For decades, dermatologists have warned that sunscreen is important for skin protection, and it seems that the message has gotten through: Data suggest that 57% of Americans use sunscreen regularly. But now, experts are urging the public to remember that while SPF is crucial, it’s a myth that sunscreen alone will completely shield you from the sun’s rays. Moreover, relying entirely on sunscreen—particularly if you’re not using enough or not reapplying frequently—can open the door to health concerns associated with sun exposure, such as skin cancer and premature skin aging, she says.

“In a perfect scenario, you would do more than apply sunscreen,” says Ife J. Rodney, M.D., founding director of Eternal Dermatology + Aesthetics. Gary Goldenberg, M.D., assistant clinical professor of dermatology at Icahn School of Medicine at Mount Sinai Hospital, agrees. “Sunscreen alone is not enough,” he says.

That doesn’t mean sunscreen is useless—far from it. But it’s possible to rely on SPF too heavily, when you could be taking a more effective approach. Below, doctors break it down.

Why isn’t sunscreen alone enough to protect your skin?

There are a few reasons for this. One is that many people who use sunscreen don’t apply enough, Dr. Rodney says. “They put on a very thin layer or accidentally leave gaps of exposed skin, which doesn’t give you enough protection,” she adds.

Adults need at least an ounce of sunscreen to cover their exposed skin, which is about the amount that it would take to fill a shot glass, according to the American Academy of Dermatology (AAD). But that’s a minimum amount. “Some people just don’t put on enough sunscreen—that’s the bottom line,” Dr. Rodney says.

Something else to consider, per Joshua Zeichner, MD, director of cosmetic and clinical research at Mount Sinai Hospital: “The SPF value of a sunscreen is measured based on a density of 2 milligrams per square centimeter. If you’re not applying that much, then you are not getting the SPF labeled on the bottle.”

Many people also fall short with reapplication, per Dr. Rodney. “If you’re out in the sun for two hours or more, you need to reapply,” she says, adding that if you’re swimming or sweating, it’s more like every 40 minutes. “If you don’t apply enough sunscreen or don’t reapply, you get to the point where you’re completely exposed.”

Additionally, even if you apply (and reapply) sunscreen perfectly, it’s not foolproof. “Even the highest SPF sunscreen does not block 100% of UV rays,” Dr. Goldenberg says. “People get a false sense of security and then are surprised when they get a sunburn even though they applied sunscreen.”

Dr. Rodney agrees: Sun exposure is still sun exposure. “You want to be in a shaded area,” she says, noting that wearing long sleeves or a broad-rimmed hat can be helpful, too.

What’s ideal for sun protection?

Wearing sunscreen daily—and reapplying as needed—is a good start, Dr. Rodney says. It just shouldn’t be the only protective measure you take. Dr. Rodney suggests seeking shade where you can, too, if you want to do all you can to shield your skin.

“If you’re going in the water and back to the sand with sunscreen on, or go for a walk, that is fine,” Dr. Rodney says. “But just hanging out in the sun isn’t ideal. You want to find shade, if you can.” That can be in the form of an umbrella, sun-protective clothing with a UPF rating, a hat, and sunglasses, Dr. Goldenberg says.

Dr. Rodney also stresses the importance of wearing sunscreen when you’re indoors. “Some of my patients will say, ‘I don’t go outside, so I don’t need to wear sunscreen,’” she says. “But those rays can still harm your skin, especially if you sit next to a window.”