DALLAS — Thirteen needles are simultaneously zinging in and out of Adam Metzger’s shoulder.

The 27-year-old is unruffled. He stares unblinkingly out the storefront window of Taboo Tattoo, a studio in the Bishop Arts District. To his right, Cody Biggs shades blue into a square of the Texas state flag. His movements are sure, even.

The buzzing suddenly falls silent. Biggs pauses to dunk the handpiece into a thimble-sized plastic cup of ink, then turns back to his canvas. Metzger’s shoulder is pink and puffy, weeping streams of ink and blood.

"How are you doing, buddy?" Biggs asks, rubbing on ointment in counterclockwise circles.

"It doesn’t feel good, man," Metzger responds. "But I’ve definitely felt worse pain."

Plenty of people know what he’s been through. As of 2012, 1 in 5 adults had a tattoo, up from 14 percent in 2008, a Harris Interactive Poll found. And when safety standards are followed, tattoos are usually trouble-free.

But tattoos can pose health risks that many people might not consider:

Unsterilized tools or contaminated ink can lead to infection, scarring, blood-borne diseases and other, less-obvious issues.

"It’s becoming much more common, but you still have to be careful," says Dr. Bryan Wasson, an internal medicine physician at Baylor Scott & White Medical Center at Irving, Texas. "A tattoo is like a minor surgery.

You clean and shave the skin like you’re going to operate. You use surgical tools. There are dangers. So be careful in your selection."

During the procedure, a gun with needles punctures the top layer of the skin, depositing pigment in a deeper layer called the dermis. As the skin heals, the ink remains trapped below the surface.

"When you get a tattoo, you bleed," said Dr. Donna Casey, an internal medical specialist at Texas Health Presbyterian Hospital. "Because you are bleeding, anything in contact with the tattoo — bacteria, viruses — can get into the wound and your entire body. It’s like having a bite on your leg or a gigantic abrasion."

Contaminated inks were the cause of an outbreak of serious infections in four states in late 2011 and early 2012. These infections were caused by a type of fast-growing bacteria that caused red, itchy bumps to severe sores requiring surgery. The 22 cases were associated with inks contaminated before distribution or just before tattooing.

Ingredients in tattoo ink vary, but they can contain metals, powders or other organic compounds in a liquid base. Problems can range from allergic reactions to scarring and the formation of bumpy knots called granulomas, more common in people with darker skin. The long-term effects of ink are still unknown.

"We know that the ink will gain access to your bloodstream," Wasson says. "I had a young gentleman come in, and he had a lymph node under his arm that was swollen. When we biopsied it, we found ink from his tattoo. We don’t really know what happens internally."

In rare cases, inks containing metallic pigments can cause swelling during magnetic resonance imaging, or MRIs.

"Tattoos are not an absolute contraindication for an MRI study," says Dr. Daihung Do, faculty director of dermatologic surgery at Harvard Medical School and director of dermatologic surgery at Beth Israel Deaconess Medical Center in Boston. "Patients should notify their radiologist that they have a decorative or permanent tattoo so that the appropriate precautions can be taken."

Tattoos can also prevent the early detection of skin cancer, says Peter Beitsch, a surgeon specializing in melanoma at Medical City Dallas Hospital. The ink can camouflage changes in asymmetry, borders, color and diameter, the "ABCDs" of melanoma detection. This is important for fair-skinned or redheaded people, who already have a higher risk of developing skin cancers.

"Sometimes when you cover up moles, the ink from the tattoo will mask changes in the mole," he says. "It’s not common. But if you cover up enough moles, some of them are going to turn bad, into a lethal kind of skin cancer."

Beitsch refers to the case of a 35-year-old man who got a large tattoo on his shoulder in honor of a brother who had died of leukemia. He did not catch changes in the mole and died of melanoma.

"It’s tragic," Beitsch says. "About half of melanoma starts in pre-existing moles. Be aware that if you cover up a mole, you need to be paying attention to it."

The Food and Drug Administration regulates tattoo ink but considers it a cosmetic and intervenes only when problems arise. The FDA has not actually approved any tattoo ink, and there is no specific requirement that explicitly says tattoo inks must be sterile.

"Tattoo inks are not highly regulated," Do says. "Many of the pigments are industrial grade, and none are currently FDA-approved. Although tattooing has been practiced for thousands of years, there are few studies regarding their safety."

Theresa Eisenman, a spokeswoman for the FDA, said this is because no sponsor has signed the required petition and provided the data needed to decide whether dye is safe for tattooing.

The easiest — and most important — way to avoid becoming a tattoo horror story is to research the tattoo parlor and review personal health history ahead of time.

"Like anything, like ear piercings, you can develop other medical problems if it isn’t at a clean place," Do says. "It all depends on who does your tattoo and whether they are cleaning their instruments in a safe manner. If you go to the wrong place, it could be very easy to contract something."

Medical experts also do not recommend tattoos for people with a history of allergies, diabetes, heart disease, skin disorders, immune system conditions, a history of infections or who are pregnant. For those with a family history of skin cancers, avoid areas that would cover up moles.

"Do your research," Metzger says. He stands in front of a mirror, examining the newly finished art on his arm.

"There are certain things everyone should check for," he says. "Find a place and an artist you like. If you don’t get a good vibe, maybe that shop is not the shop for you. Something I always look for is an autoclave machine. Disposables are OK, too. I want them to wear gloves; I want to hear that snap. If you are unsure about any part of the process, don’t do it."