The epidemiologic perspective: water hardness and cardiovascular disease.

Studies of water hardness and cardiovascular disease are briefly reviewed, and the probability that the reported associations are those of cause-and-effect is assessed. While the verdict is "not proven", further studies are desirable because of the potential benefits that might accrue from water treatment. Future studies, however, need to be much more definitive than the ecological investigations of the past. Although such studies have been useful as pilot studies, the effects of a possible water factor cannot be disentangled from the effects of many personal characteristics that also vary with geography.