Before you buy sunscreen, it’s important to understand what sun protection factor, or SPF, means. That’s the number you see on the bottle when you’re picking out your sun safety products.
A sunscreen’s SPF is a measure of how well it protects you from getting a sunburn. But a higher SPF number doesn’t necessarily mean better protection from sun damage.
In fact, choosing a sunscreen based only on a high SPF can lead you away from your sun safety goals. “A high SPF can give people a false sense of security,” says Saira George, M.D., a dermatologist at MD Anderson in Sugar Land. “There are a lot of limitations with SPF numbers. People often mistakenly think they can’t get sunburned or they can be out in the sun for much longer than is safe if they put on SPF 100 sunscreen.”
Double SPF does not always mean double protection
A sunscreen’s SPF is a measure of how many harmful ultraviolet rays it absorbs or reflects away from your skin.
There are two kinds of UV rays – UVA and UVB. Each penetrates your skin differently. The SPF rating only refers to UVB rays.
An SPF 15 sunscreen blocks 93% of UVB radiation, and SPF 30 blocks 97%. After that, the difference in protection is small. SPF 50 blocks 98%, and SPF 100 stops 99% of UVB rays from reaching your skin.
And it’s important to remember that SPF measurements are determined in laboratory settings where a precise amount of sunscreen is applied evenly to an area of skin, and then exposed to a controlled light source.
“Even when you know a sunscreen’s SPF, it’s hard to know exactly how it will do in the real world, where factors like sun intensity and application levels aren’t controlled,” says George.
SPF 30 is usually high enough
If you pick SPF 30 and reapply at least every two hours, George says you’ll be making a good choice. “A higher SPF might give you a little more wiggle room if you didn’t apply enough sunscreen, or you forgot to reapply,” says George. “But after a point, going higher doesn’t mean you are getting dramatically more protection.” Here’s why:
To get the full SPF protection noted on the bottle, you need to apply the sunscreen liberally and evenly. “Most of us tend to under-apply our sunscreens so we’re not getting to the listed level of protection at any SPF,” George says.
SPF rubs off and sweats off. “Sunscreen should be reapplied at least every two hours,” says George. “Even the highest SPF sunscreen won’t protect you when it wears off.”
Choose a broad spectrum sunscreen
Even though the SPF rating focuses on UVB rays, you do need protection from UVA rays.
UVA is present in much higher levels in sunlight than UVB and is a major cause of skin aging like wrinkles, freckles and sunspots. It also plays a role in causing some skin cancers.
“A high SPF doesn’t tell you anything about how much UVA protection you’re getting,” says George.
“Choose a sunscreen that’s at least SPF 30, broad spectrum, and water-resistant if you’re swimming or sweating,” George says. “Beyond that, with all the brands and formulations out there, pick whichever one you like that you won’t mind using regularly.”
Spray sunscreens can be convenient, but be sure to spray until you see a good sheen evenly across your skin.
Don't rely on sunscreen alone
The most important thing to know about sunscreen is that it should never be your only protection against the sun.
Seek shade or stay indoors between 10 a.m. and 4 p.m., when the sun’s rays are strongest. Sand, snow and water can reflect those rays up and still cause sun damage, even if you are in the shade.
Protect yourself with a wide-brimmed hat and sunglasses with UVA and UVB protection. Wear UV protective clothing or choose clothes that are dark and tightly woven.
And finally, avoid tanning beds which emit the same harmful rays as the sun.
“Sunscreen should be one part of a package deal,” says George. “Being sun smart means not relying on sunscreen alone for sun protection, regardless of its SPF.”