With 40 percent of Americans between the ages of 18 and 29 having at least one tattoo, long gone are the days when adorning the skin with indelible ink was an act of edgy rebellion. But that doesn’t mean it doesn’t indicate daring behavior — if for nothing else than the health risks that come along with injecting ink into the skin.

Consider the following:

1. Infections: The obvious

Infections from tattooing are nothing new, but the connection between hepatitis C and body art is more widespread than many people may be aware of. The blood-borne virus is the leading cause of liver cancer in the U.S. and is responsible for chronic liver disease in 70 percent of people infected with it. According to a study in the journal Hepatology, people with hepatitis C were almost four times more likely to report having a tattoo, even when other major risk factors were taken into account. "Tattooing in and of itself may pose a risk for this disease that can lay dormant for many, many years," said study co-author Fritz Francois.

A study published in June 2017 said that a man who went swimming in the Gulf of Mexico immediately after getting a tattoo managed to contract a skin disease called Vibrio vulnificus, also known as flesh-eating bacteria. His fresh ink made him vulnerable to it, and the infection led to his death a few days later.

2. Infections: The sneaky

While tainted needles and unsanitary conditions are often to blame for hepatitis C infections, people can get infections even in the most sterile of conditions. How? Contaminated ink. Ack! In 2014, one ink company recalled its product after testing confirmed bacteria in unopened bottles of ink. And a few years before that, 19 people who received tattoos in upstate New York suffered infections from contaminated water that was used to dilute the ink. Symptoms range from local pain to fever, shaking chills and sweats, with increased risk for anyone with pre-existing conditions.

3. Sweat changes

Woman with tattoo doing yoga A new study finds that getting a tattoo changes the physiology of your skin and the way your sweat glands work. (Photo: fizkes/Shutterstock)

According to a small new study, the amount you sweat and the saltiness of your sweat changes after you get a tattoo. The findings may be of particular importance to athletes, many of whom sport tattoos. The process of getting a tattoo changes the physiology of the skin and the way sweat glands operate. However, the study author told the New York Times that it is unlikely that tattoos would impede perspiration enough to contribute to overheating or other problems in most people, even during exercise.

4. Allergic reactions

You might think that an allergic reaction from the process would dissipate quickly enough, but such is not always the case. The most common allergic reactions come courtesy of pigments in the ink, and while they often occur soon after the tattoo, they can appear or persist for months and even years after the tattoo was received. The thought of having an allergen injected permanently into the skin does seem slightly crazy.

5. Mystery inks

Man has been ornamenting himself with tattoos since at least 6000 B.C., and with a wide array of media ranging from ash to inks that include metal salts, lead and cobalt. Today, according to the American Academy of Dermatology, many modern inks contain organic azo dyes with plastic-based pigments that also have industrial uses in printing, textiles and car paint. There isn't a lot known about how these ingredients might interact with the skin or the body in general. And that's just the top of the mystery ingredient list. Specifically, the following ingredients may be of concern.

6. Heavy metals

Tattoo inks often contain lead and other heavy metals that could be a concern for inciting allergies. Tattoo inks often contain lead and other heavy metals that could be a concern for inciting allergies. (Photo: IvanRiver/Shutterstock)

In one analysis of 17 black inks from five different manufacturers, a number of heavy metals turned up, including lead. Tattoo inks often contain lead, cadmium, chromium, nickel, titanium and other heavy metals that could be concern for inciting allergies or diseases, scientists say. Some pigments are industrial-grade colors that are "suitable for printers' ink or automobile paint," according to the Food and Drug Administration.

7. Dibutyl phthalate

Researchers have reported finding the chemical dibutyl phthalate, a common plasticizer that is likely problematic for human health, in black tattoo inks. They looked at 14 commercially available inks; they discovered low levels of dibutyl phthalate in all of them.

8. Polycyclic aromatic hydrocarbons (PAHs)

A 2010 study found that black inks often contain products of combustion called polycyclic aromatic hydrocarbons (PAHs). The PAHs included benzo(a)pyrene, which has been identified in an EPA report as "among the most potent and well-documented skin carcinogens." It is such a well-known carcinogen that it is employed in animal tests to grow tumors.

"Tattooing with black inks entails an injection of substantial amounts of phenol and PAHs into skin. Most of these PAHs are carcinogenic and may additionally generate deleterious singlet oxygen inside the dermis when skin is exposed to UVA (e.g. solar radiation)," wrote the study authors. They said the PAHs could "stay lifelong in skin" and "may affect skin integrity," which could lead to skin aging and cancer.

9. Your poor lymph nodes

Studies have shown that tattoo inks move into people's lymph nodes, one of the body’s magical ways of filtering out organisms that cause disease. Whether the migration of tattoo ink has health consequences or not is still unknown, notes a 2009 FDA consumer update, but do you really want to be cluttering up an organ that helps to keep the body free of disease?

And apologies in advance to anyone who has received this article complements of their concerned mom. (Just remember, sometimes mothers know best.)

Editor's note: This story has been updated since it was originally published in August 2014.