How Does Sunscreen Work? Mineral vs. Chemical SPF, Explained

At some point, you’ve probably been nagged by your doctor, an overly concerned parent, or a well-meaning health site (hello!) to wear SPF. Yes, sun protection really is that important. However, few of us understand the science behind how sunscreen works to save our skin.

Cream or lotion, spray or stick, sunscreen in any of its forms should be an essential part of your skin care routine. Its most important benefit is, of course, shielding you from the sun’s ultraviolet (UV) rays, which in turn can reduce your risk of developing skin cancer (including melanoma, the deadliest form) and ward off painful burns when you spend a little too much time outside.^1,2 And if all that isn’t enough to convince you that SPF is queen, research has also shown that sunscreen can prevent and lighten hyperpigmentation (a.k.a. dark spots) and diminish signs of aging, like wrinkles and loss of elasticity.^3,4

“Sunscreen is a necessary staple for all skin types, textures, and colors,” Mona Gohara, MD, board-certified dermatologist and associate professor at Yale School of Medicine, tells SELF. Yes, all skin types—whether your face is dry or oily, dark or light, quick to tan or burn. Though many of us are pretty diligent about slathering on SPF products before a beach day or an outdoor hang, you should be wearing sunscreen every single day and all year round, come rain or shine, Dr. Gohara urges.

If you’re wondering precisely why—and how—you must incorporate this holy grail into your daily routine, we’ve got you, well, covered. SELF asked dermatologists to break down sunscreen’s seemingly magical ability to deliver on both its health and beauty promises.

How does sunscreen work?| Sun tan lotion versus sunscreen | What does SPF mean? | How much sunscreen should you use? | How to reapply sunscreen

How does sunscreen work, exactly?

There are two types of sunscreen—mineral (a.k.a. physical) and chemical—and both safeguard you from the sun’s harmful UVA rays (responsible for premature aging) and UVB rays (which cause sunburns and play the biggest role in the development of skin cancer), Susan Massick, MD, dermatologist and associate professor at Ohio State University College of Medicine, tells SELF.⁵ The key difference between mineral and chemical formulas, however, are the active ingredients used to protect your skin cells.

“Mineral SPF products primarily use zinc oxide or titanium dioxide, which physically block the sun’s rays before they penetrate your skin,” Dr. Massick explains. (Hence, the term “sunblock”). Chemical ones, on the other hand, absorb the heat from UV rays, thanks to ingredients like avobenzone and octisalate, among various others.^6,7,1 Think of it like a shield (mineral) versus a sponge (chemical).

You might be wondering, Well, which one is better? The short answer: It really comes down to personal preference, as SELF previously reported. “Most people won’t have a negative or allergic reaction to mineral sunscreens, so they’re generally safe for sensitive skin types,” Dr. Gohara says. However, because zinc and titanium dioxide are white in color, and mineral sunscreens tend to be thicker, they may also leave behind an unwanted, ghostly white cast—especially on darker skin tones. (More on that below.)⁷ No matter your formula of choice, just make sure the label specifies “broad-spectrum” coverage, so you know you’re protected against both UVA and UVB rays.

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So…is suntan lotion the same as sunscreen?

Let’s be real: Some of us (me!) want to achieve a sun-kissed glow—without the damage, of course. You’d think that’s what tanning lotions are for, but sorry to burst your bubble: “Suntan lotions, which are products with very low SPF levels, between 4 and 10, are not sunscreens,” Dr. Massick clarifies. “They’re not strong enough to protect your skin from sun exposure.”

The truth is there isn’t a safe way to tan under the sun (or in a tanning bed, to be clear). Any burning and subsequent darkening of the skin—including a so-called “base tan”—is a sign of skin damage, Dr. Massick says. And even if you use sunless tanning products to get a superficial bronzing effect, yep, you still need to apply sunscreen.

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What does the SPF number on sunscreen labels mean?

You’ve probably seen those three letters sprawled across sunscreens, moisturizers, and even makeup. But what the heck does it really mean? Is higher always better?

SPF stands for sun protection factor. According to the Food and Drug Administration (FDA), this number is a measure of how much UVB radiation it takes to burn your skin when it’s slathered in sunscreen compared to the amount of UVB radiation it takes when you’re unprotected. In other words, the higher the SPF value of the product, the more sunburn (and therefore damage) protection it offers.

To get more specific, about 7% of the sun’s rays will reach your skin if you’re wearing SPF 15 versus only 2% for SPF 50.⁸ That said, a higher number may give you a false sense of security and lead you to, um, fly too close to the sun, so we’re warning you: No matter the SPF number on your sunscreen, you still need to apply enough and reapply regularly (per the guidelines below).

Even if you’re sitting in the office all day or just running a few errands, you should opt for SPF 30 at a minimum for daily wear, according to the American Academy of Dermatology (AAD). It may seem silly (I’m barely going to be outside!) but the sun’s damaging UV rays can still permeate on cloudy days or even indoors through a window, according to Dr. Gohara. (Plus, SPF can also shield your skin from the blue light from device screens, which has been shown to worsen hyperpigmentation.⁹)

For more outdoorsy activities where you’ll have lots of direct sun exposure, like an afternoon picnic in the park or pool party at your best friend’s place, both Dr. Gohara and Dr. Massick recommend SPF 50 or above.

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How much sunscreen should you use?

For your face, a nickel-sized dollop (after skin care products, like moisturizers and serums, and before makeup) is a good rule of thumb, Dr. Gohara says, as is a shot glass’s worth for your body, per the AAD.

However, people come in a variety of shapes and sizes, so if you have more skin to cover, use your best discretion and opt for more: You don’t want to use too much to the point where you’re feeling super sticky, greasy, and uncomfortable, but everyone should aim for a thick layer over all exposed areas, including often-forgotten spots like behind the ears and near the armpits—basically, if the skin will see the light of day, you’ve gotta cover it up! (Oh, and apply it 15 to 30 minutes prior to sun exposure, if you can, to give it enough time to absorb and become fully effective).

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How often should you reapply sunscreen?

Generally speaking, applying sunscreen first thing in the morning probably won’t last you all day.¹⁰ It’ll likely rub off to some degree (even if you’re staying indoors) and eventually become less effective, Dr. Gohara says.¹ That’s why the second most important step for sunscreen, above using enough in the first place, is consistent reapplication—every two to three hours when outdoors, according to Dr. Gohara. You don’t necessarily need to reapply that often if you’re inside all day, but since sunscreen can still wear off (via friction from clothing, touching your face, or washing your hands, for example) you should probably still slather up at least twice per day, she adds.¹¹

“The protection provided by most sunscreens is temporary and only lasts between two to three hours,” Dr. Massick explains. And options that say “water-resistant” on the label—which you should always choose if you’ll be sweating or getting wet in the pool or elsewhere—only guarantee protection for 40 to 80 minutes, she adds. Another thing to keep in mind: If you’re in the sun during its strongest hours (10 a.m. to 4 p.m.), you’ll burn more quickly, which, yep, makes reapplication all the more essential.

Again, the kind of sunscreen you use is up to you. What matters most is finding one (or more) you like. (Lucky for you, SELF recently rounded up plenty of options for various skin types and needs.) So if you break out easily or hate the greasy, sticky aftermath of some lotions, for example, you’ll probably be happier with a noncomedogenic (not pore-clogging) option without heavy oils, like EltaMD’s UVClear Broad Spectrum sunscreen ($38, Amazon), a Dr. Gohara pick that also contains anti-inflammatory niacinamide. Or let’s say most mineral SPF products leave your dark skin a streaky, chalky mess. In that case, we suggest a formula containing non-nano zinc oxide which can prevent a white cast due to its tinier particles, like Vacation’s Mineral Lotion SPF 30 Sunscreen ($20, Ulta).⁷ Or perhaps a chemical-based alternative like Black Girl Sunscreen ($16, Target), another Dr. Gohara fave.⁷

One last warning before we let you go: Be mindful of your sunscreen’s expiration date. Don’t see one? Or perhaps it rubbed off? As a general rule, the FDA says that you should consider your product expired (and no longer effective) three years after you buy it. However, exposure to light and extreme heat (from sitting on the beach or in a hot car all day, say) can make it go bad faster, so give it the sniff and sight test: If it smells or looks watery, chunky, or like its changed color, it’s probably past its prime and belongs in the trash, not on your face.

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