Using chemical speciation modelling to discuss variations in patch test reactions to different aluminium and chromium salts

Background: Allergic contact dermatitis to metals is diagnosed by applying a metal salt in a patch test. The bioavailability of the metal salt might depend on the choice of metal salt, the concentration, sweat composition, and pH. Objectives: The main purpose of this study was to apply chemical speciation modelling, which is based on experimentally derived input data and calculates the concentrations of chemical forms (species) in solutions, to reproduce and discuss clinical patch test results of aluminium and chromium. Methods: Joint Expert Speciation System (JESS), Hydra/Medusa, and Visual Minteq were employed to study the bioavailable fraction and chemical form of clinically applied aluminium and chromium salts as a function of salt type, applied concentration, sweat composition, and pH. Results: Investigated aluminium and chromium salts can have a very low bioavailability with a large dependency on sweat composition, pH, metal salt, and concentration. Both aluminium and chromium ions could shift the pH towards acidic or basic values based on their chemical form. Conclusions: Reported seasonal and inter-patient variability in positive reactions to aluminium is likely related to sweat pH and composition. Potassium dichromate increases the pH and aluminium and trivalent chromium chloride strongly decrease the pH, possibly increasing skin diffusion