Hardness of drinking-water and cardiovascular disease.

In the past 10 years or so evidence has been accumulating about an environmental factor which appears to be influencing mortality in particular, cardiovascular mortality and this is the hardness of the drinking-water. There is an inverse association between indices of water hardness and cardiovascular death rates ; the softer the drinking-water the higher the death rates. This association has been found in several very different countries: in Japan (Kobayashi, 1957); in the USA (Schroeder, 1960a,b, 1966; Masironi, 1970); in the UK (Morris, Crawford & Heady, 1961 ; Crawford, Gardner & Morris, 1968); in Sweden (Biorck, Bostrom, Widstrom, 1965); in Ireland (Mulcahy, 1968); and in others. Although different components of cardiovascular mortality are more clearly involved in some of these countries thanin others (e.g. cerebrovascular disease in Japan andmyocardial degeneration in Sweden), the similarity of the findings in the largest studies, those in the USA and the UK, is remarkable. The closest associations between death rates and water indices have been found in the UK (Crawford et al. 1968). In the sixty-one large county boroughs of England and Wales the correlation coefficients between cardiovascular death rates and water hardness are negative and highly significant ; they arc greatest with water calcium and the carbonate fraction; correlations with sodium are much lower and with magnesium, negligible. The main subgroups of cardiovascular disease cerebrovascular, ischaemic heart, and ‘other’ heart disease, which includes hypertensive heart disease and the myocardial degeneration deaths are all closely associated with water calcium (Table I). Bronchitis death rates show a lesser but still highly significant association with water hardness and calcium; this is of interest as many bronchitics die of cardiac complications. The rest of mortality is also negatively correlated but the coefficients are smaller. Findings at ages 65-74 years are similar. Fig. I shows the trend of cardiovascular death rates for men aged 45-64 in association with the calcium content of the drinking-water in these sixty-one county boroughs. The death rates in some of the soft-water towns are 1.5 times those in the very hard-water towns, indicating that a large number of deaths are involved in these differences. These findings are not of course evidence of a direct ‘causal’ relationship between drinking-water and mortality ; the correlations could be due to some confounding factor (or factors). In a comprehensive search for such, over a IOO indices of local