What Do SPF Numbers Mean And Which Should You Use?

Summer is all about spending more time in the sun, whether you're hanging out at a rooftop bar or chilling by the pool. More sun exposure also means more sun damage, so there's no arguing that you need to wear sunscreen every day. According to the American Academy of Dermatology Association, wearing sunscreen can protect you from "developing skin cancer, sunburn, and premature skin aging like age spots, sagging, and wrinkles." That's why it's so important to wear sunscreen, especially when you are out, though you should also be wearing sunscreen indoors.

"We are exposed to ultraviolet and infrared light every day that produces free radicals in our skin," dermatologist Diane Madfes, M.D., tells Byrdie, adding that free radicals decrease the production of collagen and elastin in our skin that are responsible for keeping our skin looking firm and young. Sunscreen is one of the best anti-aging products in which anyone can invest.

We know sunscreen is essential, but beauty aisles are overcrowded with tons of options at all price points, which can make choosing the best sunscreen complicated. Should you use physical or chemical sunscreen? Is tinted sunscreen as effective as regular sunscreen? Is powder, stick, or spray sunscreen easier for reapplication? Is it sweat and water-resistant? Of course, too, sunscreens also have an SPF number, but what do they represent?

SPF stands for sun protection factor, and while many people think the numbers relate to how much time your skin is protected, that's not the case. According to the U.S. Food & Drug Administration (FDA), SPF measures "how much solar energy (UV radiation) is required to produce sunburn," and it corresponds to the "amount of solar exposure" and not the "time of solar exposure." SPF is considered a "relative measure" of how much sunscreen can protect you from UVB rays that cause burning. For example, an hour at 9 a.m. produces the same amount of solar energy as 15 minutes at 1 p.m., which is why you'd benefit from using a higher SPF if you're out midday compared to during the morning or evening. Still, you must reapply throughout the day. 

"SPF 15 guards against 93% of UVB rays, SPF 30 protects against 97%, SPF 50 is about 98%, and SPF 100 blocks 99%," board-certified dermatologist Joshua Zeichner tells Allure, adding that SPF 30 gives twice as much sun protection than SPF 15. Therefore, people who choose a higher SPF get more sun protection. But what do the numbers actually mean? 

According to The Skin Cancer Foundation, the numbers boil down to the strength of the product. For instance, SPF 30 indicates that your skin will stay healthier for "30 times longer" compared to if you'd forgone sun protection altogether. As such, SPF 45 protects you for 45 times longer, SPF 60 for 60 times longer, and so on; however, there's no such thing as 100% protection, even if the sunscreen claims to have SPF 100 — it only delays the inevitable (even if that delay is quite long in the case of SPF 100).

As it turns out, broad-spectrum sunscreen is what we all need. In fact, Dr. David Kim, a cosmetic dermatologist at Idriss Dermatology in New York City, tells Self, "The American Academy of Dermatology and dermatologists, in general, recommend SPF 30 and higher because that provides 97% of the UVB protection." So, when a medical professional recommends a specific type of sunscreen, we're all for it.

While higher SPFs mean heightened protection from burning, SPF doesn't protect you from the sun's UVA rays. According to the Skin Cancer Foundation, you need a sunscreen with at least SPF 15 — SPF 30 or higher if you're outside — and broad-spectrum protection to protect you from the sun's harmful UV rays that are a combination of UVA, UVB, and UVC rays. The UVC rays don't make it past the ozone layer as they are shorter, so you don't need to worry about them. Both UVA and UVB rays can damage your skin's DNA, so choosing a sunscreen with broad-spectrum protection is vital so you are protected from premature signs of aging, burning, and skin cancer. It's smarter to protect yourself from all kinds of harmful UV rays than just one.

Sunscreen is non-negotiable when it comes to essential skincare. When selecting the right sunscreen for your skin type or lifestyle, the higher the SPF, the less likely you are to burn. But don't forget to make sure it has broad-spectrum protection, so you can keep premature signs of aging at bay.