How deep into the skin does a cosmetic actually go?

When an ad says a product is "absorbed deep into the skin," how deep is deep? In cosmetic advertising, "deep into the skin" means as far as the stratum corneum. The stratum corneum is the outermost layer of the skin, and it is as far as a cosmetic reaches. Going below it is not a cosmetic's job.

How deep is the "deep" a cosmetic can reach?

The stratum corneum is 10 to 20 micrometers thick (World Health Organization, 2009). It is a stack of flat cells without nuclei, with lipids filling the gaps between them, and it is the main barrier against anything coming in from outside (Bos & Meinardi, 2000). A cosmetic reaches the stratum corneum; the living epidermis and the dermis below it are not where a cosmetic goes. Korea's Cosmetics Act defines a cosmetic as a product "with a mild effect on the human body." In the advertising review standard used in Korea, the word "dermis" appears in exactly one place, and that clause forbids claiming that a product "passes through the epidermis and reaches the dermis" (Korea Cosmetic Association, 2025). Japan's advertising guideline is more direct. To use the word "penetrate," an ad must state that the site of penetration is the stratum corneum, and wording that implies action deeper in the skin, such as "deep into the skin," is not allowed even with a footnote saying "stratum corneum" (Japan Cosmetic Industry Association, 2020).

How large a molecule can pass the stratum corneum?

Whether an ingredient passes the stratum corneum is judged by the size of its molecule. Because contact allergens, topical skin drugs, and the drugs in skin patches all have molecular weights under 500 daltons, the accepted view is that molecules over 500 daltons do not simply pass the stratum corneum (Bos & Meinardi, 2000). The 500-dalton rule of thumb is enough to read how far "deep into the skin" goes in an ad. Glycerin (92 daltons), niacinamide (122), ascorbic acid (176), and retinol (286) are all under the line (PubChem). Hyaluronic acid, by contrast, ranged from 50,000 to 2,000,000 daltons in the grades tested in cosmetic creams (Pavicic et al., 2011), and PDRN, which now appears in many ads, is a set of DNA fragments whose smallest pieces are 50,000 daltons and whose largest reach 1,500,000 (Squadrito et al., 2017). That does not make large molecules useless. In the same trial, all five molecular weights of hyaluronic acid raised skin hydration and elasticity.

So what does "deep into the skin" actually mean?

High-molecular-weight hyaluronic acid does not pass the stratum corneum; it stays on the surface as a thin film and holds water (Bravo et al., 2022). Holding water at the surface is not something only hyaluronic acid does. Glycerin has long been known to raise stratum corneum hydration and support the barrier (Fluhr et al., 2008). In Korea, the ingredient list is written in order of amount for ingredients above 1% (Enforcement Rule of the Cosmetics Act, Annex 4). Hyaluronic acid is usually used at 0.2 to 1% in cosmetics (Juncan et al., 2021), so it sits near the end of the list, while glycerin usually sits near the top. The hero behind the moist feel of a "hyaluronic acid serum" is glycerin. That is why hyaluronic acid ads usually inflate the degree of what the ingredient actually does. There is no data showing that a "high concentration" of hyaluronic acid works better, "8 types" and "low-molecular-weight HA that reaches the dermis" collapse at the 500-dalton threshold of the stratum corneum, and the smallest fragment of topical PDRN is 50,000 daltons.

In short, it is hard for a cosmetic to get "deep into the skin"

The deepest a cosmetic can reach is the stratum corneum, so read "deep into the skin" as "as far as the stratum corneum." Molecules over 500 daltons do not simply pass the stratum corneum (Bos & Meinardi, 2000), and holding water and refilling lipids there is as far as a cosmetic's job goes. The collagen that supports firmness and smoothness sits in the dermis, and reaching the dermis takes an injection. We compare the two routes in our note on PDRN serum versus injection.

One step today

• When an ad says "deep into the skin," check where glycerin and hyaluronic acid sit on the ingredient list. If hyaluronic acid is near the end, read the moist feel as glycerin's work more than hyaluronic acid's.
• Filter words like "high concentration," "8 types," and "low-molecular-weight HA that reaches the dermis" through the 500-dalton rule. Expect hyaluronic acid and PDRN to hold water on top of the stratum corneum, and nothing more.