In many countries a “skin notation” is used for cautioning against skin contact in cases where the skin is significantly permeable to the chemical in question. The stratum corneum provides the greatest barrier against hydrophilic compounds, whereas the viable epidermis is most resistant to highlyl ipophilic compounds. Skin absorption depends on the physicochemical properties(e.g. octanol–water partition coefficient (Po/w), molecular weight,electron structure and dissociation constant (pKa) of the compound, but also on interactions with other compounds. Additionally, the vehicle, occlusion,concentration, exposure pattern and the site of the skin also play a role [Refs. 2, 5].
Evaluations of hazards due to skin penetration are generally complicated due to various factors that need to be considered: type of vehicle for the chemical, size of the exposed area, applied dose, etc. The simplest, semi-quantitative assessments consider the flux (inmg/cm2hr) derived from in vitro studies. Theoretically, skin absorption depends, amongst others things, on the volume of the molecule andhence on the molecular weight of a compound as well as on the hydrophobic and hydrogen bonding properties, which are often based on the Po/w [Ref. 9]. The U.S. National Institute for Occupational Safety and Health (NIOSH)has a free service that allows the calculation of a skin permeation coefficient(KP) for substances: http://www.cdc.gov/niosh/topics/skin/skinPermCalc.html. Also, skin penetration data can be obtained from the EDETOX database [http://www.ncl.ac.uk/edetox/].