Hard water is not a treatable cause of eczema you already have, but it makes every wash more damaging. Calcium and magnesium bind to the surfactants in soap and wash products, leaving deposits that rinsing does not fully remove, and those deposits increase water loss through the skin and provoke irritation (Danby et al, 2018). The most useful response is not an appliance. It is removing the soap side of that reaction and putting emollient back on damp skin.
What water hardness is, in numbers
Hardness is the concentration of dissolved calcium and magnesium that water picked up from the rock it passed through, reported in milligrams per litre of calcium carbonate. Thames Water classifies water up to 100 mg/l as soft, 100 to 150 as slightly hard, 150 to 200 as moderately hard, 200 to 300 as hard, and anything above 300 as very hard. Its own supply is hard across the region because the water passes repeatedly through chalk and limestone (Thames Water, Water hardness).
Geology sets the map. Supplies drawn through chalk and limestone are hard, which covers much of the South East, East Anglia and the East Midlands. Rather than guessing from a regional impression, look up the figure: water companies publish hardness by postcode, and that number tells you which band you are actually in.
Why hard water makes washing worse
Soap and most wash products clean through anionic surfactants. In hard water those surfactants react with calcium and magnesium to form insoluble salts, the same scum that rings a bath and dulls glassware. The precipitate does not rinse cleanly, and it holds surfactant against the surface of the skin after you have finished washing.
That mechanism has been measured directly. Danby and colleagues recruited 80 participants: healthy controls, and people with atopic dermatitis with and without filaggrin mutations. Each person's skin was washed with sodium lauryl sulfate in water of varying hardness and chlorine concentration, rinsed, then covered with chambers to isolate the effect of what was left behind. "Sites washed with hard water had significantly increased sodium lauryl sulfate deposits. These deposits increased transepidermal water loss and caused irritation, particularly in AD patients carrying FLG mutations" (Danby et al, 2018). Softening the water by ion exchange reduced those effects.
Two things follow. Surfactant that stays in contact keeps dissolving the lipids that hold the skin barrier together, so the wash continues after the tap is off. And soap residue is alkaline against a surface that is normally acidic. People carrying filaggrin mutations start with less defence, which is why the same wash costs them more: in the washing experiment, irritation from the deposits was most pronounced in that group.
What the evidence shows, and what it does not
Two findings are often merged into one claim, and they do not say the same thing.
The observational signal is in infancy. Perkin and colleagues recruited 1,303 three-month-old infants, took domestic calcium carbonate and chlorine concentrations from local water suppliers, examined each infant for atopic dermatitis and screened for filaggrin mutations. Visible eczema was more common in the high calcium carbonate, low chlorine group than in the low calcium, low chlorine baseline: an adjusted odds ratio of 1.87 (95% CI, 1.25 to 2.80; P = .002). The high calcium, high chlorine group sat at 1.61. Effect estimates were larger in children carrying filaggrin mutations, though the formal interaction test was not statistically significant. The study was cross-sectional, measured at a single point, so it establishes an association rather than a cause (Perkin et al, 2016).
The treatment test came out flat. The Softened Water Eczema Trial randomised 336 children aged 6 months to 16 years to an ion-exchange softener plus usual care, or usual care alone, for 12 weeks. Severity fell in both arms: a mean SASSAD change of 5.0 points (20% improvement) with a softener and 5.7 points (22%) without it, a mean difference of 0.66 (95% CI, -1.37 to 2.69; p = 0.53) (Thomas et al, 2011).
Read together, the risk signal sits in early life and the softener was tested on eczema that had already developed. A softener has not been shown to treat eczema that has already developed.
What to change at the tap
Wash with an emollient instead of soap. The NHS advice for atopic eczema is to "wash with an emollient instead of soap", and in a hard water area it does double duty: no anionic surfactant means no lime soap deposit for the calcium to form.
Rinse for longer than feels necessary, and keep the water cool. The residue is what causes the harm, so rinsing is the step worth extending, and the NHS also advises keeping cool because heat increases itch.
Apply emollient while the skin is still damp, at least twice a day (NHS, Atopic eczema treatment). Timing and quantity matter more than the product, which is covered in how to moisturise for eczema.
Consider a softener on its merits, not as therapy. Ion exchange swaps calcium and magnesium for sodium, which removed the deposit effect in Danby's experiment, but the trial in children with established eczema found no severity benefit over usual care.
Treat shower filters as untested for this. The trials ran on plumbed ion-exchange softeners, so a filter rests on the mechanism rather than on trial evidence.
Chlorine, and why pools are a separate question
Domestic chlorine and hardness track each other closely, which makes them difficult to separate. Perkin's group concluded that the influence of chlorine remained uncertain, and Danby's washing experiment observed no clear effect of chlorine. Tap chlorine is not the main suspect here.
Pool water is a different problem. The irritants there are chloramines, formed when chlorine meets sweat and skin cells, and the alkaline pH pools are kept at. That is a distinct set of steps before and after the water, covered in eczema and swimming.
References
- Danby SG, Brown K, Wigley AM, Chittock J, Pyae PK, Flohr C, Cork MJ. The Effect of Water Hardness on Surfactant Deposition after Washing and Subsequent Skin Irritation in Atopic Dermatitis Patients and Healthy Control Subjects. Journal of Investigative Dermatology, 2018. https://eprints.whiterose.ac.uk/id/eprint/120644/
- Perkin MR, Craven J, Logan K, et al. Association between domestic water hardness, chlorine, and atopic dermatitis risk in early life: a population-based cross-sectional study. Journal of Allergy and Clinical Immunology, 2016. https://kclpure.kcl.ac.uk/portal/en/publications/5810f63c-f581-4e68-b9f4-aebfa0b66223
- Thomas KS, Dean T, O'Leary C, et al. A Randomised Controlled Trial of Ion-Exchange Water Softeners for the Treatment of Eczema in Children. PLoS Medicine, 2011. https://journals.plos.org/plosmedicine/article?id=10.1371/journal.pmed.1000395
- NHS. Atopic eczema: treatment. https://www.nhs.uk/conditions/atopic-eczema/treatment/
- Thames Water. Water hardness. https://www.thameswater.co.uk/help/water-quality/water-hardness