Hard Water, Chlorine and Eczema: What the Research Actually Shows
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Short answer
The evidence linking hard water to eczema is reasonably strong and has a plausible mechanism: hard water leaves more surfactant residue on skin after washing, which increases water loss and irritation.[1] The evidence linking chlorine at tap-water concentrations to eczema is much weaker — the key experimental study looking for a chlorine effect did not find a clear one.[1] And the one randomised trial that installed water softeners in babies' homes was a pilot that found a difference too small and too uncertain to be conclusive.[3] Water chemistry is a real contributing variable in eczema. It is not a cause, and no shower filter is a treatment.
What this article is. A plain-language summary of the peer-reviewed research on water hardness, chlorine and atopic dermatitis, including what the studies failed to show. Every claim below is linked to a primary source with a DOI. We sell shower filters, so we have flagged the places where the evidence does not support what our own category typically claims.
The question researchers were trying to answer
Epidemiologists noticed something in the 1990s and 2000s: children living in hard water areas appeared to have higher rates of atopic dermatitis (eczema) than children in soft water areas. Correlations like that are cheap. They can reflect socioeconomics, climate, housing, diet, or a dozen other things that happen to track with geology.
So the research question became a harder one: is there a mechanism? If hard water genuinely damages the skin barrier, you should be able to demonstrate it in a controlled setting, and you should be able to intervene on it. Three pieces of work — a mechanism study, a systematic review, and a randomised trial — attempted exactly that. They are worth reading in that order.
The mechanism study: hard water and surfactant deposition
In 2018, a team led by Dr Simon Danby at the University of Sheffield, working with colleagues at King's College London, published a study in the Journal of Investigative Dermatology that is still the clearest explanation of how hard water affects skin.[1]
The design was direct. They washed skin sites on people with atopic dermatitis and on healthy controls, using hard water and softened water, with and without a surfactant (sodium lauryl sulfate, the standard experimental irritant). Then they measured what was left behind on the skin and what happened to barrier function.
The mechanism this establishes is not "hard water burns your skin." It is subtler and more interesting. Calcium and magnesium ions react with anionic surfactants to form poorly soluble complexes that rinse away badly. Wash in hard water and you leave more detergent on your skin. Detergent left on skin disrupts the lipid barrier. Barrier disruption causes water loss and irritation.
In other words: hard water is not the irritant. It changes how well you can rinse off the irritant you are already using. That distinction matters enormously for what you should do about it.
The systematic review and meta-analysis
In 2021, a group including several of the same researchers published a systematic review and meta-analysis in Clinical and Experimental Allergy, pooling the available studies on water hardness, eczema and skin barrier function.[2]
A systematic review sits above any single study in the evidence hierarchy, because it accounts for the studies that found nothing as well as the ones that found something. This review found an association between domestic water hardness and atopic eczema, consistent with the mechanism work — while also documenting the heterogeneity and methodological limits across the underlying studies.
This is roughly the state of the science: a reproducible association, supported by a demonstrated mechanism, in a field where individual studies vary in quality.
The SOFTER trial: what happened when softeners were installed
The strongest possible test is an intervention. If hard water contributes causally to eczema, then removing hardness before a baby is born should reduce how many babies develop it.
That is exactly what the SOFTER trial (Softened Water for Eczema Prevention) did. Led from Guy's and St Thomas' NHS Foundation Trust and King's College London, it installed domestic ion-exchange water softeners in the homes of families in hard water areas before birth, where the infants were at high risk of atopy, and followed the babies to six months.[3]
You will see this trial cited by water treatment companies as proof that softening prevents eczema. It is not. It is a promising, underpowered pilot, and the honest reading is "plausible, unproven, worth a bigger trial."
What the research says about chlorine specifically
Here is where our industry's marketing and the published evidence diverge most sharply, so we will be precise.
At tap-water concentrations, the direct evidence that chlorine causes eczema is weak. The Sheffield mechanism study specifically looked for a chlorine effect and reported that a clear one was not observed.[1] That is a null result from the best-designed study in the area, and it should carry weight.
At higher, sustained exposures, there is evidence of barrier effects. A 2021 study of young elite athletes found significant increases in transepidermal water loss on the forearm, antecubital flexure and hand dorsum after swimming training, compared with baseline and with non-water sports — suggesting that the swimming pool environment can impair stratum corneum barrier function.[4] Pool water, however, is a different exposure from shower water: higher chlorine concentration, far longer immersion, plus chloramines formed from organic load.
There is also older work indicating that free residual chlorine in bathing water reduces the water-holding capacity of the stratum corneum in atopic skin. This is directionally consistent with what people report anecdotally, and it is the most defensible published basis for the comfort benefit of dechlorinating a shower.
So the fair summary is: chlorine at shower concentrations plausibly affects how skin feels, and there is some evidence for effects on stratum corneum hydration. It has not been shown to cause eczema, and the best mechanism study looking for that effect did not find it.
Evidence summary table
| Claim | Evidence strength | Best available source |
|---|---|---|
| Hard water increases surfactant residue left on skin after washing | Strong — directly measured, with mechanism | Danby et al., J Invest Dermatol 2018[1] |
| Domestic water hardness is associated with atopic eczema | Moderate — consistent association across pooled studies | Jabbar-Lopez et al., Clin Exp Allergy 2021[2] |
| Installing a water softener prevents eczema in infants | Unproven — pilot trial, confidence interval crosses zero | Jabbar-Lopez et al. (SOFTER), Clin Exp Allergy 2022[3] |
| Chlorine at tap-water concentrations causes eczema | Not supported — clear effect not observed in mechanism study | Danby et al., J Invest Dermatol 2018[1] |
| Sustained chlorinated-water exposure impairs skin barrier function | Moderate, in pool conditions — not directly transferable to showers | Paciência et al., Int J Environ Res Public Health 2021[4] |
| A shower filter treats or prevents eczema | No evidence — no trial has tested this | — |
What this research does not show
Five things get claimed on the back of these studies that the studies do not support:
- That hard water causes eczema. Eczema is a complex condition with strong genetic determinants, notably filaggrin mutations. Water hardness appears to be a contributory environmental factor that interacts with existing barrier vulnerability. It is not the cause.
- That softening water prevents eczema. The only randomised trial to test this was a pilot and was not statistically conclusive.
- That chlorine in tap water is a proven skin irritant at those concentrations. The best mechanism study did not observe a clear chlorine effect.
- That pool findings transfer to showers. Concentration, duration and chemistry all differ substantially.
- That a shower filter does what a water softener does. The Sheffield and SOFTER studies both used ion-exchange softening, which removes calcium and magnesium. Shower filters do not perform ion exchange. You cannot cite softener research as evidence for a shower filter, and we will not.
Not medical advice. This article summarises published research for general information. FiltraZen products are not medical devices and are not intended to diagnose, treat, cure or prevent eczema, atopic dermatitis or any other condition. If you or your child has eczema, please speak to a GP, dermatologist or pharmacist about evidence-based management, which typically centres on emollients and prescribed topical treatment.
What this means for you
If you have eczema-prone skin and you are wondering whether your water is part of the picture, the research supports a short and unglamorous list of actions.
- Change the surfactant before you change the water. The mechanism runs through your cleanser. Switching to a soap-free, low-surfactant or syndet wash addresses the causal pathway directly, costs less than any filtration, and is what the mechanism study implies.
- Use less product, and rinse thoroughly. In hard water the temptation is to add more product to chase lather. That is the exact behaviour the research suggests is counterproductive.
- Turn the temperature down and keep showers shorter. Heat and duration affect the lipid barrier independently of water chemistry.
- Moisturise immediately after showering, while skin is still damp. This is standard dermatological advice and has a far stronger evidence base than anything on this page.
- Know your water before you spend on it. Look up your hardness and disinfectant type in the Australian hardness reference. If you are in soft-water Melbourne, hardness is not your variable.
- Treat a shower filter as a comfort purchase. If chlorine smell or post-shower tightness bothers you, dechlorination is the thing shower filtration genuinely does well, and it is reasonable to try it on those terms. It is not a treatment, and if a retailer tells you otherwise, that is a reason to shop elsewhere.
From FiltraZen
We make a shower filter. Based on the evidence above, what we will claim for it is this: it reduces disinfectant residual, sediment and some dissolved metals, which changes how shower water feels. We will not claim it treats eczema, because no one has tested that.
Common questions
Does hard water cause eczema?
No — but it appears to contribute. Research from the University of Sheffield showed hard water increases how much surfactant is left on skin after washing, which raises transepidermal water loss and irritation, particularly in people with atopic dermatitis and filaggrin gene mutations. A 2021 systematic review found a consistent association between water hardness and atopic eczema. Eczema itself is driven principally by genetic and immune factors.
Does chlorine in tap water cause eczema?
There is no good evidence that it does at Australian tap-water concentrations. The 2018 Danby study specifically examined chlorine alongside hardness and reported that a clear effect of chlorine was not observed. There is separate evidence that sustained exposure to pool water, which is far more concentrated, can impair skin barrier function.
Do water softeners prevent eczema in babies?
Not proven. The SOFTER pilot randomised controlled trial found 33% of infants in the softener group had visible eczema at six months versus 48% in the control group, but the 95% confidence interval ran from −38% to +8.3%, meaning the result is compatible with no effect. The trial's purpose was to establish whether a full-scale trial would be feasible.
Can a shower filter help eczema?
No shower filter has been tested in a clinical trial for eczema, and none should be described as a treatment. Some people with sensitive skin report that dechlorinated shower water feels less drying, which is a comfort outcome rather than a clinical one. Eczema management should be guided by a health professional.
Is a shower filter the same as a water softener?
No. A water softener removes calcium and magnesium by ion exchange and regenerates with salt. A shower filter uses media such as activated carbon, KDF and calcium sulfite to reduce disinfectant residual, sediment and some metals. The eczema research on hardness used ion-exchange softeners, so it cannot be used as evidence for shower filters.
Sources
- 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 Jan;138(1):68–77. doi:10.1016/j.jid.2017.08.037. PMID 28927888.
- Jabbar-Lopez ZK, Ung CY, Alexander H, Gurung N, Chalmers J, Danby S, Cork MJ, Peacock JL, Flohr C. The effect of water hardness on atopic eczema, skin barrier function: A systematic review, meta-analysis. Clinical and Experimental Allergy. 2021 Mar;51(3):430–451. doi:10.1111/cea.13797. PMID 33259122.
- Jabbar-Lopez ZK, Ezzamouri B, Briley A, Greenblatt D, Gurung N, Chalmers JR, Thomas KS, Frost T, Kezic S, Common JEA, Danby S, Cork MJ, Peacock JL, Flohr C. Randomized controlled pilot trial with ion-exchange water softeners to prevent eczema (SOFTER trial). Clinical and Experimental Allergy. 2022 Mar;52(3):405–415. doi:10.1111/cea.14071. PMID 34854157.
- Paciência I, Rodolfo A, Leão L, Silva D, Cavaleiro Rufo J, Mendes F, Padrão P, Moreira P, Laerte Boechat J, Delgado L, Moreira A. Effects of Exercise on the Skin Epithelial Barrier of Young Elite Athletes — Swimming Comparatively to Non-Water Sports Training Session. International Journal of Environmental Research and Public Health. 2021;18(2):653. doi:10.3390/ijerph18020653. PMID 33466624.
- National Health and Medical Research Council. Hardness (as calcium carbonate) — Australian Drinking Water Guidelines, Part 5. NHMRC, Australian Government. guidelines.nhmrc.gov.au
- King's College London. SOFTER trial investigates if water softeners can prevent eczema. kcl.ac.uk
Written by Filtrazen Editorial Team
