After I recently read a book about about words on skin, I jokingly told my husband that I wanted to get a tattoo. Now we both know that the likeliness of that is about 0.1% (firstly, needles are one of my least favorite things, and even working with them in lab sometimes gives me chills, and second I can't stand seeing blood, particularly my own), but I must have worried him somewhat, because the he gave me the September issue of "Nachrichten aus der Chemie" (kind of a German version of C&EN News, but not quite) that contains an article on tattoos. Other people have written about it--Carbon Based Curiosities comes to mind--but I hadn't really ever seen the actual structures of tattoo pigments, so it was actually quite an interesting read.
In the past, inorganic pigments such as titanium dioxide (white), cadmium sulfide (yellow) and iron oxide (black) were used as pigments. These compounds are relatively insoluble and thus produce long lasting color. Today organic compounds make up the bulk of colored tattoo pigments, while totally black tattoos are still made out of carbon black which unfortunately contains toxic impurities associated with its production. I've drawn out some of the structures of common organic tattoo pigments for you to see; most of the structures contain polycyclic azoles, which I guess isn't very surprising.
Not to scare anyone away from getting an awesome tattoo of their favorite natural product, but the bulk of the article is dedicated to the dangers associated with tattooing. Amazingly, some pigments that aren't allowed in cosmetics due to toxicity are still used in tattoos (at least in Germany, and I wouldn't be surprised if the same is true for the United States). Also, there are many dangers associated with the breakdown products of the pigments that are utilized, especially in the case of red pigments. One can imagine that these decomposition products could be oxidized in the body to produce an even larger number of unknown chemicals that could potentially be hazardous to you health. But don't let any of that stop you from getting that tattoo of the periodic table on your arm like you've always wanted--tattoos can't be any worse for your heath than washing your hands in benzene (which from what I hear was a relatively common practice for chemists 50 years ago).