Eczema has been blamed on everything from food to car exhaust fumes, but a new report shows the problem may be hard water.

Through a questionnaire, the details of the prevalence of eczema were obtained from the parents of 4,141 randomly selected primary school children and 3,499 secondary school children in southern Nottinghamshire. Geographical data was used to link the prevalence of eczema with domestic water hardness.

Among secondary school children, the trends were not significant. But among the primary school children the relationship was significant, even after adjusting for factors such as age, sex, socioeconomic status and access to health care.

The prevalence of eczema over one year was 17.3 per cent in the highest hard water areas and 12 per cent in the lowest. This translated into a lifetime prevalence of 25.4 and 21.2 per cent, respectively.

Hard water is high in both calcium and magnesium. The researchers suggest that topical exposure, rather than ingestion, of these elements (particularly calcium) may act as a direct irritant. They can also interact with other chemicals and may break down the skin's defence barrier and allow invasion of antigens (Lancet, 1998; 352: 527-31).