The World Health Organization says, "There does not appear to be any convincing evidence that water hardness causes adverse health effects in humans."
Some studies have shown a weak inverse relationship between water hardness and cardiovascular disease in men, up to a level of 170 mg calcium carbonate per liter of water.
Other studies have shown weak correlations between cardiovascular health and water hardness. The World Health Organization has reviewed the evidence and concluded the data were inadequate to allow for a recommendation for a level of hardness.
In a review by František Kožíšek, M.D., Ph.D. National Institute of Public Health, Czech Republic gives a good overview of the topic, and unlike the WHO, sets some recommendations for the maximum and minimum levels of calcium (40-80 mg/L) and magnesium (20-30 mg/L) in drinking water, and a total hardness expressed as the sum of the calcium and magnesium concentrations of 2-4 mmol/L.
Also, the National Research Council (National Academy of Sciences) advises that hard drinking water generally contributes a small amount toward total calcium and magnesium human dietary needs. It further states that in some instances, where dissolved calcium and magnesium are very high, water could be a major contributor of calcium and magnesium to the diet.
For some good information on how hard water may "stop" heart attacks, see this article at the BBC website at http://news.bbc.co.uk/1/hi/health/3396141.stm.
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