Nanoparticles (NPs) skin absorption is a wide issue, which needs to be better understood. The attempt of
this review is to summarize the scientific evidence concerning open questions, i.e.: the role of NPs intrinsic
characteristics (size, shape, charge, surface properties), the penetration of NPs through the intact or
impaired skin barrier, the penetration pathways which should be considered and the role of NPs interaction
in physiological media. The outcomes suggest that one main difference should be made between
metal and non-metal NPs. Both kinds have a secondary NPs size which is given after interaction in physiological
media, and allows a size-dependent skin penetration: NPs 6 4 nm can penetrate and permeate
intact skin, NPs size between 4 and 20 nm can potentially permeate intact and damaged skin, NPs size
between 21 and 45 nm can penetrate and permeate only damaged skin, NPs size > 45 nm cannot penetrate
nor permeate the skin. Other aspects play an important role, mostly for metal NPs, i.e., dissolution
in physiological media, which can cause local and systemic effects, the sensitizing or toxic potential and
the tendency to create aggregates. This paper suggests a decision tree to evaluate the potential risk for
consumers and workers exposed to NPs.
