Skin permeability, for all chemicals or just some?
"Skin's outermost layer, the stratum corneum, is an effective barrier to most inorganic nanosized particles."

But skin is quite permeable to many organic compounds. DMSO is one example, and it is warned that DMSO is a good solvent for other compounds that could be more toxic than DMSO.

Another example: we all (should) know that mercury is toxic. However, it's not going to hurt you much if you roll it around in your hand for a brief period (don't breathe in, though!). But organic mercury compounds are terribly toxic and skin permeable, including the infamous dimethyl mercury which killed a famous researcher of toxins.