Transepidermal Water Loss

Transepidermal water loss (TEWL) is the amount of water that passes from the body out through the epidermis and evaporates. It is measured without breaking the skin, using a probe (an evaporimeter) held against the skin. Results are given in g/m²/h. It is the standard objective measure of how well the skin barrier holds water in. Higher TEWL means a leakier barrier. TEWL is raised in Atopic Dermatitis, in both affected and unaffected skin. It is lower in the less common “intrinsic” type of AD, which has normal IgE levels and no sensitisations (Papapostolou et al., J Clin Med 2022). In the stratum corneum, Filaggrin and the water-binding Natural Moisturising Factor it breaks down into, together with lipids such as Ceramides, keep TEWL low (Agrawal & Woodfolk 2014).

Skin Barrier Dysfunction [causes] Transepidermal Water Loss Natural Moisturising Factor [prevents] Transepidermal Water Loss Ceramides [prevents] Transepidermal Water Loss

TEWL as a predictor in infants: Kelleher 2015. The key study is the Cork BASELINE birth cohort (Kelleher et al., J Allergy Clin Immunol 2015). It enrolled 1,903 infants and measured TEWL at day 2, 2 months and 6 months. 18.7% had AD at 6 months and 15.5% at 12 months. Babies in the top quarter for TEWL on day 2 were significantly more likely to have AD at 12 months (area under the ROC curve 0.81). Top-quarter TEWL at 2 months predicted even better (AUC 0.84). Babies in the bottom quarter on day 2 were protected. The effect held regardless of parental atopy, FLG mutation status, or an itchy flexural rash at 2 months. The authors concluded that “impairment of skin barrier function at birth and at 2 months precedes clinical AD”. This is direct evidence for the “outside-in” view of Skin Barrier Dysfunction: a leaky barrier comes first, and eczema follows.

Transepidermal Water Loss [precedes] Atopic Dermatitis Filaggrin [relates] Transepidermal Water Loss

Conflict: [Kelleher et al., J Allergy Clin Immunol, 2015] found that upper-quartile TEWL at day 2 and 2 months predicted AD at 12 months (AUC 0.81–0.84) in 1,903 Irish infants measured under controlled conditions. [Chittock et al., J Allergy Clin Immunol, 2024] found that TEWL “measured in ambient conditions… did not signal disease risk at any stage” in 128 English infants tested at home, where FLG alleles, low NMF and skin water content were the better signals. Unresolved — add to open_questions. (Differences in cohort size, measurement setting and disease severity, since the 2024 cases were all mild, may explain the gap.)

Food sensitisation and the atopic march. Papapostolou et al. (2022) describe barrier dysfunction and raised TEWL as “a cornerstone in atopic dermatitis pathophysiology” that “precedes both the occurrence of atopic dermatitis and food allergy”. This fits the Dual-Allergen Exposure Hypothesis, under which food proteins entering through a leaky barrier sensitise the infant, while eating the food early builds tolerance (Food Allergy, Atopic March).

Transepidermal Water Loss [relates] Food Allergy Transepidermal Water Loss [supports] Dual-Allergen Exposure Hypothesis

Practical implications for toddlers. If high TEWL comes before eczema, it might seem that moisturising newborns would prevent it. The large trials say no. The Cochrane review of skin care in infants (Kelleher et al. 2022 update; 33 RCTs, 25,827 infants, including the BEEP Trial) found that regular emollients in the first year “probably” do not prevent eczema (RR 1.03). They may increase food allergy and probably increase skin infections (Emollient Therapy). TEWL is therefore a marker of risk, not yet a treatment target for prevention. For a toddler who already has eczema, TEWL explains the bathing advice. Water from a bath evaporates fast through a leaky barrier, so an occlusive emollient should go on within about 3 minutes (Soak and Seal, Bathing Frequency; Medscape). Ceramide-dominant emollients lowered TEWL and severity scores in children in a small study cited by Medscape. TEWL testing is a research tool and is not used in routine care.

BEEP Trial [contradicts] Transepidermal Water Loss Soak and Seal [prevents] Transepidermal Water Loss Emollient Therapy [relates] Transepidermal Water Loss

Connections

• Skin Barrier Dysfunction — measurable output of a leaky barrier, source: Papapostolou, J Clin Med 2022
• Bathing Frequency — post-bath evaporation dries skin unless emollient applied, source: Cardona review 2016
• Atopic Dermatitis — high TEWL at day 2/2 months predicts AD at 1 year, source: Kelleher, JACI 2015
• Filaggrin — limits TEWL; FLG alleles reduce NMF and water content by 4 weeks, source: Agrawal & Woodfolk 2014; Chittock 2024
• Natural Moisturising Factor — water-binding NMF limits evaporation, source: Agrawal & Woodfolk 2014
• Soak and Seal — seal step within ~3 min stops bath water evaporating, source: Medscape
• BEEP Trial — neonatal emollients did not prevent eczema despite TEWL rationale, source: Cochrane, Kelleher 2022
• Ceramides — ceramide-dominant emollient lowered TEWL in children, source: Medscape
• Food Allergy — raised TEWL precedes food allergy, source: Papapostolou 2022