Tattoos have become destigmatized over time, and more people have them. More than 1 in 5 Americans now sport some body art, and if you're in the 18-29 demographic, there's an almost 40 percent chance you have one. I have two small tattoos, and I've loved them both over the two decades I've had them. No regrets.

One of my tattoos is a tree frog who sits on my shoulder and, due to her location, she's gotten quite a bit of sun over the years. After 20 years, the color has faded. I was thinking of getting my frog touched up with color, or maybe even added to.

For years, it was believed that tattoos last because they stain skin and therefore that's why they eventually fade. But a 2018 study on mice shows that ink may last because of interactions with white blood cells called macrophages.

Macrophages feed on foreign objects that enter a body (like tattoo ink) and protect the body. When the cells eventually die, they release the ink back into the body. Then, the process begins all over again with new macrophages. While this study was conducted on mice, researchers believe the same is true for humans. One of their reasons — long-term exposure to sunlight will kill macrophages that aren't replaced and is also known to make tattoos fade faster.

Tattoo ink acts as nanoparticles

Organic pigments translocate from skin to lymph nodes. Skin specimens are oriented with its surface on the right side. Identified organic pigments are indicated below each sample. Chemical structures of the organic pigments identified in the samples are displayed on the right. (Photo: Scientific Reports /Scientific Reports)

Another study from 2017 gives me pause.

Researchers have found that tattoo inks had stained the lymph nodes of the bodies they tested — meaning the substances that make up tattoo ink don't stay put. The researchers were able to see, using X-ray fluorescence spectrometry, that the colors acted as nanoparticles and moved throughout the body.

And it wasn't just the organic pigments that moved around. The scientists found that preservatives like aluminum, chromium, iron, nickel and copper had traveled to the lymph nodes, too. They found heavy metals including cadmium and mercury, both of which are highly toxic, in one of the subjects.

"When someone wants to get a tattoo, they are often very careful in choosing a parlour where they use sterile needles that haven't been used previously. No one checks the chemical composition of the colours, but our study shows that maybe they should," Hiram Castillo, one of the authors of the study wrote in a release on Phys.org.

What the heck is in these inks anyway?

Why didn't we know this? It turns out that tattoos are considered similar to cosmetics (even if they are permanent); because they're technically not ingested, they've never been tested for safety. These insoluble colors were originally created for use on cars and printers, so they weren't formulated with the human body in mind. “And the chemistry remains the same ... If somebody asks me whether it's dangerous to have a tattoo, all I can say is, 'We don't know yet,'” Wolfgang Bäumler, a professor in the department of dermatology at the University of Regensburg told Deutsche Welle.

Examples of tattoo-ink ingredients include carbon black (produced by burning crude oil like tar and rubber, it's a carcinogen). Green and blue inks contain phthalocyanines, which include copper and nickel (a common allergen). And red, orange and yellow colors which are made up of AZO compounds, which are carcinogenic and genotoxic.

Scientists are now looking at the ingredients in tattoo inks that could directly affect human health (and in what quantities) and what the long-term implications of these inks are. Size would seem to matter though, since estimates are that about two-thirds of the color (and its potentially toxic ingredients) stays in the tattoo near the skin's surface, and about one-third makes its way into the body.

I'm sure I'm not the only tattoo fan who will be waiting to see what the further research on the implications of tattoo inks will be — not to mention looking out for alternatives to the chemical-laden inks.