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Background: Eczema is a chronic inflammatory skin disease. Domestic water with high mineral content (hard water) is a risk factor for eczema in children, but this association has not been assessed in adults., Objectives: To examine the association between domestic hard water supply and eczema prevalence and incidence in adults aged 40-69 years and the contextual effect in eczema outcomes by postcode in adults in the UK., Methods: We used data from the UK Biobank study collected in 2006-10 (baseline) and 2013-14 (follow-up). Eczema prevalence at baseline (2006-10) and at follow-up (2013-14) and incidence (new onset between baseline and follow-up) were determined from the touchscreen questionnaires and nurse-led interviews. Domestic hard water information was obtained in 2005 and 2013 from the local water supply companies in England, Wales and Scotland as CaCO 3 concentrations. We fitted multilevel logistic regression models with random intercepts for postcode areas to examine the effect of domestic hard water on eczema outcomes, and we measured components of variance., Results: In total, 306 531 participants with a mean age of 57 years nested across 7642 postcodes were included in the baseline analysis, and 31 036 participants nested across 3695 postcodes were included in the follow-up analysis. We observed an increase in the odds of eczema at baseline [odds ratio (OR) 1·02, 95% confidence interval (CI) 1·01-1·04] per 50 mg L -1 of CaCO 3 increase. Furthermore, exposure to domestic hard water (> 200 mg L -1 of CaCO 3 ) was associated with increased odds of prevalent eczema at baseline (OR 1·12, 95% CI 1·04-1·22). Moreover, there was a significant linear trend (P < 0·001) in which increasing levels of hard water increased eczema prevalence risk. No association was observed with incident eczema or eczema at follow-up. The intraclass correlation coefficient for postcode was 1·6% (95% CI 0·7-3·4), which remained unexplained by area-level socioeconomic measures., Conclusions: Increasing levels of domestic hard water, as measured by CaCO 3 concentrations, were associated with an increased prevalence of eczema in adults but not increased incidence. Ongoing efforts to reduce hard water exposure may have a beneficial effect in reducing the burden of eczema in adults. Further research is needed to explore area-level factors that may lead to eczema. What is already known about this topic? Hard water is formed when minerals are dissolved in water from filtration through sedimentary rocks. Several studies have reported a higher prevalence of eczema in areas with hard water. However, all studies on this topic have assessed this in infants and school-aged children, while this association has not been explored in adults. What does this study add? Our findings suggest that exposure to higher concentrations of domestic hard water is associated with an increase in eczema prevalence in adults aged 40-69 years. Ongoing efforts to reduce hard water exposure may have a beneficial effect in reducing eczema prevalence in adults., (© 2022 The Authors. British Journal of Dermatology published by John Wiley & Sons Ltd on behalf of British Association of Dermatologists.)