Fragrance-free, dye-free and thoroughly rinsed is the sensible default, and the NHS lists washing detergent among the things that make atopic eczema worse (NHS, Atopic eczema: treatment). Two mechanisms sit behind that. Surfactants are irritants, damaging anyone's barrier at enough dose and contact time. Fragrance and preservatives are allergens, mattering only to people already sensitised. Residue links both to laundry, repeating the exposure every time you sweat into the fabric, which makes it part of your home environment.
Surfactants and what they leave behind
Sodium lauryl sulfate is what dermatology reaches for to provoke irritation deliberately. A European Society of Contact Dermatitis standardisation report on SLS exposure tests describes the "clinical and histopathological reactions caused by sodium lauryl sulfate" (Tupker et al, 1997).
A 2023 review of detergents and epithelial barrier dysfunction describes detergents as "solubilizing plasma membranes", and reports that sodium dodecyl sulfate, the same molecule as SLS, altered claudin-1 and occludin and reduced transepithelial electrical resistance at non-toxic doses. It "activates the NLRP3 inflammasome and induces caspase-dependent IL-1β secretion by human keratinocytes" (Wright et al, 2023).
Dose does not reach zero after the rinse. The same review reproduced those barrier effects using "a 1:10 dilution of residual liquid from laundered clothing", in cultured airway cells rather than skin, so the residue stays active after a normal wash. It also notes that "atopic dermatitis patients are more susceptible to SDS-induced irritant dermatitis". NICE names "soaps and detergents" among the irritants to reduce in atopic eczema (NICE CG57).
Fragrance and preservatives are the allergens that matter
Allergy is a different process. It needs prior sensitisation, produces a delayed reaction to a tiny quantity, and is confirmed by patch testing rather than inferred from a flare. A short list of named substances accounts for most positive tests, so read the full ingredient list.
Among 18,832 patients patch tested with the European baseline series across 2021 and 2022, nickel sulphate led at 18.85%, ahead of fragrance mix I at 6.39%, methylchloroisothiazolinone plus methylisothiazolinone at 5.52%, methylisothiazolinone alone at 5.28% and fragrance mix II at 3.63% (Uter et al, 2026). Fragrance is among the most common positives, not the leading one.
A retrospective study of 6,435 patients patch tested at a Copenhagen hospital from 2014 to 2023 found benzisothiazolinone climbing from 0.3% or lower to 5.0% as methylisothiazolinone fell, and records that "the most common product categories linked to contact allergy to BIT were detergents, paints, cutting fluids and other industrial chemicals" (Søgaard et al, 2025).
Be careful what you conclude. The EDEN Fragrance Study patch tested 3,119 people from five European general populations: 224 (7.2%) reacted to at least one of 21 fragrance allergens, but household product exposure was not significantly associated with fragrance contact allergy (van Amerongen et al, 2021).
Enzymes, and the trial that tested the advice
Biological detergents carry proteases, lipases and amylases, and the instruction to avoid them is older than the evidence. Twenty-five people with atopic dermatitis completed a randomised, double-blind crossover of three one-month phases: their usual detergent, then a trial detergent with added enzymes and the same detergent without them, in random order, with corticosteroid use and SCORAD as the primary measures. "Analyses of all data revealed no statistical differences in any of the primary or secondary parameters comparing treatment and placebo periods" (Andersen et al, 1998).
One month per phase in 25 people cannot rule out an individual reaction, but it undercuts the blanket rule. If non-biological changed nothing for you, look at the fragrance, the preservative or the dose.
Optical brighteners and fabric softener
Optical brighteners are fluorescent compounds that absorb ultraviolet light and re-emit it as visible blue, so white cloth looks whiter. They are engineered to bind to the fibre and survive rinsing, so they stay on the surface that meets your skin: a residue argument for a brightener-free detergent on broken skin.
Fabric softener is deliberate residue too, a cationic film built to stay on the fibre through drying and wear, and the easiest thing to drop.
Dose, water hardness and the rinse
Hard water works against you twice. Calcium and magnesium consume surfactant, so the wash performs worse and the temptation is to overdose, while the reaction leaves deposits rinsing does not clear. In an experimental study of skin washing, "sites washed with hard water had significantly increased sodium lauryl sulfate deposits", and those deposits raised water loss through the skin and caused irritation, most of all in atopic dermatitis patients carrying filaggrin mutations (Danby et al, 2018). The hard water piece has the detail.
A softener is not the answer for eczema already established. The Softened Water Eczema Trial randomised 336 children to an ion-exchange softener plus usual care or usual care alone for 12 weeks, and severity improved similarly in both arms (Thomas et al, 2011). The lever you control is at the drawer: dose to the line for your load and hardness band rather than filling the cap, and add a rinse.
How to trial a switch
Detergent is a slow exposure, so a two-day test tells you nothing. Give a change four weeks, the phase length Andersen's trial used, and change one thing at a time.
Start with bedding, where contact runs unbroken for seven or eight hours. Clear the old residue first: wash the set once with the new detergent and once with none. Put swim kit on the same setting, since a costume holds residue against skin already sitting in pool water.
Keep a written log of the date, the change, and a repeatable severity measure, such as how many nights that week scratching woke you. Without one, ordinary variation reads as success or failure depending on the day, and the food diary piece sets out the method.
References
- NHS. Atopic eczema: treatment. https://www.nhs.uk/conditions/atopic-eczema/treatment/
- NICE. Atopic eczema in under 12s: diagnosis and management (CG57), 2025. https://www.ncbi.nlm.nih.gov/books/NBK571009/
- Wright BL, et al. Allergies come clean: the role of detergents in epithelial barrier dysfunction. Current Allergy and Asthma Reports, 2023. https://pmc.ncbi.nlm.nih.gov/articles/PMC10527525/
- Tupker RA, et al. Guidelines on sodium lauryl sulfate (SLS) exposure tests. Contact Dermatitis, 1997;37(2):53-69. https://doi.org/10.1111/j.1600-0536.1997.tb00041.x
- Uter W, et al. Patch test results with the European baseline series, 2021/2022. Contact Dermatitis, 2026;95(1):17-32. https://pmc.ncbi.nlm.nih.gov/articles/PMC13238334/
- Søgaard R, et al. Trends in contact allergy to preservatives from 2014 to 2023: benzisothiazolinone on the rise. Contact Dermatitis, 2025. https://pmc.ncbi.nlm.nih.gov/articles/PMC12318914/
- Andersen PH, et al. Skin symptoms in patients with atopic dermatitis using enzyme-containing detergents. A placebo-controlled study. Acta Dermato-Venereologica, 1998;78(1):60-62. https://medicaljournalssweden.se/actadv/article/view/14550
- van Amerongen CCA, et al. Skin exposure to scented products and fragrance contact allergy in the European general population. Contact Dermatitis, 2021. https://pmc.ncbi.nlm.nih.gov/articles/PMC8247875/
- Danby SG, et al. 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/
- Thomas KS, 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