Before a day at the beach you may want to consider this: The interaction between the sun rays, our skin and our body is a complicated one. So much so that leaving the sun behind doesn’t stop its effects. According to a study published in Science, skin damage continues 3 to 4 hours after you have gone inside.

What’s more, melanin—you may know it as the stuff that makes us tan and helps protect us from the sun—might fuel the process. “It [melanin] is doing good things and it’s doing bad,” says study co-author Douglas E. Brash, PhD, a clinical professor of therapeutic radiology and dermatology at Yale School of Medicine. At the most basic level, there are two types of melanin that make up our skin color—light and dark. Which way the ratio of the two goes, determines our skin cancer risk: Dark skin has less risk than lighter skin. “And that is all true, but that isn’t the whole story.”

For a long time there has been an increasing body of evidence that melanin may be causing a bit of havoc under its halo; however, those studies were done in test tubes or there were other effects that the melanin could have. The research that Brash and colleagues published helps put together the odds and ends of science’s findings about melanin.

The moment that an ultraviolet (UV) photon penetrates our skin it starts off a chemical reaction activating two enzymes whose chemical byproducts react with each other instantly creating a third chemical compound. “If melanin is nearby that substance can break the melanin down, dissolving it,” Dr. Brash explains. “It also starts off a chain reaction which ends up with two extra oxygens on the melanin in a square ring—it is very unstable with high energy and it isn’t going to last very long. It breaks.” Think about when planets blow up in the movies, that energy flies outward. Now imagine that the planet is melanin and DNA is next door. “The DNA absorbs the energy, and the result is the same as what we have always thought happened when a UV photon entered the skin, the energy is absorbed and it puts a bend in the DNA.” This process can continue for 3 to 4 hours after exposure to UVA or UVB rays.

And it is this bend that has a few outcomes—some toward skin cancer. That bend could prevent a cell’s DNA from being copied or signal it for programmed cell death. In both cases the cell dies. Another possibility is that the cell’s DNA is copied with a mistake, giving the daughter cell a mutation. “In that case at least the cell is not dying but you get a mutation. If that happens in an oncogene (a cell that can turn into a tumor cell) or a tumor suppressor gene then that cell can start on the way to skin cancer,” Brash says.

How do they know? This recent paper built upon research that Dr. Brash and his colleauges did nearly a decade ago. Cell death is a tricky thing: It can be a preventative strategy and in many ways it keeps cancer from developing. However, a mutation in particular genes, such as P53, makes the cell resistant to cell death. He explains: “So if you happened to get a mutation in P53 during your trip to the beach last week, then cell death as a protection mechanism backfires on your second trip to the beach. The body kills damaged cells that don’t have this mutation and the cell that does survives and has room in your skin to proliferate and fill up the space left by the cells that did die. In that case, cell death favors growth of a cell with a mutation. We usually think about cell death as a way to prevent cancer but it depends on the numbers. On the one hand it prevents cells from starting down the road to cancer but on the other hand cells already going down that path are speeded up.”

But this isn’t a reason to say bye-bye to sunshine; our bodies can do amazing things including repair bent DNA. “Then you have race between making the damage and repairing it,” Brash says. “If you only sustain a little bit of damage, from the sun exposure you get while walking down the street, your body can keep up with it. But if you are lying on the beach, that isn’t what your body is designed to do and it can’t keep up.”

“If you can prevent the photon from ever getting into your skin—which is the purpose of sunscreen, then you prevent all these other things from happening,” Brash says. There isn’t a stronger case to wear sunscreen. Why gamble with your health?