It takes a brave soul (in some cases, emboldened by a strong drink or two) to get a tattoo. And while people may spend time considering what design to have pierced onto their bodies, few may consider exactly what happens to the ink once it is injected under their skin.

In fact, scientists are still investigating that question.

To make a tattoo permanent, a tattoo artist punctures the skin with hundreds of needle pricks. Each prick delivers a deposit of ink into the dermis, the layer of skin that lies below the epidermis, which is populated with blood vessels and nerves.

Once the ink is inserted into the dermis, it doesn't all stay put, research is finding. Some ink particles migrate through the lymphatic system and the bloodstream and are delivered to the lymph nodes. Research on mice suggests some particles of ink may also end up in the liver.

"When you inject particles into the skin, some travel to the lymph nodes within minutes," Ines Schreiver, a chemist with the German Federal Institute for Risk Assessment in Berlin,told Live Science. [5 Weird Ways Tattoos Affect Your Health]

Where the ink goes

To be clear, most of the tattoo pigment stays put after a person gets a tattoo. The ink that's not cleared away by special repair cells, called macrophages, stays in the dermis within trapped macrophages or skin cells called fibroblasts. It then shows through the skin, perhaps spelling out "Mom" or featuring that eagle design you spent weeks choosing.

"Normally, the ink doesn't migrate too far from where it's injected," Dr. Arisa Ortiz, a dermatologist and director of laser and cosmetic dermatology at the U.C. San Diego Health, told Live Science. "For the most part, it is engulfed [by skin or immune cells] and then kind of sticks around in the dermis."

But researchers are now taking a closer look at the tattoo ink that does travel to other parts of the body, particularly the lymph nodes.

Schreiver was part of a team of German and French scientists that performed the first chemical analyses on tattoo ink collected at human lymph nodes. The researchers analyzed the lymph nodes of four cadavers that had tattoos, as well as two cadavers that had no tattoos, which served as controls.

The researchers pointed out in their study, published in the journal Scientific Reports, that "pigmented and enlarged lymph nodes have been noticed in tattooed individuals for decades." Those reports came mostly from pathologists who began noticing unusual coloring in lymph node biopsies taken from tattooed patients.

For example, a 2015 report in the journal Obstetrics and Gynecology described how doctors at first thought a woman's cervical cancer had spread to her lymph nodes. After surgically removing the nodes, the doctors realized that what had appeared to be malignant cells were actually tattoo ink particles.

"I was very curious about the chemical side effect of tattoos," Schreiver said. "I think people are aware that you can get skin infections from a tattoo, but I don't think most are aware that there may also be risks from the ink."

To investigate these side effects, Schreiver and her colleagues used several different tests, to analyze what forms of tattoo ink were collecting in the lymph nodes and any damage that might have resulted. Among their findings was that nanoparticles — particles measuring less than 100 nanometers across — were most likely to have migrated to the lymph nodes.

Carbon black, which is one of the most common ingredients in tattoo inks, appears to break down readily into nanoparticles and end up in the lymph nodes, the study found. The team also looked at titanium dioxide (TiO2), which is a common ingredient in a white pigment usually combined with other colors to create certain shades. This type of ink does not appear to break down into particles as small as those found with carbon black, but some larger particles of TiO2 were still detected in the cadavers' lymph nodes, the study said.

Disturbingly, Schreiver and her colleagues found that some potentially toxic heavy metals originating in tattoo ink also made their way to the lymph nodes. The scientists detected particles of cobalt, nickel and chromium, which are sometimes added to organic tattoo pigment as preservatives, at the lymph nodes.

"These are not things you want to have permanently deposited in your body," Schreiver said.

Is it harmful?

Other research has shown that tattoo pigment may land elsewhere in the body. For a May 2017 study published in the journal Dermatology, researchers tattooed the backs of mice with black and red ink.

About a year later, the team found ink pigment in the mice's lymph nodes, as was found in human studies, but also within liver cells.

"It was a quite interesting and very surprising finding," said Mitra Sepehri, lead author of the research in mice and an M.D./Ph.D. candidate at the Wound Healing Centre of Bispebjerg University Hospital in Copenhagen, Denmark. "To reach the liver cells, the pigment has to go through the blood to reach the liver. So, we have shown that tattoo pigment can spread through the mouse's blood system as well as through the lymphatic system."

The ink pigment was detected inside special cells in the liver that remove toxic substances, called Kupffer cells. These cells appeared to be in the process of "eating" the pigment particles, Sepehri said. Of course, mice aren't humans, and, as Sepehri pointed out, the study did not confirm that tattooed humans can end up with pigment in their livers. Plus, she added, since mouse skin is thinner than human skin, tattoo ink may be more likely to be deposited more deeply in mice and more likely to enter the bloodstream.

"Even if we find out maybe in five or 10 years that tattoo ink can be deposited in the liver in human beings, we still don't know if it's harmful," Sepehri said. "It may pose no risk"

It's also not known if it's harmful for tattoo pigment particles to accumulate in the lymph nodes. So far, evidence suggests such deposits may cause enlargement of the lymph nodes and some blood clotting. But long-term studies in humans are needed to definitively link tattoo ink in lymph nodes to any harmful effect.

The ingredients within tattoo ink itself also remain largely unknown and under-regulated. A study from Denmark in 2011 found that 10 percent of unopened tattoo ink bottles tested were contaminated with bacteria. And a 2012 Danish Environmental Protection Agency study revealed that 1 in 5 tattoo inks contained carcinogenic chemicals.

Schreiver said she and her team hope to start raising the curtain on tattoo ink ingredients. They next plan to investigate inks associated with tattoo-related skin reactions and infections by analyzing skin biopsies of human patients. For example, it's commonly known that red tattoo ink is often associated with nasty skin reactions. But not all red inks are the same.

"As a chemist, describing a pigment as 'red' means nothing to me," Schreiver said. "We need to analyze the chemistry."

Tattoo ink manufacturing in the United States is overseen by the U.S. Food and Drug Administration (FDA), but as a cosmetic. As the FDA states, "because of other competing public health priorities and a previous lack of evidence of safety problems specifically associated with these pigments, FDA traditionally has not exercised regulatory authority for color additives on the pigments used in tattoo inks."

Ortiz said this needs to change. She works with the U.C. San Diego Clean Slate Tattoo Removal Program, which provides free care to former gang members who wish to erase their gang-associated tattoos to make it easier to enter the job market or the military. She said she sees many tattoo-related problems that can flare up again during tattoo removal.

"People have tattooed their bodies for thousands of years. Clearly, they're not going to stop," Ortiz said. "So, we need more testing on both the tattooing process and the ink to know potential reactions in the skin so we can optimize the safety of tattoos."

Originally published on Live Science.