500 Dalton Rule

A dalton is roughly the mass of a single hydrogen atom, and it is the unit molecules are weighed in. Water is about 18 of them, glycerin about 92, ascorbic acid about 176. Collagen runs to hundreds of thousands.

The rule was proposed in the dermatological literature around the turn of the century, on the observation that the topical drugs in common use, and the substances that provoke contact allergy, nearly all fall below that mass. The mechanism it points at is plausible enough: the path between the dead surface cells is a narrow and winding lipid one, and bulk is a real disadvantage in it.

It explains a good deal at the shelf. Most proteins sit far above the line, so a protein in a cream is doing its work at the surface whatever the front of the box says. “Low molecular weight” on a hyaluronic acid label is a direct appeal to this rule, and the appeal is at least coherent.

The limits matter as much. It is a line drawn through a cloud of data, not a gate with a guard on it. Mass is one of several things that decide the outcome — shape, charge, how the molecule divides between oil and water, how much is applied and in what, and whether the skin is intact. Occlusion and damage move the answer. Treat 500 as the first question, never the last one.