Possibly of interest from Scientific American:
In the Ink: Do All Tattoo Pigments Use Mercury and Other Toxic Heavy Metals? - Scientific American
"While red causes the most problems, most other colors of standard tattoo ink are also derived from heavy metals (including lead, antimony, beryllium, chromium, cobalt nickel and arsenic) and can cause skin reactions in some people.
Helen Suh MacIntosh, a professor in environmental health at Harvard University and a columnist for the website, Treehugger, reports that as a result of a 2007 lawsuit brought by the American Environmental Safety Institute (AESI), two of the leading tattoo ink manufacturers must now place warning labels on their product containers, catalogs and websites explaining that “inks contain many heavy metals, including lead, arsenic and others”"
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But perhaps the end of the article is the most use for the nervous amongst us (like me):
Also, for what it's worth, I was seeing my dermatologist for an unrelated matter and asked him about my getting a tattoo.
He had no concerns, except to say that he preferred black and blue inks as those are the easiest to remove!