The German Commission for the Investigation of Health Hazards of Chemical Compounds in the Work Area has evaluated criteria to designate substances with an “H” which can be absorbed via the skin in such amounts as to contribute significantly to systemic toxicity. Information on the uptake via the skin can be obtained from studies in humans, animals and from in vitro models preferably with human skin. Ideally, these data allow the calculation of a flux, i.e. the amount absorbed per cm2 skin and hour. If these experimental studies are lacking, fluxes based on physicochemical data are calculated with three mathematical models for an aqueous saturated solution or a read‐across to similar substances with a better database is used. For gaseous substances, the water solubility of the substance is calculated at a substance concentration in the gas phase that corresponds to the MAK value and the flux is modelled.
An exposure time of one hour and an exposed skin area of 2000 cm2 which corresponds to the area of both hands and forearms is used as a practical exposure model for liquids. For gases, the exposure time is taken as 8 hours and the exposed skin area as 1.8 m2.
With the obtained fluxes, the body burden is calculated and if it exceeds 25% of the chronic systemic NOAEL for workers, the substance is designated with an “H”. If the substance is genotoxic and no systemic NOAEL could be deduced, a qualitative proof of uptake is enough for the designation with an “H”.