Human stratum corneum penetration by nickel. In vivo study of depth distribution after occlusive application of the metal as powder.

Sequential tape stripping was implemented on three healthy volunteers to examine the surface distribution of nickel through human stratum corneum in vivo following occlusive application of the metal as powder on the volar forearm. Exposure sites were stripped 20 times at intervals from 5 min to 96 h post-dosing and the strips analyzed for metal content by Inductively Coupled Plasma-Mass Spectroscopy with a detection limit for nickel of 0.5 ppb. The gradients of nickel distribution profiles increased proportionally with occlusion time, but after the 10th strip to the 20th strip continued at constant levels. Total nickel removed with 20 stratum corneum strips to the level of the glistening layer after maximum occlusion of 96 h was 41.6 micrograms/cm2 (+/- 12.2; average n = 3). In order to normalize the nickel depth distribution profiles, stratum corneum removed by stripping of untreated skin after occlusion was determined by weighing. Following application of nickel dust over 24 h, analysis of the 20th strip still indicated nickel present at 1.42 micrograms/cm2 (+/- 0.68; average n = 3). These data indicate that, in contact with skin, nickel metal is oxidized to form soluble, stratum corneum-diffusible compounds which may penetrate the intact stratum corneum, presumably by the intercellular route, and have the potential to elicit allergic reactions.