Americans love tattoos. Nearly a quarter of them have gotten inked, compared with 12% in Europe. The personal expression that is skin deep is particularly popular with young people: 40% of American teenagers sport tattoos. Women are also more likely than men to get one.

Yet very little is known about the effects of modern day tattoo ink on the human body. The lack of research and data is worrisome because some of the key ingredients are known to make people sick or die. It also makes it difficult to regulate them.

A recent report from the European Commission warns that tattoo ink often contains “hazardous chemicals” such as heavy metals and preservatives that could have serious health consequences, including bacterial infections. A separate study issued earlier this month by the Australian government reveals that 22% of the inks tested contained chemical compounds known to cause cancer.

The European report notes that regulators are especially wary of imports from the US, which supplies the majority of tattoo inks to the world. The report highlights the health risks and provides European countries with scientific evidence so they can decide if better oversight of tattoo inks is necessary.

“The question is, what’s in the tattoo and what can it do to the body?” said Tyler Hollmig, director of Laser and Aesthetic Dermatology at Stanford University Health Care. “The answer is, we just don’t know.”

Tattoos work by injecting the ink into the second layer of skin known as the dermis, where the ink remains permanently unless it is removed using laser technology.

“The skin is very active in protecting the body from infection, and white blood cells try to eat up tattoo ink,” said Hollmig. “The body treats the ink as a foreign substance.”

The Federal Food and Drug Administration (FDA) doesn’t need to approve tattoo inks before they go on the market. The agency can regulate the pigments used by screening them beforehand, but it seldom does so “because of other competing public health priorities and a pervasive lack of evidence of safety problems specifically associated with these pigments,” said Lauren Sucher, an FDA spokesperson.

The agency will investigate an ink only when a safety issue is reported. This happened last year, when Miami-based company, A Thousand Inks voluntarily recalled some of its products after the FDA found bacterial contamination in unopened bottles of the firm’s grey wash shades. The FDA tested the ink following an outbreak of skin infections in people who had recently got tattoos in Florida. The FDA warned tattoo artists that using the contaminated inks could lead to skin irritation such as redness and swelling, and could spread throughout the body, resulting in swollen lymph nodes and even sepsis, a life threatening blood infection.

“These infections may be severe and may require extensive treatment with antibiotics, hospitalization or surgery,” warned the FDA.

This kind of public health scare and the popularity of tattoos and emergence of health problems are forcing the FDA to consider more oversight over this $2.3bn industry. It’s currently investigating how tattoo ink is broken down in the body and its long term safety at the National Center for Toxicological Research in Arkansas. Researchers are examining some pigments, like Yellow 74, which fade in the sunlight.

“We want to know what happens to the ink,” said Paul Howard, the lead research chemist. “Where does the pigment go?” Howard said the pigment may still be there, and that it could be toxic. His team is also looking at the possibility that the body digests and secretes ink in the same way that it destroys bacteria when fighting infection.

Meanwhile, ink manufacturers already have to contend with rules from some state or local governments. In 2005, a California judge ruled that two major tattoo ink makers – Huck Spaulding Enterprises and Superior Tattoo Equipment – must include a label on their products warning California customers the ink contains heavy metals that could cause “cancer, birth defects and reproductive harm”.

Some companies claim to use safer ingredients in their ink. Kuro Sumi, a Japanese manufacturer, and US-based Eternal Ink say they use organic pigments that are vegan-friendly. But since there are no industry regulations for terms like “organic” and “natural”, it’s hard to know whether products containing these ingredients are indeed safer.

Tattoo ink is generally sold ready-to-use, and typically contains a number of ingredients, including colorants, preservatives, binding agents and fillers. Colorants, more commonly called pigments, can constitute up to 60% of an ink by weight, according to the EU study. The pigments used by manufacturers aren’t specifically made for use in tattoos, and many are “industrial-grade colors suitable for printers’ ink or automobile paint”, according to the FDA.

The overall longevity and potential toxicity may differ depending on the color. To make black ink, for instance, manufacturers might use soot or powdered jet, or cinnabar and common rust to make red. Some of the ink ingredients, like the metal cadmium, are known carcinogens, while others, like carbon black, are “possibly carcinogenic”, according to the International Agency for Research on Cancer, an arm of the World Health Organization (Who).

Yet this doesn’t necessarily mean those ingredients are dangerous to human health, said Hayley Goldbach, a resident physician in dermatology at UCLA Health, a healthcare system affiliated with the University of California at Los Angeles.

“Just because something can cause cancer doesn’t mean that it does,” she said. “As far as I know there have not been any studies convincingly or conclusively linking tattoo pigment to an increased risk for cancer.”

A 2012 paper by Finnish researchers, who analyzed previous studies, found the number of skin cancers inside tattoos was “seemingly low”, and that any link had to be considered “thus far as coincidental”.

Tattoos can camouflage moles, however, making it difficult to monitor tattooed patients for skin cancer. And tattoo ink ingredients like cadmium have been found in lymph nodes, which help filter waste from the body. According to the EU study, these substances could spread from the lymph nodes to the rest of the body, although more research is needed to support this theory, wrote the authors.

Hollmig said he sees skin cancer in and around tattoos, but it’s hard to say whether it’s the ink causing the illness.

“You figure that you inject the carcinogen and skin cancer grows there, but we don’t have data on that,” he said.

The most common tattoo-related complaints he sees in his practice are skin irritations, usually caused by red ink, which typically contains mercury, a known allergen.

“Just the red area of the tattoo would have swollen, tender and really itchy reactions,” he said, adding that these adverse health effects can occur months and even years after a person gets the tattoo.

A 2015 study from New York University’s Langone Medical Center found that at least 6% of tattooed New Yorkers suffered a serious reaction like swelling and severe itching that lasted for more than four months. Almost half of these reactions were to red ink, while one-third of the cases were linked to black ink.

As for tattoo artists, most do care about the safety of the pigments that they use on their clients, said Mike Martin, a veteran tattoo artist and president of the Alliance of Professional Tattooists, a nonprofit that promotes safe tattooing.

“Pigments are super important to the finished product and our products are what people judge us on,” said Martin. “Great color, great line work, quick healing, trouble-free, no weird skin reactions – these are all hallmarks of good professional tattooing.”

Without strong regulations, however, problematic inks will continue to show up at tattoo parlors. For example, tattoo manufacturers can use different recipes and make their own blends, which could contain different levels of carcinogens and allergens. This makes it “very hard to study them for safety”, said Goldbach.

“The composition of tattoo ink has changed over time so it also means that new and potentially dangerous agents could be introduced into tattoo ink at any time,” she said.