the Broad Street pump, a founding moment for epidemiology. Infrastructure remains fragile: urban pipe networks can leak half their water, and rich countries may need up to 200 billion dollars a year to replace ageing systems. A UNICEF and UNESCO report ranked Finland's drinking water the best in the world.
Source: Drinking water
