Why should you care about the 500 Dalton Rule? Big Molecules Can't Penetrate Skin You may or may not have heard of a concept in skin care pharmacology called the Dalton 500 rule. This rule was proposed in the year 2000 to describe the upper limit molecular weight for a molecule to be able to penetrate into the skin. This "rule" explains why some ingredients just seem to sit on the skin, while other ingredients can seep right into the lower layers of the skin. To explain this concept a little...