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If you've ever opted for gel polish at the salon, you're probably used to drying your nails beneath a UV lamp. And perhaps you've found yourself waiting and wondering: How safe are these?

Researchers from the University of California San Diego and the University of Pittsburgh had the same question. They set out to test the UV-emitting devices using cell lines from humans and mice and published their findings last week in the journal Nature Communications.

They found that chronic use of the machines can damage DNA and cause mutations in human cells that could increase the risk of skin cancer. But, they caution, more data is needed before being able to state that conclusively.

"Future large-scale epidemiological studies are warranted to accurately quantify the risk for skin cancer of the hand in people regularly using UV-nail polish dryers," the study says. "It is likely that such studies will take at least a decade to complete and to subsequently inform the general public."

Maria Zhivagui, a postdoctoral researcher at UC San Diego and the first author of the study, told NPR in a phone interview that she was alarmed by the strength of the results — especially because she was in the habit of getting gel manicures every two to three weeks.

"When I saw these results, I decided to kind of put a hold on it and just mitigate as much as possible my exposure to these risk factors," Zhivagui said, adding that she — like many other regulars — even has a UV dryer at home, but now can't foresee using it for anything other than maybe drying glue.

The study confirms concerns about UV dryers that the dermatology community has had for several years, says Dr. Shari Lipner, a dermatologist and director of the Nail Division at Weill Cornell Medicine.

In fact, she says, many dermatologists were already in the habit of advising gel regulars to protect their skin with sunscreen and fingerless gloves.

"I think even before the study, that was the way to go," Lipner told NPR. "And in light of the study, I think it should convince people even more to use caution."

What the study shows

Zhivagui's team decided to pursue this research for several reasons, she explained.

Over the years there have been sporadic cases of frequent gel polish users — like beauty pageant contestants — reporting rare cancers on their nails and fingers, which some dermatologists attributed to UV exposure (Lipner says the few studies on this have been mixed).

Plus, the UV devices are widely marketed as safe (even by the FDA), but she was unable to find any studies that actually investigated their effect on cells to confirm that.

"We wanted to devise this study and design it in order to ... address questions about the potential harms of these artificial UV lamps," says Zhivagui, who focuses on public health and cancer prevention research.

The researchers tested three different cell types, two from humans and one from mice, with various levels of exposure.

Under acute exposure, the petri dishes were placed in a UV machine for 20 minutes, then taken out for an hour and placed back in for another 20 minutes. Under chronic exposure, they were placed under the machine for 20 minutes a day for three days.

Researchers found that a single 20-minute session resulted in 20-30% cell death, while three consecutive sessions caused 65-70% of the exposed cells to die. UV exposure also caused mitochondrial and DNA damage in the remaining cells.

Zhivagui notes that nail salons typically have UV devices that are more potent than the one used in the study, meaning that even if they are used for less time in the salon than in the lab, the energy of the exposure could be higher.

The study shows that repeated exposure to UV light has a damaging effect on human cells. But it can't conclusively link it to an increased risk of skin cancers — though it does cite prior studies that link gene mutations to an increase in cancer risk.

There are limits to what one study can show. Lipner points out it would be unethical to replicate it with actual humans, if there's any risk volunteers could develop skin cancer.

"So sometimes cell line studies, even though they're not exactly what's happening in the human, [are] kind of our best shot to see what's going on," Lipner says.

Both she and Zhivagui say large-scale epidemiological studies are needed. That could involve following many regular gel polish users over the years and then running analyses on anyone who does get sick.

There's a lot we don't yet know

Gel manicures are relatively new, and DNA damage often takes time to build up — so it's entirely possible that more skin cancer cases associated with UV dryers will emerge in the coming years, Lipner says.

"And so what I would recommend is that people who have a long history or regular history of doing the gel nails ... every few weeks for years, they should partner with a board certified dermatologist who can check their skin regularly looking for early signs of skin cancer," Lipner says.

While people may be used to monitoring their moles for signs of melanoma, Lipner stresses that the type of skin cancer linked to UV dryers is actually another form known as squamous cell carcinoma, which looks very different and is noticeably rough to the touch.

"In lighter patients, it could look more pink or red," she says. "In people with darker skin types it may look more purple to brown."

Zhivagui notes that certain people are at higher risk of skin cancer, like older people or those with fair skin or red hair.

And the FDA, which does classify UV lamps as "low risk when used as directed," says you may want to avoid UV lamps if you take certain medications or supplements that can make you more sensitive to UV rays, including some antibiotics and oral contraceptives.

Lipner says without data, it's impossible to say how much UV exposure is too much — but less is better.

"I would say, in general, look for alternative ways to do manicures," she says. "If you are going to get a gel manicure, use sunscreen and fingerless gloves; that will certainly add a ton of protection. And maybe if you're going once a month, you can limit that to a few times a year."

She adds that there's another reason people may not want to get gel polish: The acetone needed to remove it has been shown to thin out and weaken nails, which is both a cosmetic issue and a functional one.

Consider precautions and alternatives

There are steps you can take to protect yourself during gel manicures and pedicures — as well as alternatives to them.

The FDA advises limiting dryer use to 10 minutes per hand and removing cosmetics, fragrances and skin care products in advance, since some of those products can increase your sensitivity to UV rays. The one major exception is sunscreen.

Experts recommend applying a broad-spectrum sunscreen to your hands about 20 minutes before they are exposed to UV light. Lipner says people should use a broad-spectrum sunscreen of SPF 30 or higher, and make sure they cover the tops of their hands, feet, fingers and toes as needed.

And, perhaps surprisingly, she says there's no need to worry about covering your nails, as studies have shown nails are "practically impermeable" to UV light.

That's why experts also suggest investing in a pair of fingerless, UV-absorbing globes that leave only your nails exposed.

And you'd rather not spend any (or at least as much) time under UV dryers, there are some nail polish alternatives to consider.

Some brands make long-lasting, fast-drying gel polish that don't need to be cured by UV light. You can also opt for dip powders, which are applied directly to the nail, air dry and can last for several weeks (though they typically cost more than gel).

And don't forget about good, old-fashioned regular nail polish, which will harden and eventually dry without any UV light. The Skin Cancer Foundation says the safest bet is to use an air blower or fan for your regular manicure, or let them air dry without any tools at all.