Here’s a general rule most people live by: Don’t get poked with a needle unless absolutely necessary — and even then only by a medical professional.

But there’s a big exception to that rule: tattoos. In 2016, a poll found that three in 10 American adults had permanent ink splayed on their skin; Pew has put that number as high as four in 10 among Millennials.

And yet, tattoo artists aren’t considered medical professionals. (Okay, there’s no reason a tattoo artist couldn’t be a retired doctor or nurse; it’s just not the norm.) And while tattooing is certainly art, it’s an art form deeply intertwined with dermatological science.

Major medical sources like the FDA have pointed to ink ingredients being similar to that of printer toners, but that’s muddled the science a bit too, in that tattoos aren’t necessarily made of unsafe ink that will slowly poison your body.

tattoo albert einstein dan gold science
Tattooist Dan Gold creates a tattoo based on an image of Albert Einstein in his studio.

University of Rochester Medical Center dermatologist and skin cancer expertSherrif Ibrahim explains the science of tattooing, and busts common myths of tattooing.

But first, how do tattoos work?

tattoo samoan pe'a
Tattooist Kasala Laei Sanele (R) crafts the pe'a, a form of traditional Samoan tattooing, in Auckland, New Zealand.

Imagine an ink so thick and binding and sticky that once it’s applied to a surface — any surface — that surface is stained forever. It can never be cleaned.

Now dump that ink over someone’s arm. It would fix to their skin, a splatter stain that would last for weeks or months. But soon enough it would disappear.

“If you look at the epidermis, or outer layer of the skin, it looks like a nice brick wall,” Ibrahim said. “Those bricks and mortar are cells — keratinocytes and melanocytes — and there’s a lot of activity going on within those cells.”

skin cells epidermis tattoo
A diagram shows skin at 40x magnification under a microscope.

Like every cell in a body, skin cells have life cycles — and quick ones. “Pretty much every couple months you have a new epidermis. So anything in the epidermis would never be permanent,” Ibrahim said.

That’s why tattooists use needles to dig under the epidermis to deposit pigment a full layer down, in the dermis.

The dermis is mostly made up of connective tissues like collagen and elastin, with a few cells known as fibroblasts strong between them. With less cellular activity going on down there, balls of ink can slip in among the local tissues and hang out for years.

Tattoo microscope dermis
Tattoo pigment (in black) is visible trapped within the dermis.

The pigment doesn’t have to lodge that deep in the skin to stick around, Ibrahim said. On a person’s back, for example, the epidermis doesn’t get more than 150 microns thick, just a fragment of a millimeter.

That’s why “you’ll never see something that’s really black,” Ibrahim said. “[A black tattoo] almost has a bluish tint to it, because you get some refraction of the light as the black passes through the superficial dermis and the epidermis.”

So is all that jabbing and inking actually dangerous?

Ask Google whether tattoos cause cancer, and you’ll get some alarming results.

A 2016 article in The Independent opens with an alarming claim: “Tattoos can cause cancer and mutations — and one color is potentially more toxic than others, according to scientists.” The Guardian is a bit more measured, reporting that tattoo inks may contain cancer-causing chemicals.

These articles are sourced to a single report that finds certain tattoo inks contain chemicals associated with health problems, including cancers.

tattoo ink cancer
A tattoo artist dips his needle in ink as he applies a tattoo on a customer.

Here’s the deal: While in many places there are laws requiring tattoo shops to meet basic health and safety guidelines in how they use needles, there’s little regulation covering the inks those needles inject. The report in question takes a stab at examining those links, and opens the door to future research into possible risks from certain brands.

There’s no reason, in other words, to think getting a tattoo is going to cause cancer to bloom in a person’s skin. A deep review of the research on skin cancer and tattoos published in The Lancet found a grand total of 50 cases of cancer growing on tattoos, despite concerns about carcinogenic ink. That puts the relationship between the ink and cancers squarely in “coincidental” territory, the authors write.

Asked whether he worries about tattoos causing cancer for his patients, Ibrahim said, simply, “No.”

“I think years ago there was a much higher risk for communicative diseases, in particular hepatitis from dirty needles. I don’t really think people are going to those types of tattoo places anymore. There’s risk any time you break the skin of infection. There are risks of changing your mind — that’s the biggest one I think, regret,” Ibrahim said.

tattoo sailors
One sailor gives another a tattoo aboard the American battleship USS New Jersey in 1944.

The most serious danger of a tattoo is a bad allergic reaction to the ink itself. “There are really allergic reactions, where a particular pigment causes thickening or keloiding of the area. And that looks and feels terrible. But that’s pretty rare.” Red pigments in particular seem to risk causing a bad reaction, Ibrahim said.

Ibrahim says that the most worrisome side effect of a tattoo is actually far more practical but overlooked. “What I see as someone who treats skin cancer, especially in people who have sleeve tattoos, large areas on the back, and so forth, is that you can’t always see a skin cancer. It’s not that the tattoo caused it, it’s just that it can mask it,” he said. The more area covered in tattoo, the more serious that risk is, and the more the tattooed person should pay attention to minute changes within their skin.

Does the sun actually fade a tattoo?

Someone Googling tattoos would also learn that that for inked-up folks, sun is the enemy. Conventional wisdom in tattoo shops holds that the sun’s ultraviolet light will penetrate the epidermis and wreck the pigment buried deep in the dermis.

Ibrahim is skeptical, though. “It’s a question of where it is in your body and whether that UV can reach it. UV can reach the superficial dermis, but not I don’t think [it would fade a tattoo],” he said. “The UV from sun and other sources just don’t make it that deep.”

tattoo outdoors sun beach
You should protect your skin from the sun, but not to save your tattoos.

Most of a tattoo’s fading over time is the result of the body’s natural processes. Microphages — cells that wander the body looking for gunk to clean out — will eventually clear many of the globs of ink out of the dermis. But that can take decades.

That said, it can’t hurt to protect skin from UV light — and it will save lives. Even people who tan but never burn are at risk of deadly diseases if they let their skin darken without sunscreen.

Let’s say you’re regretting that tat of your everlasting love to your ex. What do you do? Laser removal has proven promising, which involves shooting lasers, tuned to the pigment of the target ink, deep into the skin. “[The laser] heats the tattoo particles very quickly, very rapidly, so [the balls of pigment] basically explode into very, very, very, tiny particles,” Ibrahim said. Notably, the technology for doing so has improved dramatically over the years, with super-short laser bursts heating the pigment faster, requiring fewer sessions and therefore less pain.

That doesn’t mean a person should view a tattoo as temporary: Tattoo removal comes with some steep expenses, along with risks of pain, scarring, and the removal process simply not working.

The science of tats are evolving and slowly improving. One myth that seems to stand the test of time? The fact that tattooed dudes are deemed pretty freaking hot. That, combined with research indicating that tattoos might actually be a boon for the immune system, make getting inked a lifestyle choice that might not be so bad after all.

Photos via Getty Images / Oli Scarff, Getty Images / Sandra Mu, Gregor T. Overny, Yale Skin Lab, Getty Images / Joe Raedle, Photographed by Fenno Jacobs. Department of Defense. Department of the Navy. Naval Photographic Center., Getty Images / Michael Dodge, Giphy, Getty Images / Wang He