Nickel is perhaps the most insidious and toxic metal in our environmental exposures, since it is pervasive, can absorb right through the skin, usually doesn't show up in hair analyses, and comes out primarily in the sweat and in the bowels. Because of its propensity to reabsorb, it can be considered to be a carcinogenic tattoo and a likely cause of both skin and alimentary tract cancers, especially those of the bowels and colon.
My palm pompholyx (pin-head-sized blisters) from metal eyeglass frames and stainless steel vacuum bottles was about 10 times worse than this, produced in patients intentionally-poisoned by a dermatologist to prove that nickel causes this rash. Feeding carcinogenic metals to animals is unethical!