Some more interesting facts:
The U.S. Food and Drug Administration (FDA), which approves the color additives used in foods, cosmetics and drugs, does not regulate the inks used for tattooing, and no color additive has ever been approved for injection into the skin.
Mercury is also known to promote Alzheimer's disease, dementia and other nervous system disorders.
Just google metals in tattoo ink to read more.
Because the inks are not sold directly to consumers, manufacturers are not required to list the components on the product label, according to the FDA.