Does any of this actually get in?

Most of your routine works in the top 20 microns

The outermost layer of skin, the stratum corneum, is about 10 to 20 micrometres thick. That is roughly a fifth of the width of a human hair, and it is made of cells that are already dead. It is also where 89 of the 200 ingredients in our glossary do their work: every humectant, every emollient, every exfoliant, every barrier lipid.

This is not a disappointment. A moisturiser that reached your bloodstream would be a drug with a side-effect profile, not a moisturiser. The reason to know where something works is not to be impressed by depth, it is to spot when a product is charging you for a journey it never makes.

See the cross-section in 3D

Every weight we can verify, heaviest first

127 ingredients, each weight computed from the molecular formula of the same compound our structure diagrams are drawn from. Nothing here is estimated. The ones missing from this table are missing for a reason: plant oils, extracts, ferments and polymers are mixtures, so there is no single molecule to weigh. Hyaluronic acid and collagen are both in that group.

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Heaviest: THD Ascorbate at 1130 daltons. Lightest: Denatured Alcohol at 46.

The same number, two opposite meanings

This is where the rule gets misused, in both directions. Being over 500 daltons is not a criticism, and being under it is not a promise. What decides whether the number matters is where the ingredient is supposed to work.

A ceramide at 993 daltons belongs in the outer layer and never had to go anywhere. A peptide at a similar weight, sold for the collagen deep under your skin, has a journey to explain. Same physics, entirely different claim.

Where the 500 Dalton rule does not apply

Quoting it as though it settled everything would make us no better than the brands quoting it when convenient. Four honest limits:

• It is a rule of thumb. The original paper argued the case for a limit around 500 daltons in drug development. It describes a tendency, not a wall, and formulation chemistry exists precisely to bend it.
• Particles are not molecules. Mineral sunscreen filters like zinc oxide and titanium dioxide sit on skin as particles. Weighing them tells you nothing.
• Mixtures have no single weight. Oils, botanical extracts, ferments and polymers are many compounds at once, which is why they are absent from the table rather than assigned a number we invented.
• The pore is a shortcut. A follicle is an opening straight through the outer layer, which is how ingredients like benzoyl peroxide reach a target the surface would otherwise keep them from.

There is one more limit worth saying plainly: for 43 of our 200 ingredients we could not find good evidence of where they act at all, so we say so rather than placing them on a diagram. Most are botanical extracts and antioxidants.