Cosmetics: what is the 500 Dalton rule?

The skin is a genuine interface between the body and its environment, able to limit the entry of potentially harmful substances while allowing, to some extent, the passage of certain molecules. This barrier function relies mainly on the epidermis, and more specifically on its most superficial layer: the stratum corneum. The latter is composed of dead cells, the corneocytes, embedded in a lipid “cement” made up in particular of ceramides. Added to this is the surface hydrolipidic film, which contributes to the chemical and microbiological protection of the skin. This particular organisation, often described as a “brick-and-mortar” structure, gives the skin 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 stages. The first of these 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, where lipophilic molecules diffuse between cells by following the lipid network. The transcellular route, 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, in particular the pilosebaceous follicles, which can act as a reservoir and enable deeper diffusion.

In this context, the question of molecular 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 mass lower than 500 Daltons (Da) in order to pass through the stratum corneum. Beyond this threshold, its diffusion through the skin becomes severely restricted, if not impossible under normal conditions. The 500‑Da rule is based on several observations from pharmacology and dermatology, one of which concerns allergens. Indeed, the great majority of known allergens have a molecular mass 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, however, be regarded as an absolute boundary.

In practice, cutaneous penetration depends on a whole set of physico‑chemical and biological parameters. The lipophilicity of a molecule, for example, plays a decisive role: a molecule capable of integrating into the lipid “cement” of the stratum corneum will diffuse more readily than a strictly hydrophilic molecule. Its three‑dimensional structure and its ability to interact with epidermal lipids also influence its passage. Conversely, certain large molecules, although unable to cross the skin barrier, can nevertheless exert surface effects or act indirectly by modulating the cutaneous environment, for example 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, ageing skin is also more permeable. The application site is also important, as 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 presents an impaired barrier, which significantly increases the penetration of substances, whether beneficial or harmful.

The product characteristics also play a role. The pharmaceutical form (cream, gel, lotion, serum, etc.) determines the release of the active substance and its ability to interact with the skin. Likewise, the amount applied, the frequency of use and the area of application modulate cutaneous exposure and therefore the diffusion of active ingredients. Penetration therefore results from a balance between the formulation, the mode of application and the condition of the skin.

The 500-Dalton rule provides a useful scientific benchmark, but it is a simplifying one.

Formulators are not restricted to this constraint and develop various strategies to optimise the diffusion of active ingredients, including when their size exceeds this theoretical threshold. Among the most widely used approaches are encapsulation techniques (liposomes, nanoparticles, vectorised delivery systems), which make it possible to “transport” the active ingredient across 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 the ability of formulation to modulate the bioavailability of active ingredients by acting on parameters other than their molecular mass.

• MEINARDI M. M. & al. The 500 Dalton rule for the skin penetration of chemical compounds and drugs. Experimental Dermatology (2000).

• 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).