Scientists found that water hardness was more strongly linked with the skin disorder than traffic pollution or any other suggested hazard.
The study, reported in The Lancet medical journal, involved 4,141 primary school children and 3,499 secondary school children in southern Nottinghamshire.
Researchers led by Dr Nick McNally, from the University of Nottingham, found that 17.3 per cent of primary school children living in areas with the hardest water had suffered eczema for a year, and 25.4 per cent had had the disorder all their lives.
This compared with 12 per cent of primary school children in soft-water areas who had eczema for a year, and 21.2 per cent who had lifetime eczema. The same association between hard water and eczema was not seen among children of secondary school age.
Hard water has been suggested as a risk factor for eczema before, but until now the link had not been scientifically established.
The researchers said the association for primary school children was "highly significant" both before and after adjustment for confounding factors.
Unadjusted lifetime prevalence of eczema was 4.2 per cent higher for areas with the hardest water than for areas with the softest water. Adjustment for age, sex, socio-economic status and distance from the nearest health centre had little effect on the results.
The researchers said: "The effect of water hardness on eczema prevalence is stronger than the reported effects of traffic pollution or any other spatially disparate suggested risk factors for eczema."
The scientists said water hardness may worsen or prolong existing eczema due to calcium and magnesium in the water acting as chemical irritants. Alternatively, there may be an indirect association because more soap and shampoo was needed to obtain a lather in hard water.
The lack of an association between eczema and secondary school children suggested that the effect may be age-related.Reuse content