Hard water is defined as water containing a high concentration of calcium and magnesium ions. However, hardness can be caused by several other dissolved metals including aluminum, barium, strontium, iron, zinc, and manganese.
WHO in 2003 reported that hard water has no known adverse health effects.
There does not appear to be any convincing evidence that water hardness causes adverse
health effects in humans. In contrast, the results of a number of epidemiological studies have suggested that water hardness may protect against disease. However, the available data are inadequate to prove any causal association.
Exposure to hard water is considered to be a risk factor for eczema by studies on atopic eczema among Japanese, Nottinghamshire and Spanish children. Water hardness may increase the risk of atopic dermatitis among elementary-school children in Japan, as well as in the United Kingdom.
Potential health impacts on consumption of hard water are discussed here.
Although, there is some evidence from epidemiological studies for a protective effect of magnesium or hardness on cardiovascular mortality, the evidence is being debated and does not prove causality. Further studies are being conducted. In spite of this, drinking water may be a source of calcium and magnesium in the diet and could be important for those who are marginal for calcium and magnesium intake. Where drinking-water supplies are supplemented with or replaced by dematerialized water that requires conditioning, consideration should be given to adding calcium and magnesium salts to achieve concentrations similar to those that the population received from the original supply. There are insufficient data to suggest either minimum or maximum concentrations of minerals at this time, and so no guideline values are proposed.