It turns out that tatoo ink hasn’t ever had much scrutiny, and nobody knows what’s in some of them:

The Food and Drug Administration has the authority to regulate tattoo ink. But until recently, it hadn’t, citing more pressing public health problems and a lack of consumer complaints. FDA chemist Dr. Bhakti Petigara Harp says that recently, the agency has started to see an increase in consumer complaints.

Maybe they should add a warning tatoo to each tatoo, like with cigarettes. “Warning! This Tatoo Could be Hazardous to Your Health!”