The EU/UK regulatory framework 

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Cosmetic products distributed within the European Economic Area and Great Britain are governed by comprehensive regulatory frameworks designed to ensure public health protection. In the EU, the primary governing instrument is Regulation (EC) No 1223/2009 (European Parliament and Council of the European Union, 2009). Following Brexit, this regulation was retained and adapted into domestic law for Great Britain through Schedule 34 to the Product Safety and Metrology etc. (Amendment etc.) (EU Exit) Regulations 2019, in force from 1 January 2021 (The Product Safety and Metrology etc. (Amendment etc.) (EU Exit) Regulations 2019).

 

Under Article 3 of Regulation (EC) No 1223/2009 and its retained UK equivalent, a cosmetic product placed on the market must be safe for human health when used under normal or reasonably foreseeable conditions of use (European Parliament and Council of the European Union, 2009). Legal responsibility for compliance rests on a designated "Responsible Person" (the manufacturer, importer, or an authorised representative established within the EU or UK) whose duties are enumerated in Article 5, and include ensuring the product meets the Regulation's safety obligations, holding the Product Information File, and cooperating with the competent authorities.
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Legally safe does not mean reaction-free

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Despite mandatory pre-market safety screening, many skincare products contain ingredients with the potential to trigger adverse skin reactions. Established reference lists identify such ingredients: the fragrance allergens EU law requires to be declared (Regulation (EC) No 1223/2009, Annex III), the substances included in standard patch-test series used to diagnose allergic contact dermatitis, and further ingredients documented as sensitisers in the literature. 

 

An analysis of 88 products formulated for sensitive skin, from 19 multinational brands sampled on the European market, found that 95% contained at least one non-fragrance ingredient appearing on these lists; of these, 78% contained two or more, and 27% contained five or more (Martins, M.S., et al., 2022). Presence on such a list does not mean a product commonly provokes reactions: the authors note that most of the ingredients they found are rare sensitisers responsible for few reported cases. Fragrance exposure, however, was largely undisclosed. Recognised fragrance allergens were individually declared on only 7% of labels, yet 64% carried the catch-all terms "parfum" or "aroma", which allow a whole fragrance blend to be declared under a single word without naming its constituents (European Parliament and Council of the European Union, 2009, Annex III). Since fragrances are a common cause of allergic contact dermatitis from cosmetics (Cheng, J. and Zug, K.A., 2014), the low individual-declaration figure likely understates fragrance-allergen exposure: most of these products are scented, but the composition of that scent is largely hidden on the label.

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‘Natural’ products can also sensitise

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A widespread consumer misconception is that "natural", "organic", "clean", or "green" skincare carries an inherent safety advantage over synthetic formulations. Unlike pure synthetic molecules made to precise purity specifications, botanical ingredients (essential oils, cold-pressed plant extracts, and flower absolutes) are chemically complex mixtures containing many distinct compounds, predominantly volatile terpenes (monoterpenes and sesquiterpenes) alongside related compounds such as esters. Several of these are recognised fragrance allergens. 

 

Clinical patch testing shows a statistically significant association between sensitisation to fragrance mixtures and reactivity to essential oils, a cross-reactivity grounded in their shared terpene chemistry (Paulsen, E. and Andersen, K.E., 2005), with the risk compounded by oxidation (Matura, M., et al., 2003).

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Why legal safety and skin reactions can coexist

 

The safety assessment a product must pass evaluates each ingredient's toxicological profile, including its potential to irritate and to sensitise, against the concentration and exposure anticipated in use, and concludes whether the product is safe under normal and reasonably foreseeable conditions. Contact allergy, however, behaves in ways that a pre-market assessment cannot fully resolve for every user.

 

Allergic contact dermatitis develops in two phases: an induction phase, in which exposure to an allergen primes the immune system, usually with no visible reaction, and a later elicitation phase, in which a subsequent exposure provokes the dermatitis in someone who has become sensitised (Kimber, I., et al., 2008). The two phases have different dose thresholds, and the relationship between them is not fixed: the more strongly a person has been sensitised, the lower the dose required to elicit a reaction (Kimber, I., et al., 2008). An amount of an ingredient that does not sensitise the general population can therefore still elicit dermatitis in an individual who is already sensitised to it.

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Whether elicitation occurs also depends on conditions that vary between individuals, product types, and between uses. The state of the skin matters: damage or irritation can significantly lower the threshold at which a reaction is provoked (Arts, J.H.E., et al., 2006). The dose per unit area of skin, the vehicle the ingredient is carried in, occlusion, and the frequency of exposure matter, too (Kimber, I., et al., 2008). 

 

A product can therefore meet every requirement of its safety assessment and still provoke a reaction in a particular person, without the assessment having failed.

 

What about comedogenic ingredients?

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The concept of "acne cosmetica" was introduced in 1972 by the dermatologists Kligman and Mills, who linked certain cosmetic ingredients to comedone formation (Kligman, A.M. and Mills, O.H., 1972). The 0 to 5 comedogenicity rating that followed, developed largely by Fulton and colleagues, graded pore-plugging by applying concentrated raw ingredients under occlusion to rabbit ears and human backs (Fulton, J.E., et al., 1984).

 

The 0 to 5 ratings have been widely criticised. The rabbit-ear assay proved overly sensitive and inconsistent when applied to human skin, and subsequent research reappraised both the assay and the concept, concluding that raw-ingredient ratings don't predict finished-product behaviour in humans (Kligman, A.M., 1996; Draelos, Z.D. and DiNardo, J.C., 2006).

 

Formulators nonetheless still use the scale to screen out potential high-risk raw materials when developing non-comedogenic products. Individual ingredient ratings correlate most directly with non-inflammatory lesions (blackheads and whiteheads) and have limited predictive value for inflammatory acne. They also do not account for the chemistry of the finished vehicle, in which an ingredient's behaviour can change, or for individual differences between users.

 

When people report "breaking out" within 24 to 48 hours of applying a new product, this is unlikely to be true comedogenesis. Comedone formation is slow and initially invisible. It begins with abnormal build-up of keratin and sebum that forms a microcomedone beneath the skin surface, the subclinical precursor to acne lesions, which then takes weeks to mature into a visible comedone (Cunliffe, W.J., et al., 2004). Eruptions appearing within a day or two are more consistent with an irritant or folliculitis-type reaction (direct chemical irritation of the follicle rather than a clogged pore) which single-ingredient comedogenicity scales neither measure nor predict.

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What Aelon IQ analyses 

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For irritation and sensitisation, our model classifies each ingredient as an established or a suspected irritant or allergen, based on an assessment of the underlying evidence: 'established' reflects human clinical or regulatory data, whereas 'suspected' reflects weaker evidence such as animal studies, in-vitro tests, case reports, or inference from chemical structure. It considers both which ingredients are flagged and how many, and reflects this in the product's score.

 

For comedogenicity, the model considers which ingredients carry a rating on the 0 to 5 comedogenic scale and reflects this in the score as well. As set out above, that scale is a coarse screening tool rather than a precise predictor.

 

Neither measure predicts whether a given person will react to a finished product. Contact reactions depend on an individual's own history of sensitisation, their skin barrier, the formulation the ingredient sits in, none of which the presence of flagged ingredients can capture. What this analysis offers is narrower: a view of how heavily a formulation draws on ingredients that carry a recognised potential to irritate, sensitise, or clog pores, so that a formulation's overall load of such ingredients is visible.

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References 

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Arts, J.H.E., Mommers, C. and de Heer, C. (2006) 'Dose-response relationships and threshold levels in skin and respiratory allergy', Critical Reviews in Toxicology, 36(3), pp. 219–251. 

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Cheng, J. and Zug, K.A. (2014) 'Fragrance allergic contact dermatitis', Dermatitis, 25(5), pp. 232–245. 

 

Cunliffe, W.J., Holland, D.B. and Jeremy, A. (2004) 'Comedone formation: etiology, clinical presentation, and treatment', Clinics in Dermatology, 22(5), pp. 367–374. 

Draelos, Z.D. and DiNardo, J.C. (2006) 'A re-evaluation of the comedogenicity concept', Journal of the American Academy of Dermatology, 54(3), pp. 507–512. 

 

European Parliament and Council of the European Union (2009) Regulation (EC) No 1223/2009 of the European Parliament and of the Council of 30 November 2009 on cosmetic products. Official Journal of the European Union, L 342, 22.12.2009, pp. 59–209. Available at: https://eur-lex.europa.eu/eli/reg/2009/1223/oj

Fulton, J.E., Pay, S.R. and Fulton, J.E. III (1984) 'Comedogenicity of current therapeutic products, cosmetics, and ingredients in the rabbit ear', Journal of the American Academy of Dermatology, 10(1), pp. 96–105. 

 

Kimber, I., Dearman, R.J., Basketter, D.A., Ryan, C.A., Gerberick, G.F., McNamee, P.M., Lalko, J. and Api, A.M. (2008) 'Dose metrics in the acquisition of skin sensitization: thresholds and importance of dose per unit area', Regulatory Toxicology and Pharmacology, 52(1), pp. 39–45.

 

Kligman, A.M. and Mills, O.H. (1972) 'Acne cosmetica', Archives of Dermatology, 106(6), pp. 843–850.

 

Kligman, A.M. (1996) 'Petrolatum is not comedogenic in rabbits or humans: a critical reappraisal of the rabbit ear assay and the concept of "acne cosmetica"', Journal of the Society of Cosmetic Chemists, 47, pp. 41–48.

 

Martins, M.S., Ferreira, M.S., Almeida, I.F. and Sousa, E. (2022) 'Occurrence of allergens in cosmetics for sensitive skin', Cosmetics, 9(2), 32. 

 

Matura, M., Goossens, A., Bordalo, O., Garcia-Bravo, B., Magnusson, K., Wrangsjö, K. and Karlberg, A.-T. (2003) 'Patch testing with oxidized R-(+)-limonene and its hydroperoxide fraction', Contact Dermatitis, 49(1), pp. 15–21. 

 

Paulsen, E. and Andersen, K.E. (2005) 'Colophonium and Compositae mix as markers of fragrance allergy: cross-reactivity between fragrance terpenes, colophonium and Compositae plant extracts', Contact Dermatitis, 53(5), pp. 285–291. 

 

The Product Safety and Metrology etc. (Amendment etc.) (EU Exit) Regulations 2019, SI 2019/696, Schedule 34. Available at: https://www.legislation.gov.uk/uksi/2019/696/schedule/34/made​​​