Cosmetics: what is the 500-Dalton rule?

The skin is a true interface between the body and its environment, able to limit the entry of potentially harmful substances while still allowing, to some extent, the passage of certain molecules. This barrier function relies mainly on the epidermis, and more specifically on its outermost layer: the stratum corneum. This layer is made up of dead cells, the corneocytes, embedded in a lipid matrix composed in particular of ceramides. Added to this is the surface hydrolipidic film, which contributes to the chemical and microbiological protection of the skin. This specific organization, often described as a “brick and mortar” structure, gives the skin a low permeability, which is essential for maintaining its hydration and preventing the entry of external agents.

However, this barrier is not completely impermeable.

When a cosmetic product is applied, its components can interact with the skin surface, provided they pass through certain steps. The first step is dissolution in the hydrolipidic film, which determines how well the active ingredient adheres to the surface of the skin. Once this step is completed, several penetration pathways are possible. The most common is the intercellular route, in which lipophilic molecules diffuse between cells by following the lipid network. The transcellular route, which is more direct but more demanding, involves passage through the cells themselves; it is more often used by hydrophilic molecules, provided the skin is sufficiently hydrated to facilitate their diffusion. Finally, a more marginal but interesting pathway is that of the skin appendages, particularly the pilosebaceous follicles, which can act as a reservoir and allow deeper diffusion.

In this context, the question of molecule size becomes central. It is precisely on this point that the so‑called 500 Dalton rule is based, a principle often cited to explain the ability of an active ingredient to cross the skin barrier. According to this empirical rule, a molecule must have a molecular weight lower than 500 Daltons (Da) in order to pass through the stratum corneum. Beyond this threshold, its diffusion through the skin becomes highly limited, or even impossible under normal conditions. The 500 Da rule is based on several observations from pharmacology and dermatology, one of which concerns allergens. Indeed, the vast majority of known allergens have a molecular weight below 500 Da, suggesting that only small molecules can penetrate deeply enough to interact with the immune system.

While the 500 Dalton rule provides an interesting theoretical framework, it should not be regarded as an absolute boundary.

In practice, skin penetration depends on a whole set of physicochemical and biological parameters. The lipophilicity of a molecule, for example, plays a decisive role: a molecule that can integrate into the lipid “cement” of the stratum corneum will diffuse more easily than a strictly hydrophilic molecule. Its three-dimensional structure and its ability to interact with epidermal lipids also influence its passage. Conversely, some large molecules, although unable to cross the skin barrier, can nevertheless exert surface effects or act indirectly by modulating the skin environment, such as the microbiome.

The effectiveness of an active ingredient does not depend solely on its ability to penetrate the skin.

Beyond the intrinsic characteristics of the molecules, many factors related to the skin itself modulate this permeability. Age, for example, influences the hydration and lipid composition of the stratum corneum, which can alter the diffusion of active ingredients. More fragile, aged skin is also more permeable. The application site is also important, since thin skin, such as that of the eyelids, is more permeable than thick skin such as that of the palms or soles. The condition of the skin also determines permeability, because damaged or irritated skin has an impaired barrier, which significantly increases the penetration of substances, whether beneficial or harmful.

The product characteristics also play a role. The dosage form (cream, gel, lotion, serum, etc.) determines how the active ingredient is released and its ability to interact with the skin. Likewise, the amount applied, frequency of use, and the surface area of application all modulate skin exposure and therefore the diffusion of active substances. Penetration thus results from a balance between the formulation, the mode of application, and the condition of the skin.

The 500 Dalton rule is a useful scientific benchmark, but a simplified one.

Formulators are not limited by this constraint and develop different strategies to optimize the diffusion of active ingredients, including when their size exceeds this theoretical threshold. Among the most commonly used approaches are encapsulation techniques (liposomes, nanoparticles, targeted delivery systems), which make it possible to “transport” the active ingredient through the stratum corneum by improving its solubility and stability. Microemulsions and nanoemulsions, thanks to their very small droplet size, also promote interaction with skin lipids and can enhance penetration. These strategies do not strictly “bypass” the 500-Dalton rule, but they illustrate how formulation can modulate the bioavailability of active ingredients by acting on parameters other than their molecular weight.

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• SCHEPKY A. & al. Measurement of the penetration of 56 cosmetic relevant chemicals into and through human skin using a standardized protocol. Journal of Applied Toxicology (2019).

• PICARD C. & al. Skin surface physico-chemistry: Characteristics, methods of measurement, influencing factors and future developments. Advances in Colloid and Interface Science (2019).

• Thèse de Mélanie CHAPUT. Pénétration transcutanée des actifs appliqués par voie topique : Comparaison des formes cosmétiques et pharmaceutiques (2023).

• NISHIZAWA M. & al. Breaking the 500-Dalton rule of transdermal drug delivery by locally stretching skin with porous micropost array. The Electrochemical Society (2024).