Water hardness traditionally measures the capacity of water to react with soap, since hard water requires considerably more soap to produce lather. It is not caused by a single substance but by a variety of dissolved metallic ions, mainly calcium and magnesium. Hardness usually is expressed as milligrams of calcium carbonate per liter. Water containing less than 60 mg of calcium carbonate per liter is generally considered to be "soft," while levels above 250 mg/L are outside the Federal limit for "hard" water.

Water has solvent properties and tends to dissolve, suspend, and/or exchange trace elements it contacts in its travels through the environment. One natural cause of hardness in drinking water is seepage and runoff through limestone, which contains calcium and magnesium.

There is no convincing evidence that hard water causes adverse health effects in humans, and no health-based guidelines for water hardness have been proposed.

Along with water taste and increased soap consumption, hard water is mainly a nuisance issue. When hard water combines with soap it forms a "scum" that cannot be rinsed off, leaving spots on dishes and glassware. Hard water also contributes to limescale, a rock-hard deposit that accumulates around water fixtures. Combined with other factors such as pH, hard water can also cause problems in the distribution system, clog pipes, and ruin water heaters.

Neilson Research Corporation routinely tests for water hardness and other physical parameters in our drinking water packages. See "Drinking Water Testing Packages" for more information.

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