Lynch, I., Gheorghe, L. C., Hassas, N., Lee, R. G., Milligan, J., & DiGirolamo, L. (2026). The Permeable Lip: A Critical Knowledge Gap for Systemic Exposure to Lip Cosmetic Chemicals. Integrated Environmental Assessment and Management. https://doi.org/10.1093/inteam/vjag128
Lynch I, Gheorghe LC, Hassas N, Lee RG, Milligan J, DiGirolamo L. The Permeable Lip: A Critical Knowledge Gap for Systemic Exposure to Lip Cosmetic Chemicals. Integrated Environmental Assessment and Management. 2026. doi:10.1093/inteam/vjag128
Lynch, Iseult, et al. "The Permeable Lip: A Critical Knowledge Gap for Systemic Exposure to Lip Cosmetic Chemicals." Integrated Environmental Assessment and Management, 2026.
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RRM Academy Synopsis
Lip tissue is more permeable than facial skin, a review argues
A regulatory review argues that lip tissue is more permeable than facial skin. Lip products are assessed under skin frameworks, and EU and US exposure estimates use usage data from 2005, so the authors say internal dose may be underestimated for some ingredients. They describe the overall health risk as appearing low.
Key Findings
The vermilion (the transitional zone between lip skin and inner mucosa) has a stratum corneum of 3-5 cell layers that is incompletely keratinized, with capillaries just beneath the thin epithelium.
Buccal mucosa showed 14-, 989- and 389-fold higher permeability than keratinized skin for benzoic acid, caffeine and hydrocortisone, respectively. Comparative permeability data for the labial site remain limited.
EU guidance assumes lipstick or lip salve is applied twice per day. A 2025 estimate puts average lip salve use at 10-14 applications per day.
In a cited 32-product study, lead released in simulated digestion exceeded the FDA's allowable candy level in every sample. Estimated typical daily intake stayed below limits; the authors cautioned about chronic use.
Cited surveys indicate roughly 80-90% of conventional lipsticks and glosses carry a microplastic ingredient. The authors' best estimates put the unabsorbed share of such particles well above 99%.
Interpretation
This is a regulatory review of published studies and guidance documents. The authors state that they generated no data. Their argument rests on tissue structure, the pharmacology of oral mucosa and exposure surveys. The microplastic uptake figures are approximate, since precise human data are unavailable. Absorption data for the outer vermilion are scarcer than for inner oral mucosa. The review states that it does not assert lip cosmetics are unsafe in themselves. The 2025 usage estimate is cited to a 2026 review by several of the same authors. Several authors lead or serve on a nonprofit that advocates lip product reform, and two co-founded a commercial lip care brand.
RRM Context
Restorative reproductive medicine looks for the causes of reproductive problems. Daily chemical exposure is part of the setting in which hormones work. The review reports no human hormone, fertility or pregnancy outcomes. It examines how regulators estimate exposure and how old their usage data are.
Our editorial summary of this paper, not the article's abstract.
Abstract
The pervasive use of lip cosmetics results in chronic, low-dose exposure to complex mixtures of chemicals via multiple routes, yet current cosmetic safety regulations treat lip products as conventional topical applications to skin. While the EU safety assessment framework acknowledges ingestion as an exposure route for lip products, standardised lip-specific testing models do not currently exist. The US requires no mandatory pre-market safety review for cosmetic ingredients and systemic exposure consideration is confined to voluntary industry assessment (except for colour additives). In both jurisdictions, exposure estimates are based on usage data from 2005 that do not reflect current real-world usage patterns of lip cosmetics, including frequent reapplication, product layering, and marketing based on food characteristics. The unique physiology of the lips, which represent a transitional tissue between external skin and oral mucosa provides an additional exposure route: the vermilion and labial mucosa are characterized by a markedly thinner and incompletely keratinised epithelium, reduced lipid barrier components, high vascularisation, and constant moisture-collectively conferring substantially higher permeability than facial skin. Evidence from pharmacology demonstrates that oral mucosal tissues are deliberately exploited for rapid systemic drug delivery. By comparing how the oral mucosal exposure route is evaluated across cosmetic, food, and pharmaceutical regulatory regimes, this review identifies a critical regulatory gap: chemicals applied to the lips are assessed under frameworks designed for low-permeability skin, despite a biological reality more consistent with mucosal exposure. This mismatch undermines the accuracy of current risk assessments and may lead to systematic underestimation of internal dose for certain lip cosmetic ingredients including microplastics. Addressing this gap will require lip-specific exposure models, physiologically relevant in vitro systems, and updated regulatory guidance to ensure that cosmetic safety assessment accurately reflects the unique exposure profile of lip products.
