Some of the reasons are obvious: If you head into a pool or the ocean, the water washes your sunscreen away, which is why most of them also recommend that you reapply immediately after swimming, too. However, even if you’re just laying still and catching some rays, your clothes, your towel, and even the sand can all rub it off, to say nothing of how much you lose through sweating.
But there’s a bigger reason why you need to reapply your sunscreen every couple of hours. “Chemical sunscreens absorb UV rays and disperse them, but the active ingredients don’t stay on the skin and continue doing their job,” says dermatologist Anthony Rossi. Basically, sunscreen gets used up in the process of blocking the sun’s harmful ultraviolet-B rays. That’s why you need to keep reapplying it, at least if you don’t want a sunburn.
As always, make sure you’re applying a sunblock with an SPF of at least 30. “If you were using an SPF 30, it would take you 30 times longer to burn than if you weren’t wearing sunscreen,” says Rossi. Unless you’re vigorously rubbing sand into your skin, of course, in which case you might want to rethink more than just your sunscreen habits.