If you live in the US and are planning on getting a tattoo any time soon, we've got some potentially unwelcome news. Many common commercial tattoo inks have either different ingredients than those listed on the label or additional substances that are not listed at all, according to a new paper published in the journal Analytical Chemistry. And there are other scientific studies suggesting that some of those ingredients could have adverse health effects, either in the form of allergic reactions or skin or other cancers.

“Our goal is to empower artists and their clients," said co-author John Swierk, a chemist at Binghamton University. "Tattoo artists are serious professionals who have dedicated their lives to this craft, and they want the best possible outcomes for their clients. We’re trying to highlight that there are some deficiencies in manufacturing and labeling.”

Typical tattoo ink contains one or more pigments (which give the ink its color) within a "carrier package" to help deliver the pigments into the skin. The pigments are the same as those used in paints and textiles. They can be either small bits of solids or discrete molecules, such as titanium dioxide or iron oxide (for white or rust-brown colors, respectively). As for the carrier packages, most ink manufacturers use grain or rubbing alcohol, sometimes with a bit of witch hazel added to the mix to help the skin heal after the tattooing process. There may also be other additives to adjust the viscosity and keep pigment particles suspended in the carrier package.

Swierk was initially interested in tattoos as tools for medical diagnostics. This shifted to an interest in tattoo laser removal, specifically how laser light causes tattoos to fade—i.e., break down chemically—which in turn inspired him to learn more about the chemical composition of tattoo inks, which is also not well understood. Even the tattoo artists Swierk's team interviewed knew very little about the chemical composition of their favorite inks.

Thus the online project What's In My Ink was born. According to the project's mission statement:

In modern times, tattooing is often associated with being young and reckless. However the truth is that tattooing is an ancient art having been around for roughly 5000 years. Within the last decade, the frequency of Americans getting a tattoo has increased by 9 percent with 30 percent of Americans now having a tattoo. Despite the vast history of tattooing, and the increased frequency of tattoos in modern society, tattoo inks are generally not regulated by the government, nor does the government have any specific knowledge about the molecular composition of most tattoo inks.

Swierk et al. use various methods, including Raman spectroscopy, nuclear magnetic resonance spectroscopy, and electron microscopy, to analyze a broad range of commonly used tattoo inks. This enables them to identify specific pigments and other ingredients in the various inks. They presented preliminary results at a 2022 meeting of the American Chemical Society. They found that many ingredients didn't appear on the manufacturers' labels, such as one ink that contained ethanol even though it was not listed on the label. And 23 of the inks analyzed by that point showed evidence of an azo-containing dye. Such pigments are usually inert, but exposure to bacteria or UV light can cause them to degrade into a nitrogen-based compound that could potentially cause cancer.