Dermatologists break down the science behind sun protection.

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In 2020, one thing we know for sure is that SPF is an important part of skin care. It’s important to apply not just when we’re at the beach but on a daily basis—even on days we don’t get outside. Most times, some level of SPF can be found in our BB creams, foundations, setting sprays, and powder makeup, but of course, the most obvious displays are found on the sunscreen we slather ourselves with while sitting by the pool. Without it, we run the risk of exposing our skin to general sun damage, premature aging, hyperpigmentation, wrinkles, and more.

But just how much SPF is essential? When it comes to understanding what those numbers on the sunscreen bottles actually mean, things can get a little confusing. Though it would seem obvious that higher SPF means higher-level protection, that might not be entirely true. To clear up any confusion about SPF numbers and what they mean, we asked a dermatologist to explain. This way, we can find out once and for all whether or not a higher number means better sun protection.

What does SPF stand for?

SPF stands for “sun protection factor.” It measures how much the product shields ultraviolet rays from your skin.

What does SPF protect from?

SPF is essentially a measure of the sunscreen's ability to protect your skin from UVA and UVB rays. These are the two types of ultraviolet rays that can damage the DNA in your skin cells and lead to skin cancer, so when looking for a good sunscreen, you want to make sure the product protects from both. Usually, this is denoted by the phrase “broad spectrum” on a sunscreen product’s label.

  • UVB rays cause sunburn and play a key role in developing skin cancer. A sunscreen’s SPF number refers mainly to the amount of UVB protection it provides.
  • UVA rays cause skin damage that leads to tanning as well as skin aging and wrinkles. The shortest wavelengths of UVA rays also contribute to sunburn.

Ideally, you'll want to use a sunscreen that has both UVA and UVB protection.

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What does the SPF number mean?

According to the Skin Cancer Foundation,“The SPF number tells you how long the sun’s UV radiation would take to redden your skin when using the product exactly as directed versus the amount of time without any sunscreen.”

New York-based board-certified dermatologist Hadley King, M.D., breaks it down further by explaining the basic calculation. She says, “If it takes one minute for your unprotected skin to start turning red in the sun, using an SPF 15 sunscreen should prevent reddening 15 times longer. So the number of minutes it takes for your unprotected skin to turn red multiplied by the SPF gives you the theoretical number of minutes that your protected skin will take before it turns red.”

Because of this, dermatologists like Dr. King follow the guidelines set by The American Academy of Dermatology, which recommend using a sunscreen with an SPF of at least 30. “SPF 30 blocks about 97 percent of the sun's UVB rays,” she says. “Higher-number SPFs block slightly more of the sun's UVB rays, but remember: No sunscreen can block 100 percent of the sun's UVB rays.”

Does a sunscreen with a high SPF protect skin better than one with a lower SPF?

The effectiveness of your SPF, no matter how high, depends on how well (and how often) you apply it. Dermatologists typically recommend a quarter-sized dollop for your face and a shot-glass-sized portion for your body. So keep in mind that just because you slather on SPF 50 or 100 does not mean you can spend additional time outdoors without reapplication.

The American Academy of Dermatology explains that high-number SPFs last the same amount of time as low-number SPFs. Per the organization's recommendation, all sunscreens should be reapplied approximately every two hours when outdoors (even on cloudy days), while staying indoors, and after swimming or sweating, according to the directions on the bottle.

However, research published in the Journal of the American Academy of Dermatology did prove that SPF 100+ was significantly more effective at protecting you from sunburn. These results are consistent with the fact that SPF 50 sunscreens allow twice as many damaging UV rays to reach the skin compared with SPF 100 sunscreens. The study found that after about six hours of sun exposure, more than half of the participants had more sunburn on the side of their face to which they'd applied an SPF 50+ sunscreen compared to only 5% who had more sunburn on the SPF 100+ side.

To put it in perspective, an SPF of 15 correlates with 93.3% of UVB absorption, whereas SPF 30 correlates with 96.7%, SPF 45 correlates with 97.8%, and SPF 50 correlates with 98% UVB absorption, according to a study published in The Journal of  Clinical and Aesthetic Dermatology. If you're prone to sunburn or have a skin condition, you should still opt for the higher number, but The American Academy of Dermatology recommends everyone use sunscreen that at least offers the following:

  • Broad-spectrum protection (protects against UVA and UVB rays)
  • SPF 30 or higher
  • Water resistance

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Additionally, Jamal Downer, a Walgreens pharmacist and sun protection expert, tells HelloGiggles that higher-SPF sunscreens are often thicker in consistency, which can make them less likely to be applied properly. Because of this, sometimes a lower SPF that is applied more frequently can offer similar protection.

Sunscreen is also just one part of protecting skin from the sun. "It's also important to avoid peak UV hours, seek shade, wear protective clothing and a broad-brimmed hat, and wear UV-blocking sunglasses," says Dr. King.