Nanoparticles do not penetrate human skin--a theoretical perspective.

The penetration of intact particles in the nanometer range (nanoparticles, [NP]) through human skin is a controversial topic, which has attracted much interest from both the pharmaceutical and the personal care industries. Concerns have also been raised about the possible implications that dermal exposure to NP may have for human health, particularly from physical sunblock formulations. Here we use a theoretical approach to determine the feasibility of NP penetration of healthy human skin. The maximum flux of NPs of various dimensions is calculated based on two algorithms that have been developed to model passive diffusion of molecules through skin. The results confirm that NPs are too large to permeate skin by this mechanism. Although components of NPs may dissolve in the skin and measurable amounts have been detected in body fluids, this is not indicative of actual NP transport through the skin. The possible roles for NP formulations in drug permeation enhancement are also considered but are not associated with the penetration of intact NP.