The World Health Organization says that "there does not appear to be any convincing evidence that water hardness causes adverse health effects in humans".[1] In fact, the United States National Research Council has found that hard water can actually serve as a dietary supplement for calcium and magnesium.[9]
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 litre of water. The World Health Organization has reviewed the evidence and concluded the data was inadequate to allow for a recommendation for a level of hardness.[1]