A Permeable Skin Barrier: What EU Studies Really Show About Penetrating Ingredients

Note: image partly or fully created with AI.

The short answer

The skin barrier isn't an absolutely impermeable shield, but selectively permeable: molecules under 500 daltons that are small and fat-soluble can overcome the top layer of the skin. The EU Cosmetics Regulation therefore requires certain substances to be tested for skin absorption. Most INCI ingredients, however, stay on the surface – what matters are the exceptions.

How permeable is the skin really?

The top layer of the skin, the stratum corneum, works on the so-called ‘bricks and mortar’ principle: skin cells (bricks) are embedded in a lipid matrix (mortar), together forming a very effective but not absolute barrier. Substances can penetrate in three ways: between the skin cells through the lipid layer, through the cells themselves, or via hair follicles and sweat glands. As a rough rule of thumb, dermatology uses the so-called 500-dalton rule: molecules over 500 daltons practically can't overcome an intact stratum corneum, and almost all known contact allergens and topically active substances are well below this. Besides molecular size, fat solubility, concentration, contact time, occlusion (e.g. under dressings) and the condition of the skin itself determine the extent of absorption – damaged, inflamed or very dry skin is generally more permeable than an intact barrier.

What does the EU Cosmetics Regulation say?

The EU Cosmetics Regulation (EC) No 1223/2009 requires manufacturers to carry out a safety assessment that explicitly includes dermal absorption and possible systemic effects before a product is placed on the market. This is specified in the ‘Notes of Guidance’ of the Scientific Committee on Consumer Safety (SCCS), most recently revised in the 12th revision in 2023: they set out how skin absorption is measured and that, for leave-on products, complete absorption of the amount used should be assumed in case of doubt unless data shows otherwise. Since 2023, there has also been a separate, updated SCCS guideline for nanomaterials that requires separate testing of penetration. The EU microplastics restriction (EU) 2023/2055, in force since October 2023, and the PFAS restriction revised by ECHA in 2025 also show that the question of whether and how strongly a substance is absorbed through the skin or accumulates in the environment is an active, evolving area of regulation – not a closed topic.

Which substances are really in focus? A critical assessment

Differentiation rather than generalisation is worthwhile here. The often-cited myth that ‘60 percent of all cosmetic ingredients enter the bloodstream’ isn't scientifically tenable – most INCI substances are simply too large, too water-soluble or too unstable to penetrate the body to a relevant extent. That's exactly why the pharmaceutical development of transdermal active ingredients is a separate, complex field of research: deliberately penetrating the skin is technically difficult, not trivial.

However, there is a clearly definable group of substances for which systemic absorption has been independently proven. A much-cited FDA study published in the journal JAMA in 2020 showed that common chemical UV filters such as oxybenzone, octocrylene and homosalate were detectable in the blood after repeated use – in some cases even three weeks after the last application. The authors expressly stressed that this is no reason to stop using sunscreen, but is a reason for further safety testing. In diffusion cell studies, parabens are considered ‘moderately’ skin-permeable, with weak oestrogen-like activity in cell experiments. For the PFAS compound PFOA, a self-experiment by the German Federal Institute for Risk Assessment (BfR) showed actual absorption through the skin from sunscreen-like formulations. On the other side are mineral zinc oxide and titanium dioxide, as required in certified natural cosmetics: according to the SCCS assessment, these particles remain predominantly in the stratum corneum even in nano form and are not absorbed to a relevant extent. So the difference doesn't lie in whether a substance sounds ‘chemical’ or ‘natural’, but in molecular size, solubility and independently verified data.

In practice: what to look for in the INCI list

Cleansing without unnecessary additives

Cleansing is already a good starting point: The Facial Cleanser is formulated to be skin-friendly and does without aggressive sulphates, so the barrier isn't damaged before other products are applied.

Choose mineral formulations for UV filters

If you follow the discussion about chemical UV filters critically, The Sun Kiss SPF 30 offers a mineral alternative based on zinc oxide which, according to current SCCS data, doesn't penetrate the skin to a relevant extent.

An intact barrier is the best protection

Since damaged skin is generally more permeable, a stable skin barrier also reduces the uncontrolled absorption of unwanted substances. The Hydra Barrier Repair Set supports exactly this with ceramides and soothing ingredients.

Frequently asked questions about the permeability of the skin barrier

Does this mean I should generally use fewer cosmetics?

Not across the board. Most ingredients are harmless because they don't overcome the barrier to a relevant extent. Rather than giving things up, it makes more sense to check the INCI list specifically for the few independently identified problem groups, such as certain chemical UV filters, parabens or PFAS.

Are natural cosmetics ingredients automatically less permeable?

Not automatically, but certified natural cosmetics standards such as COSMOS specifically exclude groups of substances for which independent studies show relevant skin absorption or hormonal effects, such as certain chemical UV filters, PEG compounds and mineral-oil-based additives.

How reliable is the EU safety assessment of cosmetic ingredients?

The EU Cosmetics Regulation requires a mandatory safety assessment including skin absorption before a product is placed on the market, and the SCCS guidelines are updated regularly. As the ongoing debates about PFAS and microplastics show, however, this is an evolving field – new findings also lead to subsequent changes for substances that have already been approved.

Conclusion

The skin barrier protects reliably but isn't an absolute wall – it lets small, fat-soluble molecules through to varying degrees, while the vast majority of cosmetic ingredients stay on the surface. It's exactly this differentiation that makes a conscious look at the INCI list worthwhile, without falling into a blanket fear of ‘chemicals’. With a barrier-friendly routine such as Cosmenia's Hydra Barrier Repair Set, you can put this approach straight into daily practice.