UK Biobank: As hard water levels increase, so does the burden of eczema

Exposure to domestic hard water in high concentrations appears to go with a corresponding increase in the prevalence of eczema in adults, as reported in a UK study.

Using data from the UK Biobank, a group of researchers from the University of Melbourne in Australia included 306,531 participants (2006–2010; mean age 57 years) nested across 7,642 postcodes and 31,036 participants (2013–2014) nested across 3,695 postcodes for the baseline and follow-up analyses, respectively. They defined hard water in terms of calcite (CaCO₃) concentrations

Every 50-mg L⁻¹ increase in CaCO₃ concentrations was found to be associated with greater odds of eczema at baseline (odds ratio [OR], 1.02, 95 percent confidence interval [CI], 1.01–1.04). Moreover, exposure to domestic hard water (>200 mg L⁻¹ of CaCO₃) positively correlated with prevalent eczema at baseline (OR, 1.12, 95 percent CI, 1.04–1.22). [Br J Dermatol 2022;doi:10.1111/bjd.21771]

There was a significant linear trend in which increasing levels of hard water occurred in parallel with increased eczema prevalence (p<0.001).

“The absence of an association found between domestic hard water and incident eczema may be due to a lack of statistical power,” the researchers pointed out.

“Furthermore, the estimates of clustering effect by postcode in eczema outcomes, although small, remain mostly unexplained by area-level socioeconomic measures, so further research is needed to explore which geographical factors may lead to eczema,” they added.

Damaging skin barrier

“One of the proposed environmental risk factors for eczema is domestic hard water, which is water with a high mineral content. Hard water is formed when minerals are dissolved in water from filtration through sedimentary rocks,” according to the researchers. [Expert Rev Clin Immunol 2017;13:15-26; J Chem Appl 2018;4:4]

Hard water contains an abundance of dissolved mineral particles, mostly calcium in the form of calcite and dolomite (CaMg[CO₃]₂). According to the World Health Organization (WHO), domestic water is hard when the concentration of CaCO₃ is >200 mg L⁻¹. At these concentrations, the use of excess soap is required to achieve lather or the formation of soap film (calcium stearate) on skin and clothes. [Br J Dermatol 2020;183:285-293]

The researchers specified a number of biological mechanisms by which hard water may damage the skin barrier, which in turn contributes to the development and progression to more severe and persistent eczema. The most recognized mechanism, they said, involves the increased use of products that contain sodium lauryl sulphate (SLS) and its buildup on the skin.

“SLS causes skin irritation and skin-barrier impairment, the extent of which is dependent on the hardness of the wash water. SLS residues left on the skin alter protein secondary structure, solubilize stratum corneum lipids, and elevate skin-surface pH in a dose-dependent manner,” they explained. [J Invest Dermatol 2018;138:68-77]

“Soap also reacts with calcium in hard water to form small chalk particles that can irritate the skin. In addition, calcium and magnesium are alkaline metals that form a basic solution with water, which can increase skin pH (normally mildly acidic) and compromise barrier function. Increased concentrations of the metal ion (Ca²⁺) in hard water may also alter calcium signalling in the epidermis and contribute to impaired skin-barrier function,” they continued. [Br J Dermatol 2020;183:285-293; J Invest Dermatol 2018;138:68-77; Ann Dermatol 2018;30:265-275]

All of these mechanisms are said to facilitate increased allergen penetration and bacterial colonization of the skin, both of which are risk factors for eczema development and progression. [Allergy 2009;64:abstract 1458]

“To the best of our knowledge this is the first study to evaluate the association between domestic hard water and eczema in a cohort of adults… Ongoing efforts to reduce hard water exposure in adults may have a relatively small beneficial effect in reducing eczema prevalence in adults,” the researchers said.