What is hard water?
Water from aquifers and other underground sources collects dissolved minerals from rock--particularly calcium, magnesium carbonate, and manganese.
These minerals give water undesirable characteristics that are dubbed "hardness."
The severity of hardness is measured by grains (of mineral) per gallon (GPG) or, in some cases, by parts per million (PPM). These two measurements are related: 17.1 PPM equals 1 GPG.
Technically, any water that contains more than 1 GPG of dissolved hardness minerals is considered hard, but realistically, water with from 0 to 3.5 GPG is relatively soft. Water with more than 10.5 GPG is very hard. Between those extremes is typical, moderately hard water.

Hard water is less an issue of health than one of potential expense. Many of the problems created by hard water are hidden until some type of malfunction occurs in your home's plumbing system or in water-using appliances.

When heated, dissolved hard-water minerals re-crystallize and form scale that eventually clogs plumbing. Eventually, this reduces water flow through pipes.
Scale and lime deposits take their toll on other water-heating appliances such as dishwashers and coffee makers, increasing the need for repairs. Worse, scale cakes onto interior surfaces of water heaters, making them less efficient and more likely to fail.
According to a study at New Mexico State University, commissioned by The Water Quality Research Council, water heaters operate from 22 to 30 percent less efficiently when plagued with hard-water scale.

Hard water problems are more obvious as a nuisance where you cook and bathe. Calcium and magnesium react with many soaps and detergents, diminishing their lathering or cleaning capability and forming a scum--sometimes called "soap curd"--that is difficult to rinse away.
In the kitchen, this translates to spotted dishes and scale on cookware. In the bath, it appears as bathtub ring and tile scum. In the laundry, it means gray, stiff clothing. And in house cleaning it means more scrubbing and rinsing.
When bathing, you generally need more soap or shampoo and must rinse more thoroughly. Additionally, certain hard-water minerals, such as iron and manganese, can also have an undesirable appearance, odor or taste.

Hard water does enter the health arena in one area: People who have it are more prone to rashes and skin problems because it changes the skin's pH and soap remains on the skin, clogging pores.