Calcium and magnesium levels combine to determine the water's "hardness." Bottled water is considered soft, thanks to low levels of calcium and magnesium. When higher levels are found in municipal water, it is often "softened" — particularly in the U.S. — for ease of home use. The taste of water is heavily impacted by softening. Hard tap water makes cleaning more difficult and more dependent on soaps and synthetic detergents. Scaling in boilers and teakettles comes from hard water. But hard water does not pose any danger to your health: According to the U.S. National Research Council, the magnesium and calcium in hard water can contribute to your daily dietary requirements.
Hardness can be calculated with this formula (calcium and magnesium should be measured in milligrams per liter):
HARDNESS = (calcium x 2.5) + (magnesium x 4)
The Institute of Medicine (IOM), responsible for establishing the "Daily Values" on food packaging, has determined that we need more calcium as we get older. From age 19 to 50, the IOM recommends 1,000 milligrams and 1,200 milligrams for those 50 years old and older.
One of the most critical healthful minerals in mineral water is magnesium, although few know its health benefits. Magnesium promotes hundreds of bodily functions. Its benefits include bone strength, higher immunity, heart, muscle, and nerve function. The IOM recommends these daily intakes of magnesium: For men ages 19 to 30, 400 milligrams; men men age 30 and older, 420 milligrams; women age 19 to 30, 310 milligrams; and women age 30 and older, 320 milligrams.
Water is softened with an ion-exchange water softener, which adds sodium (salt) to the water. About 8mg/l of sodium is added for every grain of hardness (17.1mg/l) taken out. Water softening means that most Americans are accustomed to slightly salty water.
In the U.S., water hardness isn't regulated by the U.S. Environmental Protection Agency (EPA). Instead, the U.S. Department of the Interior and the international Water Quality Association (WQA) offer the following classifications: