The 500 Dalton Rule: What Can Actually Get Into Your Skin

Mycena Skin: Ingredient Science Series · Deep Dive: Delivery Science

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Almost every serum claims to "penetrate deep." Most of the time, nobody says how deep, or why. There is a simple, well-cited rule of thumb that gets you most of the way to an answer, and it comes down to one number: molecular weight.

This article explains the 500 Dalton rule: where it came from, why it works, where it breaks down, and where the most common skincare ingredients and peptides actually sit. We finish by holding our own label up against it.

First, what's a dalton?

A dalton (Da) is a unit of mass for molecules. One dalton is about the mass of a single hydrogen atom, and a molecule's weight in daltons is the same number as its molar mass in grams per mole. Water is 18 Da. Glycerin is 92. The copper peptide GHK-Cu is about 402. A strand of cosmetic-grade hyaluronic acid can top 1,000,000.

Those numbers span five orders of magnitude, and skin notices the difference.

Where the rule comes from

In 2000, dermatologists Jan Bos and Marcus Meinardi published a short paper in Experimental Dermatology titled "The 500 Dalton rule for the skin penetration of chemical compounds and drugs." They pointed out three facts that all lined up at the same limit:

1. Almost all common contact allergens, the chemicals that sensitize skin on contact, weigh under 500 Da. Larger molecules rarely cause contact allergy, because they can't get in far enough to meet the immune system.
2. The pharmacological agents used in topical dermatology are all under 500 Da.
3. Every drug delivered through a transdermal patch is under 500 Da.

Their conclusion: above about 500 Da, a compound's ability to pass through intact skin drops off sharply. It's a rule of thumb, not a law of physics, but it has held up well for more than two decades.

They also offered a telling example. Cyclosporine, at about 1,200 Da, works well as a pill for psoriasis but failed as a topical. The drug is effective. It just can't get through the skin.

Why size matters: the brick wall

The main barrier is the stratum corneum, the outermost layer of the epidermis. It's only about 10 to 20 micrometers thick, but it's built like a brick wall. Flattened, dead skin cells (corneocytes) are the bricks, and a dense mix of ceramides, cholesterol and free fatty acids is the mortar.

Most molecules that cross it take the long way, winding through the lipid mortar around the bricks. That path is narrow, crowded and oily. Small molecules slip through. Big ones get stuck.

A widely used model from Potts and Guy (1992) puts numbers on it. It predicts a molecule's skin permeability from just two properties: how oil-loving it is (its octanol-water partition coefficient, log P) and its molecular weight. With lipophilicity held constant, every extra 100 Da cuts predicted permeability roughly four-fold. Add 500 Da and you've lost about a thousand-fold. That's why the drop-off feels like a wall rather than a slope.

Where common skincare ingredients sit

The good news is that most of the classic small actives are well under the line. Humectants, alpha- and beta-hydroxy acids, niacinamide, caffeine, panthenol and the retinoids all sit under 300 Da.

Two things stand out.

The lipids sit near or over the line, and that's fine. Ceramides, cholesterol, squalane and tocopherol are there to reinforce the stratum corneum itself: the mortar in the brick wall. They don't need to go deeper to do their job.

Small doesn't mean it all gets through. Ascorbic acid is only 176 Da, but in its pure form it's water-soluble and charged at skin pH, which is why so much vitamin C formulation work goes into low pH and oil-soluble derivatives. Size gets a molecule to the starting line. Chemistry decides how far it runs.

Peptides: where it gets interesting

Peptides are chains of amino acids, and each added amino acid adds about 110 Da on average. Short peptides sit comfortably under the line. Longer ones don't.

• GHK (340 Da), GHK-Cu 1:1 (402 Da) and AHK-Cu (about 416 Da) are all under 500. This is one reason the copper tripeptides are among the most practical peptides for topical use.
• GHK-Cu 2:1 (about 742 Da), two GHK chains sharing one copper ion, is well over. It's better thought of as a surface- and follicle-level ingredient.
• The palmitoyl peptides (the Matrixyl family) cross 500 Da on purpose. A 16-carbon fatty acid is attached to make the peptide far more oil-loving, which helps it move into the lipid layers even though it weighs more. It's a trade: more mass for better solubility in the mortar.
• Acetyl hexapeptide-8 (Argireline, 889 Da) and acetyl octapeptide-3 (SNAP-8, about 1,075 Da) are well above the line, which is worth keeping in mind when these are marketed as "topical Botox."
• BPC-157 is even larger, so there's effectively no skin penetration.

The giants: collagen, hyaluronic acid and polysaccharides

On a linear chart, the biggest skincare ingredients wouldn't fit on the page. On a log scale, where each gridline is ten times the one before, you can see just how far past the line they sit.

• Collagen in a cream can't replace collagen in your skin. An intact collagen molecule is around 300,000 Da. Hydrolyzed collagen fragments in cosmetics are usually thousands of daltons. Both act as film-formers and humectants on the surface.
• High-molecular-weight hyaluronic acid, often 1,000,000 Da or more, doesn't penetrate. It holds water on the surface and forms a smooth, hydrating film, which is exactly what it's good at. Lower-weight fragments get somewhat further, but even "low molecular weight" HA is usually tens of thousands of daltons.
• Tremella polysaccharide, the "snow mushroom" long used in skincare, is also a very large molecule. Like hyaluronic acid, it works as a surface humectant and film.
• Botulinum toxin, at about 150,000 Da, is the clearest example of why some things have to be injected.

None of this makes large molecules useless. It means they do a different job. A big humectant working at the surface and a small active moving through the barrier are complementary, not competing.

Beyond molecular weight

Molecular weight is the most useful single number, but it isn't the only one. Five other factors decide how far a molecule actually gets:

1. Lipophilicity. Molecules that dissolve in both oil and water cross best. A log P of roughly 1 to 3 is the sweet spot. Too water-loving and a molecule can't enter the lipid mortar. Too oil-loving and it gets stuck there.
2. Charge. Ions cross poorly. Formulators adjust pH to keep an active in its uncharged form, or add a lipid tail to mask the charge.
3. Hydrogen bonding and melting point. Molecules that form many hydrogen bonds, or that melt at high temperatures, dissolve less readily in the barrier lipids and move more slowly.
4. Barrier condition. Damaged skin lets more through. Tacrolimus, at 804 Da, works topically mainly because the eczema-affected skin it treats has a compromised barrier.
5. Other routes. Hair follicles and sweat ducts offer shortcuts that bypass part of the brick wall. They make up a small share of the skin's surface, but they matter for larger molecules and for scalp care.

Concentration and the vehicle matter too. Delivery is driven by the concentration gradient, so the same molecule at 2% will move more than at 0.1%, and the base it's dissolved in can speed it up or hold it back.

Holding our own label to the rule

We think any brand that talks about penetration should be willing to show where its own ingredients sit. Here's our Copper Theory serum, ingredient by ingredient, in label order.

Six of the nine ingredients, including the 2% copper tripeptide, are under 500 Da. The two big polysaccharides, tremella and sodium hyaluronate, aren't meant to go in. They hold water on the surface so skin looks and feels hydrated while the smaller ingredients do their work. The formula has no fragrance, alcohol, silicones or parabens.

How to use this when you read a label

Three quick questions will tell you more than most product copy:

1. What's the active, and how big is it? Look it up. If it's well over 500 Da, it's probably working at the surface, whatever the marketing says.
2. If it's big, is that the point? Humectants and barrier lipids are supposed to stay near the top. That's not a flaw.
3. If it's a peptide, is it modified? A palmitoyl or acetyl group is a sign the formulator has thought about delivery.

Molecular weight won't tell you whether a product is good. It will tell you whether its claims are plausible.

References

• Bos JD, Meinardi MM. The 500 Dalton rule for the skin penetration of chemical compounds and drugs. Experimental Dermatology, 2000.
• Potts RO, Guy RH. Predicting skin permeability. Pharmaceutical Research, 1992.
• Hostynek JJ, Dreher F, Maibach HI. Human skin penetration of a copper tripeptide in vitro as a function of skin layer. Inflammation Research, 2011.
• Elias PM. Epidermal lipids, barrier function, and desquamation. Journal of Investigative Dermatology, 1983.
• Lintner K, Peschard O. Biologically active peptides: from a laboratory bench curiosity to a functional skin care product. International Journal of Cosmetic Science, 2000.

This article is for educational purposes only. Cosmetic products are intended to change how skin looks and feels, not to treat or prevent disease.

These statements have not been evaluated by the Food and Drug Administration. Mycena Skin products are cosmetics and are not intended to diagnose, treat, cure, or prevent any disease. Ingredient research is provided for education and describes the ingredients, not claims about finished-product performance.