Drinking hard water may protect against heart disease, researchers have claimed.
Researchers from the Geographical Survey of Finland looked at 19,000 men who had suffered heart attacks.
They found for every unit increase in water hardness, there was a 1% decrease in the risk of having a further attack.
Writing in the Journal of Epidemiology and Community Health, the researchers said the findings explained regional variations in heart attack rates.
They said the differences of up to 40% between areas could not be explained solely by lifestyle or genetic factors.
The team looked at men aged between 35 and 74, who had had an initial heart attack in the years 1983, 1988, and 1993.
They also examined national geological survey data on water hardness and trace elements, divided up into 10 by 10 kilometre grids.
Hard water is any water which contains an appreciable quantity of dissolved minerals
The researchers looked at measurements levels of calcium, magnesium, fluoride, iron, copper, zinc, nitrate and aluminium from almost 12,500 groundwater samples.
They suggest higher fluoride levels were protective, with every one milligram of fluoride per litre of household drinking water was associated with a 3% decrease in the risk of a heart attack.
But for every microgram of iron per litre, risk increased by an average of 4%, and for every microgram of copper per litre of water, it increased by 10%.
Writing in the Journal of Epidemiology and Community Health, the researchers, led by Dr Anne Kousa, said: "The large geographical variations and changes in the incidence of heart attacks in Finland cannot be explained by individual lifestyle or genetic factors alone.
"Environmental exposures must also contribute to the development of the disease."
But Professor Jeremy Pearson, Associate Medical Director at the British Heart Foundation, told BBC News Online: "There have been several studies going back more than 35 years examining the relationship between incidence of coronary heart disease and hardness of local water supplies, with inconsistent results.
"It is not implausible that water hardness might affect disease rates, since it relates to the levels of trace elements that may be important for nutrition. However, the contribution of drinking water to the total intake of these elements is usually low."
He added: "This study concludes that the incidence of acute myocardial infarction is significantly lower in areas of the country where water is harder.
"However, it is clear that any effect that there might be is small by comparison with the well-known major risk factors, such as poor diet, physical inactivity, smoking and high alcohol intake."