Divalent nickel has been shown to penetrate the skin fastest at sweat ducts and hair follicles where it binds to keratin and accumulates in the epidermis. However, the surface area of these ducts and follicles is small; hence, penetration through the skin is primarily determined by the rate at which nickel is able to diffuse through the horny layer of the epidermis (Grandjean et al., 1989). Nickel penetration of skin is enhanced by many factors including sweat, solvents, detergents, and occlusion, such as wearing gloves (Malten, 1981; Fischer, 1989; Wilkinson and Wilkinson, 1989).
Although dermal exposure to nickel-containing products constitutes an important route of exposure for the public, the amount of nickel absorbed from such products is unknown. In a study using excised human skin, only 0.23 percent of an applied dose of nickel chloride permeated non-occluded skin after 144 hours, whereas 3.5 percent permeated occluded skin in the same period (i.e., skin with an airtight seal over the test material on the epidermal side). Nickel ions from a chloride solution passed through the skin approximately 50 times faster than nickel ions from a sulfate solution (Fullerton et al., 1986).