In theory, SPF 50 and SPF 100 are very similar sunscreens: one blocks 98 per cent of UVB rays, and the other blocks 99 per cent. The problem is, nobody applies the amount of sunscreen that will get you the SPF listed on the bottle. So in reality you're getting very different protection out of those two bottles.
And no, the answer isn't just to apply more (although it will certainly help). Have you ever tried actually applying a shot glass of sunscreen? Amy Marturana did, and wrote about the experience for Self. She managed to get the sunscreen on, but it took forever to rub in, she wasn't able to adequately reapply, and she wrote of the experience: "My skin felt drenched in lotion all day. Even inside when I was no longer sweating, I felt almost weighed down." This is not a realistic amount of sunscreen to wear.
So why is it the recommended amount? A shot glass is enough to give you 2 milligrams per square centimeter of skin. That's the amount that will get you the SPF listed on the bottle, because that's the amount that's used in SPF testing. "This was considered to be the lowest application density that could be applied consistently for clinical SPF testing purposes," explains a review on sunscreen testing. In other words, their experiments require that they test a thicker layer than people actually apply.
Real World Results
When you apply sunscreen normally — or even in what feels like a thick layer — you're getting less protection than the people who tested the sunscreen in the first place. That's fine, as long as you take steps to protect yourself. One of those is choosing a higher SPF. In a recent study published in the Journal of the American Academy of Dermatology, researchers asked 199 people at a Colorado ski resort to apply an SPF 50+ sunscreen to one side of their face, and SPF 100+ to the other side. (They didn't know which was which; they were given bottles marked "right" and "left.") The volunteers carried the bottles with them throughout the day and reapplied however they liked.
The result: 14 per cent of volunteers had redness the next day on the SPF 100+ side of their face, and 41 per cent on the SPF 50+ side.
So the higher SPF really did work better, in real-world conditions. Dermatologist Dr. Fayne Frey agrees that higher SPF is preferable, but she has a warning. Sunscreens are only tested for whether they prevent redness (sunburn), which is caused by UVB rays, but skin also gets other kinds of sun damage from UVA rays and from waves outside the UV spectrum, including visible and infrared light.
She says, "If the result of not getting red or sun burnt results in the person spending more time in the sun getting exposed to more ultraviolet light (even SPF 100+ sunscreen doesn't protect 100 per cent), visible and infrared light ... then more harm than good is the result."
So preventing sunburn doesn't mean you're preventing every type of skin damage. Sunscreen is necessary, but ultimately the best protection is keeping the sun off your skin entirely, through hats and clothing and staying in the shade.